Is it possible to operate a 12 VDC relay from a 24 VAC suppl

[HC]

On Feb 15, 10:38 pm, John Popelish <jpopel...@rica.net> wrote:

HC wrote:
Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

(snip)

For the same coil power, the 24 volt relay will need about
1/4 of the copper to operate over the distance. AC relays
are not quite as efficient as DC relays are and have an
inductive current component added to that which powers the
coil, so the actual improvement may be closer to 3 to 1 or
even 2 to 1.

But 100 feet is not so far for a couple watt 12 volt coil.
How fine a wire do you want to use to drive the coil?
Measure the coil resistance and use a wire table to figure
out how many ohms you can afford in series with that (maybe
1/20th of the coil resistance for 200 feet of wire.

For instance if your have a relay that has a 2 watt coil
(many are lower than this) that would be about a 288 ohm
coil. So you would loose only about 5% of the drive voltage
if the wire loop added another 288/20=14.4 ohms. You could
use higher resistance wire if you had a bit more than 12
volts to drive the loop.

Taking a look at a wire table,http://www.pupman.com/listarchives/1998/April/msg00189.html
the size that has less then 14.4 ohms for 200 feet would be
AWG 28, which is really fine. Something like cheap 22 AWG
speaker wire would have a resistance of only about 3.2 ohms
for the 200 feet, and that would drop only
12*3.2/(288+3.2)=0.13 volts or about 1% of the 12 volts.

You really do not have much of a problem to solve.

--
Regards,

John Popelish

Hey, John, thank you for your reply. That wire link is cool and I've
bookmarked that.

What I've been intending to use as the "messenger" wire for the relay
is some 24 gauge wire I have already (from where and when I don't
recall; it's followed me for about 10 years now).

If I understand you correctly, the resistance of the wire for this
distance and for this load (the relay coil) is not significant enough
to worry about, within some parameters. I went and measured my
relay's coil resistance and found it to be 308 ohms. I then applied
11.89 volts (according to my DVM) from my proto-board and measured
37.7 mA (using the same DVM) across the relay coil with that voltage.
I guess all I needed to measure was the 308 ohms, but since I was
there it seemed useful to measure the rest. I ran it through the
equations I have for power (Watts) and Ohm's Law, just to see if my
measurements were right and got numbers that seem good.

1/20th of the 308 would be 15.4 ohms, so, you're saying I could add up
to 15.4 ohms of round-trip "messenger" wire to the relay, on a 12VDC
supply, and it would be okay? So, looking at the chart you linked to,
24 gauge has 38.958 feet per ohm so I could run, round-trip, roughly
600 feet?

I really am an amateur at this stuff and I've seen your posts all over
the place helping people so I ask the following with no ill-will or
attitude towards you; you said that a 2W coil would be about a 288 ohm
coil. I tried to get that (because I want to be able to calculate
this information for other relays and to better understand this stuff)
assuming a 12 volt DC supply and I did a calculation for current with
P = I E so I = 2W / 12V and got I = 0.1667A. I put that into E = I R
solving for R = 12V / 0.1667A, R = 72 ohms. What have I done wrong?

Thank you very much again for your reply. It helped a lot. I will
set up a test rig to verify that the numbers I work out on paper are
accurate but I'm confident this will work. This is going to simplify
my project quite a bit (not having to use a 24 VAC supply to try to
run 12 VDC relays).

--HC

 

[ehsjr]

HC wrote:

Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC

John Popelish gave you the answer - you won't
have a problem driving those 12 volt relays
with 100 feet of wire.

But as a learning/experimental/fun thing, you could
try using a zener & transistor circuit with the 24 volt
approach you mentioned.

Ed

 

[Ross Herbert]

On Fri, 15 Feb 2008 20:19:36 -0800 (PST), HC <hboothe@gte.net> wrote:

:Hey, all, I'm not sure if this can be done but here's what I am trying
:to do and how I've tried to go about doing it.
:
:I want to be able to control 120VAC devices some distance away from a
:controller, say, up to 100 feet or so. I would like to run the 120VAC
:to the device through a switch (relay) at the device with no other
:switches or control devices in line from the breaker. Then I would
:like to have the controller turn that relay on and off to control the
:device. The idea is that I could run the thicker, high-voltage lines
:directly to the device and then use smaller wire to operate a relay at
:the device to turn it on and off instead of running the high-voltage
:wire to each switch I would like to use.
:
:Since I have a boat-load of low-cost 12VDC relays that can switch up
:to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
:be a lot more expensive and harder to come by (they seem to be, in my
:searching, related to HVAC and other "industrial" uses; they're not
:like the overly-abundant 12VDC relays we have for our cars and such) I
:would like to use a 12VDC relay at the device. However, I'm afraid
:that if I attempt to use 12VDC to control these relays over a distance
:like I mention of up to 100 feet that the line-loss will be
:significant (on 12VDC). I was thinking that using 24VAC would be much
:better (it's higher voltage and it's AC, so line-loss should be quite
:a bit less than 12VDC).
:
:I tried this: I took the 24VAC and rectified it with a single diode
half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
:that buffered output through a LM78L12 (with input and output caps as
:detailed in the datasheet I was reading) and that output to the 12VDC
:relay I wanted to operate. When I apply 24VAC to the circuit the
:relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
:the relay turns off like a marshmallow; slow and makes some light
:clicking noises. So, the input cap (2,200 uF) is still powering the
:relay coil and is letting it down slowly; at least, that's my hack-boy
:assessment: I'm not an expert at any of this stuff. I tried whatever
:I could including "pull-down" resistors (if I'm using that term
:correctly); I put a 10k resistor from the relay input to ground. I'm
:afraid the slow turn-off is going to cause arcing and fry the relay
:contacts.
:
:I don't have a lot of caps that are rated at 35 volts or higher (which
:could handle the 24VAC, rectified) so experimentation was limited in
:various cap sizes (like, could a 1,000 uF input cap allow the relay to
:turn off quickly?); somewhere I got the idea that if you put more
:voltage across an electrolytic cap than it can handle it can "explode"
r "pop" or "blow-up" or whatever so I'm reluctant to use 16V
:electrolytics on what should be 24V or higher.
:
:Anyway, I hope I've done a good job of explaining what I'm trying to
:do and what I've tried to do to achieve it. Is there an effective way
:to run a 12VDC relay from a supply circuit of 24VAC?
:
:Thanks in advance.
:
:--HC


The first thing to sort out is whether your "boat load of 12V relays" is
suitable for controlling 120Vac devices. Current rating sounds reasonable but
the critical detail is "isolation voltage rating" between the 12Vdc side(coil)
and the 120Vac side (contacts). Any relay which is to be used for this purpose
must meet certain standards and automotive types are not suitable. Many
"industrial" types will be suitable.

Your approch to producing the dc coil operating voltage is ok and your reasoning
for the long release time is also correct. The release time problem is easy to
overcome. When you want the relay to release, you don't switch off the 24V
supply at the ac wall switch, you simply disconnect the relay coil from the dc
output voltage of your power supply.

 

[John Popelish]

HC wrote:

Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).
(snip)

For the same coil power, the 24 volt relay will need about
1/4 of the copper to operate over the distance. AC relays
are not quite as efficient as DC relays are and have an
inductive current component added to that which powers the
coil, so the actual improvement may be closer to 3 to 1 or
even 2 to 1.

But 100 feet is not so far for a couple watt 12 volt coil.
How fine a wire do you want to use to drive the coil?
Measure the coil resistance and use a wire table to figure
out how many ohms you can afford in series with that (maybe
1/20th of the coil resistance for 200 feet of wire.

For instance if your have a relay that has a 2 watt coil
(many are lower than this) that would be about a 288 ohm
coil. So you would loose only about 5% of the drive voltage
if the wire loop added another 288/20=14.4 ohms. You could
use higher resistance wire if you had a bit more than 12
volts to drive the loop.

Taking a look at a wire table,
http://www.pupman.com/listarchives/1998/April/msg00189.html
the size that has less then 14.4 ohms for 200 feet would be
AWG 28, which is really fine. Something like cheap 22 AWG
speaker wire would have a resistance of only about 3.2 ohms
for the 200 feet, and that would drop only
12*3.2/(288+3.2)=0.13 volts or about 1% of the 12 volts.

You really do not have much of a problem to solve.

--
Regards,

John Popelish

 

[HC]

Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC

 

[John Fields]

On Wed, 20 Feb 2008 06:39:06 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 20 Feb 2008 07:52:51 -0600, John Fields
jfields@austininstruments.com> wrote:

On Mon, 18 Feb 2008 17:20:45 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:


Did you actually test a bunch of relays just to prove me wrong?

---
Of course. With your proclivity for argumentative generalization
and the ol' bob and weave I sometimes find it necessary to use
experimentally derived real-world data in order to pin you down.
---

I'm flattered.

---
Sorry, that wasn't my intention.
---

Better would be to put the resistor *before* the bridge, so
the diodes clamp the coil voltage close to zero.

---
You've lost me there.

What are you talking about?


The L/R thing. Look it up.

---
Better yet, look at this:

Version 4
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SYMATTR Value MURS120
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SYMATTR InstName R7
SYMATTR Value 50
TEXT -1248 400 Left 0 !.tran .05 uic

Note that either way (with the 50 ohm resistor internal or external
to the bridge) clamps the coil to a couple of diode drops _below_
ground, so I still don't know what you're talking about.

The difference is in the ripple current in the coil.

---
Come on John, knock off the bullshit.

You wrote:

"Better would be to put the resistor *before* the bridge, so
diodes clamp the coil voltage close to zero."

Which states, as I read it, that without the resistor being placed
before the bridge the coil voltage wouldn't be clamped close to zero
volts.

The truth is that either configuration clamps the coil voltage to a
couple of diode drops _below_ zero volts, so your premise is flawed.
---

The current difference is modest for the L/R values in your model,
especially with two diode drops. One interesting thing about relays
is that the inductance increases, often by a huge factor,
between the de-energized and pulled-in state [1].

---
Huge???

Got an example?
---

---
The de-energized value, and I chose that because it was the rest
inductance of the coil of the only relay which worked in real life
with your half-wave driver.

It's configured as a solenoid, so I suspect the reason it _did_ work
is because the energy stored in the inductance with the plunger
fully engaged didn't decay to the point where the plunger disengaged
once the driving signal was removed and later reapplied.
---

However, there _does_ seem to be an advantage to using the external
resistor.

Can you tell what it is?

Dunno. Tell us.

---
OK.

Assuming that the relay has a drop-out current of 60mA (25% of the
nominal 240ma; I'll measure it later if it's important to you) the
external resistor lets the ripple current fall to about 112mA, while
the internal one allows the ripple current to fall to about 94mA.

So, either on will work fine, but the external one has the edge when
the coil heats up.
---

which gives power AC relays a huge advantage.

---
Over what?
---

It would be awful to do a fully time-accurate model of a relay
pulling in, what with L being a function of the varying magnetic loop
reluctance. Ugh.

---
Sounds kinda fun to me; maybe I'll make that my next project!

--
JF

 

[John Larkin]

On Thu, 21 Feb 2008 07:14:22 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 20 Feb 2008 06:39:06 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 20 Feb 2008 07:52:51 -0600, John Fields
jfields@austininstruments.com> wrote:

On Mon, 18 Feb 2008 17:20:45 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:


Did you actually test a bunch of relays just to prove me wrong?

---
Of course. With your proclivity for argumentative generalization
and the ol' bob and weave I sometimes find it necessary to use
experimentally derived real-world data in order to pin you down.
---

I'm flattered.

---
Sorry, that wasn't my intention.
---

Better would be to put the resistor *before* the bridge, so
the diodes clamp the coil voltage close to zero.

---
You've lost me there.

What are you talking about?


The L/R thing. Look it up.

---
Better yet, look at this:

Version 4
SHEET 1 960 932
WIRE -1168 -48 -1248 -48
WIRE -1024 -48 -1088 -48
WIRE -704 -48 -1024 -48
WIRE -240 -48 -464 -48
WIRE 176 -48 -240 -48
WIRE -1024 -16 -1024 -48
WIRE -704 -16 -704 -48
WIRE -240 -16 -240 -48
WIRE 176 -16 176 -48
WIRE -1024 128 -1024 48
WIRE -1024 128 -1088 128
WIRE -960 128 -1024 128
WIRE -848 128 -880 128
WIRE -704 128 -704 48
WIRE -704 128 -768 128
WIRE -240 128 -240 48
WIRE -240 128 -304 128
WIRE -192 128 -240 128
WIRE -64 128 -112 128
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WIRE -704 368 -704 304
WIRE -704 368 -1024 368
WIRE -464 368 -464 320
WIRE -240 368 -240 304
WIRE -240 368 -464 368
WIRE 176 368 176 304
WIRE 176 368 -240 368
FLAG -304 176 0
FLAG -1088 176 0
SYMBOL ind -80 144 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L1
SYMATTR Value .148
SYMBOL voltage -464 224 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 34 60)
SYMATTR InstName V1
SYMBOL diode 160 -16 R0
WINDOW 0 -45 31 Left 0
WINDOW 3 -109 -7 Left 0
SYMATTR InstName D1
SYMATTR Value MURS120
SYMBOL res -96 112 R90
WINDOW 0 -38 58 VBottom 0
WINDOW 3 -34 59 VTop 0
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL diode 192 304 R180
WINDOW 0 47 34 Left 0
WINDOW 3 23 -2 Left 0
SYMATTR InstName D2
SYMATTR Value MURS120
SYMBOL diode -224 48 R180
WINDOW 0 -44 32 Left 0
WINDOW 3 -107 64 Left 0
SYMATTR InstName D3
SYMATTR Value MURS120
SYMBOL diode -256 240 R0
WINDOW 0 39 34 Left 0
SYMATTR InstName D4
SYMATTR Value MURS120
SYMBOL res 160 112 R90
WINDOW 0 -38 58 VBottom 0
WINDOW 3 -34 59 VTop 0
SYMATTR InstName R4
SYMATTR Value 50
SYMBOL ind -864 144 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L3
SYMATTR Value .148
SYMBOL voltage -1248 224 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 34 60)
SYMATTR InstName V3
SYMBOL diode -720 -16 R0
WINDOW 0 -45 31 Left 0
WINDOW 3 -109 -7 Left 0
SYMATTR InstName D5
SYMATTR Value MURS120
SYMBOL res -864 112 R90
WINDOW 0 -38 58 VBottom 0
WINDOW 3 -34 59 VTop 0
SYMATTR InstName R5
SYMATTR Value 50
SYMBOL diode -688 304 R180
WINDOW 0 47 34 Left 0
WINDOW 3 23 -2 Left 0
SYMATTR InstName D7
SYMATTR Value MURS120
SYMBOL diode -1008 48 R180
WINDOW 0 -44 32 Left 0
WINDOW 3 -107 64 Left 0
SYMATTR InstName D8
SYMATTR Value MURS120
SYMBOL diode -1040 240 R0
WINDOW 0 39 34 Left 0
SYMATTR InstName D9
SYMATTR Value MURS120
SYMBOL res -1072 -64 R90
WINDOW 0 -38 58 VBottom 0
WINDOW 3 -34 59 VTop 0
SYMATTR InstName R7
SYMATTR Value 50
TEXT -1248 400 Left 0 !.tran .05 uic

Note that either way (with the 50 ohm resistor internal or external
to the bridge) clamps the coil to a couple of diode drops _below_
ground, so I still don't know what you're talking about.

The difference is in the ripple current in the coil.

---
Come on John, knock off the bullshit.

No BS. LT Spice lets you see currents. Look at the currents in R5 and
R1. The R1 current is decaying faster. The advantage will be more
pronounced when the coil L is higher, as it will be for bigger relays
and/or when when the armature is seated.

You wrote:

"Better would be to put the resistor *before* the bridge, so
diodes clamp the coil voltage close to zero."

Which states, as I read it, that without the resistor being placed
before the bridge the coil voltage wouldn't be clamped close to zero
volts.

The truth is that either configuration clamps the coil voltage to a
couple of diode drops _below_ zero volts, so your premise is flawed.

What I said wasn't a "premise", it was a fact. If there's current in
the coil, and there's a resistor directly in series with the coil,
there must be voltage drop in the resistor, so that changes the
voltage in the coil. L/R drops in half if you double R, which is
exactly what happens if you put the resistor after the bridge.

What's being clamped is the voltage across the coil+resistor series
pair, which is not the voltage across the coil.

---

The current difference is modest for the L/R values in your model,
especially with two diode drops. One interesting thing about relays
is that the inductance increases, often by a huge factor,
between the de-energized and pulled-in state [1].

---
Huge???

Got an example?
---

Which L did you use in your sim? [2]

---
The de-energized value, and I chose that because it was the rest
inductance of the coil of the only relay which worked in real life
with your half-wave driver.

Well, coil L increases a lot when the armature pulls in and closes up
the air gap.

It's configured as a solenoid, so I suspect the reason it _did_ work
is because the energy stored in the inductance with the plunger
fully engaged didn't decay to the point where the plunger disengaged
once the driving signal was removed and later reapplied.
---

However, there _does_ seem to be an advantage to using the external
resistor.

Can you tell what it is?

Dunno. Tell us.

---
OK.

Assuming that the relay has a drop-out current of 60mA (25% of the
nominal 240ma; I'll measure it later if it's important to you) the
external resistor lets the ripple current fall to about 112mA, while
the internal one allows the ripple current to fall to about 94mA.

Thanks! That's exactly the point I've been making.

So, either on will work fine, but the external one has the edge when
the coil heats up.

How so?

---

[1] which gives power AC relays a huge advantage.

---
Over what?

Over DC-coil relays, especially in demanding industrial applications.
You can design an AC-coil relay to have a huge initial coil current,
to really slam the contacts closed against a powerful spring. When the
armature fully seats, inductance increases, coil current drops, and
the thing stays nicely latched at low coil current. A DC relay with
similar mechanics would fry the coil, because current is determined by
the copper resistance and doesn't drop when the armature seats.

I have seen DC relays and solenoids with auxiliary contacts that
simulated this effect.

John

 

[HC]

On Feb 19, 8:20 pm, whit3rd <whit...@gmail.com> wrote:

On Feb 15, 8:19 pm, HC <hboo...@gte.net> wrote:



I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so.

To do it safely, there are some concerns about the '12V' relays; if
they are
intended for auto use, for instance, the application of 120VAC may not
be wise.

Why not use the canned solution, an X-10 appliance module and suitable
remote controllers? It has the advantage of UL approval. In case
of mishap your insurance provider might find that interesting.

Whit3rd, thank you for your reply. I'm sorry for the delay in my
response; I let life run me a bit this week and I hadn't been able to
get the materials I needed, until yesterday, to test some of the other
suggestions so I hadn't been following this closely.

I have not found an answer to the question of whether or not the auto
relays can handle the 120VAC, and without that answer I will not use
them for safety reasons, as you mention. I do have ones that are
rated for 120VAC and 250VAC and 300VAC, so I still have something to
play with.

I looked up the X-10 stuff, thank you. What I see is that the devices
are mostly used in connection with other X-10 devices whereas what I'm
looking for is automation; can I control this, ultimately, with my
computer? What I'm envisioning here is that the devices will be
installed at my ceiling lights and other permanent power devices in my
home and controlled via low-voltage switches, including ones that can
be driven by my computer (via the parallel port (I have built these
simple devices along the lines of the simple Kit 74 that somebody
makes), or USB).

I have found some good information through testing of the suggestions
made to my posting that I will summarize in one post in reply to my
original post to centralize the information for anyone who wants it.

Thank you again.

--HC

 

[John Popelish]

HC wrote:

Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.

I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.
(snip)

Thanks for adding your conclusions to the thread. It will
make it more useful to those who eventually find it, in a
search to solve similar problems. I'm glad you have gotten
useful answers and added to your understanding.

--
Regards,

John Popelish

 

[John Fields]

On Sun, 24 Feb 2008 11:11:14 -0800 (PST), HC <hboothe@gte.net>
wrote:

On Feb 15, 10:19 pm, HC <hboo...@gte.net> wrote:
Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC

Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.

I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.

First, John Popelish suggested running the relays with 12VDC and that
the line-loss would be negligible. I did the math on the line-loss,
which I had not conceptualized or understood correctly before his post
and it seemed like a good idea. I had some crappy wire already run
out (I took a headphone jack to RCA Y-cable and cut it in two and
spliced some thin wire between the pieces to make a long-distance
auxiliary input for the stereo in my shop to run off the stereo in my
house). The cable is probably thinner than what John and I had
bounced back and forth about and the RCA audio stuff is pitifully
thin. The overall cable length is about 80 feet. I ran 12VDC across
the cable to a Shrack RP510012 relay with a coil resistance of 327
ohms. The coil snapped briskly in and out when the voltage was
applied. John was absolutely correct, the line-loss was insignificant
for the resistance of the coil. As I told John I would, I checked the
cut-in voltage of the relay. I started with it hooked up and engaged
at 12VDC and then turned the voltage down until it dropped out. I
then removed the relay and checked the voltage. Cut-out voltage was
2.79 VDC. I then hooked the relay back up and turned the voltage up
until it engaged and then disconnected and checked the voltage. Cut-
in was very soft at 6.5 VDC. I upped the voltage until I had crisp
cut-in which I got at 6.9 VDC. There seems to be no problem with a
very long-distance run to control these relays. Thank you, John
Popelish.

Next was John Larkin who suggested running the relays directly off 24
VAC with a couple of diodes. I used his diagram and two 1N4004
diodes. One diode came from the AC input to the relay, one went
across the relay coil. On the Shrack RP 50012 with a 327 ohm coil and
my 24 VAC source (RMS meter says it's 26.9 VAC) the relay chattered
constantly. I tried a Zettler AZ8-1CH-12D with a 312 ohm coil. It
also chattered. There seems to be some discussion as to why this
would or would not work and, honestly, it's beyond my current skill
level with electronics. But, FWIW, that's what I tried and that's
what I got. Thank you, John Larkin.

Next, Ed suggested I try something involving a Zener and a
transistor. I formulated my own idea based on this and then, in
response to that, Ed suggested a schematic. It tried his way and
mine. His way was to take the 24 VAC (which I full-wave rectified and
then buffered with a 2,200 uF 63V electrolytic) and make a voltage
divider with two 1k ohm resistors to drive a transistor. I hooked up
a LM78L12 exactly as (and the app notes) said, and the voltage divider
exactly as he said. I ran the voltage divider output to the base of a
2N4401 transistor. Applying 24VAC (same power supply as used above)
gave me a crisp turn-on of the relay and a crisp turn-off. However,
the turn-off came about 1.5 seconds after I disconnected the power
from the circuit indicating that the capacitor was still carrying the
circuit for a little bit, but at least it was a crisp turn-off. I
only tried this with the Schrack relay. I then tried the circuit the
way I had postulated in response to Ed's suggestion before he gave me
the schematic. I did it exactly as he suggested except that I ran the
rectified and buffered 24 VAC (about 35 VDC) trough a 30V Zener to the
base of the 2N4401. I got the same crisp on and off but the off came
much more quickly after I disconnected the power from the circuit. I
did the math on the current through the relay coil (12 / 327) and got
that it should be 36.7 mA. I measured it in actual use and got about
35 mA. In either case, the current should be well below the maximum
current capability of the LM78L12 (100 mA, 140 mA peak), however, the
voltage regulator was getting very, very hot (enough it was painful to
touch and I could smell the chip). I checked the actual current
through the voltage regulator to the coil of the relay (which is where
I got the number above; 35 mA). At 12 volts that should be about 0.4
watts, which I thought the LM78L12 could handle but the heat seems
excessive. I may need to put a heat sink on it. Regardless, the idea
works. Thank you, Ed.

Thank you all for your help.

---
Geez, what am I, chopped liver? ;(

--
JF

 

[HC]

On Feb 15, 10:19 pm, HC <hboo...@gte.net> wrote:

Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC

Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.

I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.

First, John Popelish suggested running the relays with 12VDC and that
the line-loss would be negligible. I did the math on the line-loss,
which I had not conceptualized or understood correctly before his post
and it seemed like a good idea. I had some crappy wire already run
out (I took a headphone jack to RCA Y-cable and cut it in two and
spliced some thin wire between the pieces to make a long-distance
auxiliary input for the stereo in my shop to run off the stereo in my
house). The cable is probably thinner than what John and I had
bounced back and forth about and the RCA audio stuff is pitifully
thin. The overall cable length is about 80 feet. I ran 12VDC across
the cable to a Shrack RP510012 relay with a coil resistance of 327
ohms. The coil snapped briskly in and out when the voltage was
applied. John was absolutely correct, the line-loss was insignificant
for the resistance of the coil. As I told John I would, I checked the
cut-in voltage of the relay. I started with it hooked up and engaged
at 12VDC and then turned the voltage down until it dropped out. I
then removed the relay and checked the voltage. Cut-out voltage was
2.79 VDC. I then hooked the relay back up and turned the voltage up
until it engaged and then disconnected and checked the voltage. Cut-
in was very soft at 6.5 VDC. I upped the voltage until I had crisp
cut-in which I got at 6.9 VDC. There seems to be no problem with a
very long-distance run to control these relays. Thank you, John
Popelish.

Next was John Larkin who suggested running the relays directly off 24
VAC with a couple of diodes. I used his diagram and two 1N4004
diodes. One diode came from the AC input to the relay, one went
across the relay coil. On the Shrack RP 50012 with a 327 ohm coil and
my 24 VAC source (RMS meter says it's 26.9 VAC) the relay chattered
constantly. I tried a Zettler AZ8-1CH-12D with a 312 ohm coil. It
also chattered. There seems to be some discussion as to why this
would or would not work and, honestly, it's beyond my current skill
level with electronics. But, FWIW, that's what I tried and that's
what I got. Thank you, John Larkin.

Next, Ed suggested I try something involving a Zener and a
transistor. I formulated my own idea based on this and then, in
response to that, Ed suggested a schematic. It tried his way and
mine. His way was to take the 24 VAC (which I full-wave rectified and
then buffered with a 2,200 uF 63V electrolytic) and make a voltage
divider with two 1k ohm resistors to drive a transistor. I hooked up
a LM78L12 exactly as (and the app notes) said, and the voltage divider
exactly as he said. I ran the voltage divider output to the base of a
2N4401 transistor. Applying 24VAC (same power supply as used above)
gave me a crisp turn-on of the relay and a crisp turn-off. However,
the turn-off came about 1.5 seconds after I disconnected the power
from the circuit indicating that the capacitor was still carrying the
circuit for a little bit, but at least it was a crisp turn-off. I
only tried this with the Schrack relay. I then tried the circuit the
way I had postulated in response to Ed's suggestion before he gave me
the schematic. I did it exactly as he suggested except that I ran the
rectified and buffered 24 VAC (about 35 VDC) trough a 30V Zener to the
base of the 2N4401. I got the same crisp on and off but the off came
much more quickly after I disconnected the power from the circuit. I
did the math on the current through the relay coil (12 / 327) and got
that it should be 36.7 mA. I measured it in actual use and got about
35 mA. In either case, the current should be well below the maximum
current capability of the LM78L12 (100 mA, 140 mA peak), however, the
voltage regulator was getting very, very hot (enough it was painful to
touch and I could smell the chip). I checked the actual current
through the voltage regulator to the coil of the relay (which is where
I got the number above; 35 mA). At 12 volts that should be about 0.4
watts, which I thought the LM78L12 could handle but the heat seems
excessive. I may need to put a heat sink on it. Regardless, the idea
works. Thank you, Ed.

Thank you all for your help.

The best, I think, is going to be to use the 12 volts DC from the
beginning as it is simpler and seems to work very well. With what I
think I understand now, I should be able to easily calculate the
maximum run of wire for the 12 VDC so it should be easy to know if
whatever run I choose will work or not. If, for some reason, that
doesn't work for the distance I might want to run, then I can switch
to the 24 VAC with the voltage regulator/transistor/zener.

Thanks again.

--HC

 

[ehsjr]

HC wrote:

On Feb 15, 10:19 pm, HC <hboo...@gte.net> wrote:

Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC


Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.

I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.

First, John Popelish suggested running the relays with 12VDC and that
the line-loss would be negligible. I did the math on the line-loss,
which I had not conceptualized or understood correctly before his post
and it seemed like a good idea. I had some crappy wire already run
out (I took a headphone jack to RCA Y-cable and cut it in two and
spliced some thin wire between the pieces to make a long-distance
auxiliary input for the stereo in my shop to run off the stereo in my
house). The cable is probably thinner than what John and I had
bounced back and forth about and the RCA audio stuff is pitifully
thin. The overall cable length is about 80 feet. I ran 12VDC across
the cable to a Shrack RP510012 relay with a coil resistance of 327
ohms. The coil snapped briskly in and out when the voltage was
applied. John was absolutely correct, the line-loss was insignificant
for the resistance of the coil. As I told John I would, I checked the
cut-in voltage of the relay. I started with it hooked up and engaged
at 12VDC and then turned the voltage down until it dropped out. I
then removed the relay and checked the voltage. Cut-out voltage was
2.79 VDC. I then hooked the relay back up and turned the voltage up
until it engaged and then disconnected and checked the voltage. Cut-
in was very soft at 6.5 VDC. I upped the voltage until I had crisp
cut-in which I got at 6.9 VDC. There seems to be no problem with a
very long-distance run to control these relays. Thank you, John
Popelish.

Next was John Larkin who suggested running the relays directly off 24
VAC with a couple of diodes. I used his diagram and two 1N4004
diodes. One diode came from the AC input to the relay, one went
across the relay coil. On the Shrack RP 50012 with a 327 ohm coil and
my 24 VAC source (RMS meter says it's 26.9 VAC) the relay chattered
constantly. I tried a Zettler AZ8-1CH-12D with a 312 ohm coil. It
also chattered. There seems to be some discussion as to why this
would or would not work and, honestly, it's beyond my current skill
level with electronics. But, FWIW, that's what I tried and that's
what I got. Thank you, John Larkin.

Next, Ed suggested I try something involving a Zener and a
transistor. I formulated my own idea based on this and then, in
response to that, Ed suggested a schematic. It tried his way and
mine. His way was to take the 24 VAC (which I full-wave rectified and
then buffered with a 2,200 uF 63V electrolytic) and make a voltage
divider with two 1k ohm resistors to drive a transistor. I hooked up
a LM78L12 exactly as (and the app notes) said, and the voltage divider
exactly as he said. I ran the voltage divider output to the base of a
2N4401 transistor. Applying 24VAC (same power supply as used above)
gave me a crisp turn-on of the relay and a crisp turn-off. However,
the turn-off came about 1.5 seconds after I disconnected the power
from the circuit indicating that the capacitor was still carrying the
circuit for a little bit, but at least it was a crisp turn-off. I
only tried this with the Schrack relay. I then tried the circuit the
way I had postulated in response to Ed's suggestion before he gave me
the schematic. I did it exactly as he suggested except that I ran the
rectified and buffered 24 VAC (about 35 VDC) trough a 30V Zener to the
base of the 2N4401. I got the same crisp on and off but the off came
much more quickly after I disconnected the power from the circuit. I
did the math on the current through the relay coil (12 / 327) and got
that it should be 36.7 mA. I measured it in actual use and got about
35 mA. In either case, the current should be well below the maximum
current capability of the LM78L12 (100 mA, 140 mA peak), however, the
voltage regulator was getting very, very hot (enough it was painful to
touch and I could smell the chip). I checked the actual current
through the voltage regulator to the coil of the relay (which is where
I got the number above; 35 mA). At 12 volts that should be about 0.4
watts, which I thought the LM78L12 could handle but the heat seems
excessive. I may need to put a heat sink on it. Regardless, the idea
works. Thank you, Ed.

Thank you all for your help.

The best, I think, is going to be to use the 12 volts DC from the
beginning as it is simpler and seems to work very well. With what I
think I understand now, I should be able to easily calculate the
maximum run of wire for the 12 VDC so it should be easy to know if
whatever run I choose will work or not. If, for some reason, that
doesn't work for the distance I might want to run, then I can switch
to the 24 VAC with the voltage regulator/transistor/zener.

Thanks again.

--HC

Nice! Thanks for letting us know what you did and the
results you got.

Measuring the pull in and drop out voltages can be an
eye opener - amazing to many that a 12 volt relay is
still energized at 3 volts, until that observation prompts
some thinking.

The heat you noticed in the 7812 can also be an eye opener.
Some people think "I'm using it a well below the max,
whyinthehell is it getting hot". You went the right way and
figured the watts. The 7812 will handle that .4 watts, but
you need to take the heat away for it to do so.

The time delay you found is expected, and a good observation
to post as you did. You can experiment to get rid of that and
learn something about time constant, if you do not already know
it.

There's a lot of people who can benefit from your example
of investigating various alternatives, if they do the same.

Thanks again for posting your results. Often people ask a
question, get answers, but don't follow up to say how they
made out. Nice to hear when someone was as successful as
you were,

Ed

 

[HC]

On Feb 24, 1:39 pm, John Fields <jfie...@austininstruments.com> wrote:

On Sun, 24 Feb 2008 11:11:14 -0800 (PST), HC <hboo...@gte.net
wrote:



On Feb 15, 10:19 pm, HC <hboo...@gte.net> wrote:
Hey, all, I'm not sure if this can be done but here's what I am trying
to do and how I've tried to go about doing it.

I want to be able to control 120VAC devices some distance away from a
controller, say, up to 100 feet or so. I would like to run the 120VAC
to the device through a switch (relay) at the device with no other
switches or control devices in line from the breaker. Then I would
like to have the controller turn that relay on and off to control the
device. The idea is that I could run the thicker, high-voltage lines
directly to the device and then use smaller wire to operate a relay at
the device to turn it on and off instead of running the high-voltage
wire to each switch I would like to use.

Since I have a boat-load of low-cost 12VDC relays that can switch up
to 250VAC and 15 amps and since 24VAC relays seem, in my searching, to
be a lot more expensive and harder to come by (they seem to be, in my
searching, related to HVAC and other "industrial" uses; they're not
like the overly-abundant 12VDC relays we have for our cars and such) I
would like to use a 12VDC relay at the device. However, I'm afraid
that if I attempt to use 12VDC to control these relays over a distance
like I mention of up to 100 feet that the line-loss will be
significant (on 12VDC). I was thinking that using 24VAC would be much
better (it's higher voltage and it's AC, so line-loss should be quite
a bit less than 12VDC).

I tried this: I took the 24VAC and rectified it with a single diode
(half-wave) then buffered it to "ground" with a 2,200 uF cap. I ran
that buffered output through a LM78L12 (with input and output caps as
detailed in the datasheet I was reading) and that output to the 12VDC
relay I wanted to operate. When I apply 24VAC to the circuit the
relay turns on like a mousetrap: SNAP! But, when I remove the 24VAC
the relay turns off like a marshmallow; slow and makes some light
clicking noises. So, the input cap (2,200 uF) is still powering the
relay coil and is letting it down slowly; at least, that's my hack-boy
assessment: I'm not an expert at any of this stuff. I tried whatever
I could including "pull-down" resistors (if I'm using that term
correctly); I put a 10k resistor from the relay input to ground. I'm
afraid the slow turn-off is going to cause arcing and fry the relay
contacts.

I don't have a lot of caps that are rated at 35 volts or higher (which
could handle the 24VAC, rectified) so experimentation was limited in
various cap sizes (like, could a 1,000 uF input cap allow the relay to
turn off quickly?); somewhere I got the idea that if you put more
voltage across an electrolytic cap than it can handle it can "explode"
or "pop" or "blow-up" or whatever so I'm reluctant to use 16V
electrolytics on what should be 24V or higher.

Anyway, I hope I've done a good job of explaining what I'm trying to
do and what I've tried to do to achieve it. Is there an effective way
to run a 12VDC relay from a supply circuit of 24VAC?

Thanks in advance.

--HC

Hey, all, thank you for your replies and information. I am sorry for
the delay my response; I let life run me this week and only got to my
electronics shop yesterday.

I had several suggestions on how to solve this and I'll go over them
here; one to provide feedback to the replies and two, to offer this
information to maybe help others who come along and read it with the
same question I had.

First, John Popelish suggested running the relays with 12VDC and that
the line-loss would be negligible. I did the math on the line-loss,
which I had not conceptualized or understood correctly before his post
and it seemed like a good idea. I had some crappy wire already run
out (I took a headphone jack to RCA Y-cable and cut it in two and
spliced some thin wire between the pieces to make a long-distance
auxiliary input for the stereo in my shop to run off the stereo in my
house). The cable is probably thinner than what John and I had
bounced back and forth about and the RCA audio stuff is pitifully
thin. The overall cable length is about 80 feet. I ran 12VDC across
the cable to a Shrack RP510012 relay with a coil resistance of 327
ohms. The coil snapped briskly in and out when the voltage was
applied. John was absolutely correct, the line-loss was insignificant
for the resistance of the coil. As I told John I would, I checked the
cut-in voltage of the relay. I started with it hooked up and engaged
at 12VDC and then turned the voltage down until it dropped out. I
then removed the relay and checked the voltage. Cut-out voltage was
2.79 VDC. I then hooked the relay back up and turned the voltage up
until it engaged and then disconnected and checked the voltage. Cut-
in was very soft at 6.5 VDC. I upped the voltage until I had crisp
cut-in which I got at 6.9 VDC. There seems to be no problem with a
very long-distance run to control these relays. Thank you, John
Popelish.

Next was John Larkin who suggested running the relays directly off 24
VAC with a couple of diodes. I used his diagram and two 1N4004
diodes. One diode came from the AC input to the relay, one went
across the relay coil. On the Shrack RP 50012 with a 327 ohm coil and
my 24 VAC source (RMS meter says it's 26.9 VAC) the relay chattered
constantly. I tried a Zettler AZ8-1CH-12D with a 312 ohm coil. It
also chattered. There seems to be some discussion as to why this
would or would not work and, honestly, it's beyond my current skill
level with electronics. But, FWIW, that's what I tried and that's
what I got. Thank you, John Larkin.

Next, Ed suggested I try something involving a Zener and a
transistor. I formulated my own idea based on this and then, in
response to that, Ed suggested a schematic. It tried his way and
mine. His way was to take the 24 VAC (which I full-wave rectified and
then buffered with a 2,200 uF 63V electrolytic) and make a voltage
divider with two 1k ohm resistors to drive a transistor. I hooked up
a LM78L12 exactly as (and the app notes) said, and the voltage divider
exactly as he said. I ran the voltage divider output to the base of a
2N4401 transistor. Applying 24VAC (same power supply as used above)
gave me a crisp turn-on of the relay and a crisp turn-off. However,
the turn-off came about 1.5 seconds after I disconnected the power
from the circuit indicating that the capacitor was still carrying the
circuit for a little bit, but at least it was a crisp turn-off. I
only tried this with the Schrack relay. I then tried the circuit the
way I had postulated in response to Ed's suggestion before he gave me
the schematic. I did it exactly as he suggested except that I ran the
rectified and buffered 24 VAC (about 35 VDC) trough a 30V Zener to the
base of the 2N4401. I got the same crisp on and off but the off came
much more quickly after I disconnected the power from the circuit. I
did the math on the current through the relay coil (12 / 327) and got
that it should be 36.7 mA. I measured it in actual use and got about
35 mA. In either case, the current should be well below the maximum
current capability of the LM78L12 (100 mA, 140 mA peak), however, the
voltage regulator was getting very, very hot (enough it was painful to
touch and I could smell the chip). I checked the actual current
through the voltage regulator to the coil of the relay (which is where
I got the number above; 35 mA). At 12 volts that should be about 0.4
watts, which I thought the LM78L12 could handle but the heat seems
excessive. I may need to put a heat sink on it. Regardless, the idea
works. Thank you, Ed.

Thank you all for your help.

---
Geez, what am I, chopped liver? ;(

--
JF

John, I'm very sorry, the omission was not intentional. Thank you for
your input. You were correct, the relays did chatter. I chose not to
use the series resistor because it seems something about it runs
contrary to my nature (I'm not even sure how to put that into words
better). I like the idea of using two relays in series with the load
in parallel, but the loads I intend to run won't need that kind of
current. :-/

Again, I'm sorry. Thank you for your time and help.

--HC