neg voltage relay driver (for an idiot)

Ok. All they had at the RS I went to was 555's. The web site said
they had 339's. I, of course, saw your post after I got back, and I
was in a hurry so I just grabbed the 555's because they are pretty
useful for all kinds of stuff.

I know a 555 can be used as a comparator, can I swap that in instead of
the comparators you spec'd? They can source a lot of current, so that
could be a help. I could find a configuration out there I'm sure for a
555 comparator implementation.

Thanks again. Deciphering these circuits and knowing the application
is helping me learn a lot about this stuff.

Chris

 

Ok.

I decided to try a TL072 for the comparator, since they are in abundant
supply at my studio, as the console is full of them.

First of all, let me thank you again, John, for all the help and
direction in this endeavor.

It does sort of work. I had to use 15k resistors instead of 10's,
maybe that is why. I tried 33k in place of 20k also. I did not use
the pull up resistors on the opamp output.

It works in that output 1 goes high when +24VDC is present, and output
2 goes high when 0VDC == GND is present, but output 1 goes high in both
cases.

This could be ok, since I could put relay 1's power leads through an
extra set of contacts on relay 2, thus disabling relay 1 when relay 2
activates.

Maybe this is what was supposed to happen? If so, I can totally work
with it, but I wanted to see if I did something wrong.

Reading with a meter, pin 1 on the opamp goes to ~24V on both states,
pin 5 goes to 24V on 0VDC==GND.

Any more suggestions?

Chris

 

[John Fields]

On 1 Sep 2005 16:03:49 -0700, crm0922@rocketmail.com wrote:

Ok. All they had at the RS I went to was 555's. The web site said
they had 339's. I, of course, saw your post after I got back, and I
was in a hurry so I just grabbed the 555's because they are pretty
useful for all kinds of stuff.

I know a 555 can be used as a comparator, can I swap that in instead of
the comparators you spec'd?

---
Dunno.
---

They can source a lot of current, so that
could be a help. I could find a configuration out there I'm sure for a
555 comparator implementation.

---
OK
---

Thanks again. Deciphering these circuits and knowing the application
is helping me learn a lot about this stuff.

---
You're welcome.

--
John Fields
Professional Circuit Designer

 

Thanks John.

Did you see my other post with my results using a TL072? There was a
little strangeness I though you might have some insight into...

Chris

 

[John Fields]

On 2 Sep 2005 03:41:10 -0700, crm0922@rocketmail.com wrote:

Ok.

I decided to try a TL072 for the comparator, since they are in abundant
supply at my studio, as the console is full of them.

First of all, let me thank you again, John, for all the help and
direction in this endeavor.

It does sort of work. I had to use 15k resistors instead of 10's,
maybe that is why. I tried 33k in place of 20k also. I did not use
the pull up resistors on the opamp output.

It works in that output 1 goes high when +24VDC is present, and output
2 goes high when 0VDC == GND is present, but output 1 goes high in both
cases.

---
That's because the TL072 (as well as the TL082 I suggested in error)
aren't rail-to-rail output opamps, so when their outputs are
supposed to go to 0V, they can actually be a couple of volts above
ground. If they can source enough current under those conditions,
they can keep the relay drivers turned on. It's iffy, so I wouldn't
try the relay inhibit scheme you mention next, because you can't
really tell which relay is going to stay on and which one isn't.
---

This could be ok, since I could put relay 1's power leads through an
extra set of contacts on relay 2, thus disabling relay 1 when relay 2
activates.

Maybe this is what was supposed to happen?

---
No. If it's not rock-solid, predictable, and reproducible, it's no
good.
---

If so, I can totally work
with it, but I wanted to see if I did something wrong.

Reading with a meter, pin 1 on the opamp goes to ~24V on both states,
pin 5 goes to 24V on 0VDC==GND.

---
I don't know what's up with that, unless you made a wiring error.
---

Any more suggestions?

---
Email me with a physical address and I'll send you a couple of
LM393's.

Or, if you want something that can drive a relay directly, try a
Linear Technology LT1017.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1004,C1013,P1227,D1268

They offer on-line ordering or maybe you can talk them into a
sample? and do this:


+24
|
+--------+-----+----+-------+
| | | |K |
| | | [DIODE] [COIL]
| | | | |
[100.0k] [10.0k] | +-------+
| | | |
| +----|-\ |
| | | >--+
Vin---+-------------|+/LT1017
| |
| |
| |
| | +24
| [20.0k] |
| | +-------+
| | |K |
| | [DIODE] [COIL]
| | | |
| | +-------+
| | |
+-------------|-\ |
| | | >--+
| +----|+/LT1017
[100.0k] | |
| [10.0k] |
| | |
+--------+-----+
|
GND

Also, I believe an LM311 can drive a relay directly. Check
National's site for a data sheet and app notes and samples.

Also, Mouser sells onesies and Jameco does too...

--
John Fields
Professional Circuit Designer

 

All I meant to show by my meter readings was that both go high for
0V==GND.

The relays were both turning on all the time at first. The idle output
voltage was about 1.3V. I just used a much bigger resistor before the
relay driver and that apparently dropped in current enough to not turn
on. They are very low current relays.

It really is quite predictable, it's just that the side triggered by
+24V is goes high in both states, at the opamp outputs.

I can get the radio shack comparator tomorrow and ground the extra pins
as you suggested.

Thanks for the offer to send the chip. I wouldn't want to trouble you
more than I already have been with all my questions!

Chris

 

[John Fields]

On 2 Sep 2005 12:03:34 -0700, crm0922@rocketmail.com wrote:

All I meant to show by my meter readings was that both go high for
0V==GND.

The relays were both turning on all the time at first. The idle output
voltage was about 1.3V. I just used a much bigger resistor before the
relay driver and that apparently dropped in current enough to not turn
on. They are very low current relays.

It really is quite predictable, it's just that the side triggered by
+24V is goes high in both states, at the opamp outputs.

---
But it's not _supposed_ to, and therein lies the problem!
---

I can get the radio shack comparator tomorrow and ground the extra pins
as you suggested.

---
OK. One other thing you might want to consider is how much current
you need to hit the driver with to turn on the relay. A good rule
of thumb is to force the driver to have a gain of 10, and you do
that by determining how much collector current it needs to supply,
and then forcing 1/10th of that current into the base of the driver.

In your case, the collector current is the relay coil current, so
what you would do would be to take one tenth of that current and
divide it into the difference between the supply voltage and the
driver's base-to-emitter saturation voltage to get the value of the
comparator's output pullup resistor. For example if the relay coil
current was, say, 10mA, then the current into the driver would be
1mA, and the value of the pullup would be:

Vcc - Vbe(sat) 24V - 1V
R = --------------- = ---------- = 23000 ohms
Ib 1E-3A


22k is a standard 5% value, and that would be fine.

Thanks for the offer to send the chip. I wouldn't want to trouble you
more than I already have been with all my questions!

---
Not a problem


--
John Fields
Professional Circuit Designer

 

I had originally calculated the resistor value as 40k or so, using a
similar formula that derives from the same thing. It is a pretty
sensitive relay, maybe 6mA required I think.

Unfortunately, with the original driver circuit, this was still enough
base current to collapse the power supply. It seemed weak to me, but
raising the value to 80k+ got it working predictably. I just raised
the resistors a few times until the voltage would hold up.

Shouldn't I be able to put a meter on the comparator outputs and read
the outputs going high when the corresponding voltage appears at Vin?
The relay driver can be left out until later, correct?

Chris

 

[John Fields]

On 2 Sep 2005 13:58:12 -0700, crm0922@rocketmail.com wrote:

I had originally calculated the resistor value as 40k or so, using a
similar formula that derives from the same thing. It is a pretty
sensitive relay, maybe 6mA required I think.

Unfortunately, with the original driver circuit, this was still enough
base current to collapse the power supply. It seemed weak to me, but
raising the value to 80k+ got it working predictably. I just raised
the resistors a few times until the voltage would hold up.

Shouldn't I be able to put a meter on the comparator outputs and read
the outputs going high when the corresponding voltage appears at Vin?
The relay driver can be left out until later, correct?

---
Yes. With 24V on Vin, the output of the top comparator should be
high and the output of the bottom one should be low.

With 0V on Vin, the output of the top one should be low and the
output of the bottom one high.

With nothing on Vin the comparator outputs should both be low.

Notice, though, that with a TL071 in there, the outputs won't go to
zero volts, and that's where the problem lies. That is, the outputs
go high enough above zero volts to turn on the drivers unless you
"starve" their inputs by increasing the value of the base current
limiting resistor to the point where the transistors aren't passing
enough current through the relay to turn it on. The problem with
that method is that you're running the transistor wide open and
subjecting its output to the vagaries of temperature and just about
everything else you can imagine which can change its gain. For
instance, the first time you power up the circuit both relays might
be off, but after a while you might find that you can't turn off one
of the relays. Possibly becuse it warmed up a little and its gain
increased, allowing its quiescent "OFF" base current keep it turned
on. The right way to do it is to make sure you use a device with an
output voltage less than the "ON" threshold of the driver
transistor. You could also use a voltage divider on the output of
the TL071, and make sure that with the worst case voltage out, when
it's low, the voltage into the driver won't be high enough to turn
it on.

--
John Fields
Professional Circuit Designer

 

Ok. So the TL071 could work with a voltage divider calculated to go
near to zero from the TL071's offset voltage.

However, that doesn't explain why both cmprtr outputs are going high
when Vin is 0V. Could it be because I am using 15k's instead of 10k?
I've checked the wiring about 10 times, but I will check it again. Now
I'm at home and I think I have some 10k's here, or I could pick some up
tomorrow.

Could the 0V input actually have a small voltage against ground,
resulting in a small positive potential to ground? would that even
turn on the top relay, were it the case?

Chris

 

[John Fields]

On 2 Sep 2005 15:04:02 -0700, crm0922@rocketmail.com wrote:

Ok. So the TL071 could work with a voltage divider calculated to go
near to zero from the TL071's offset voltage.

However, that doesn't explain why both cmprtr outputs are going high
when Vin is 0V. Could it be because I am using 15k's instead of 10k?
I've checked the wiring about 10 times, but I will check it again. Now
I'm at home and I think I have some 10k's here, or I could pick some up
tomorrow.

Could the 0V input actually have a small voltage against ground,
resulting in a small positive potential to ground? would that even
turn on the top relay, were it the case?

---
After looking at the data sheet for the TL071, I found that they
don't have rail-to-rail inputs (mea culpa for making that asumption)
but, rather, have an input common mode range of +/- 11V with a +/-
15V supply, which means that you have to stay 4V away from the rails
in order to have the thing work right. With a single 24V supply,
that means (I assume) that both sets of inputs should be between 4V
and 20V. The reference divider satisfies that requirement since its
high output is at +18V and its low output at +6V, but if Vin is
being fed into the working divider from a low impedance source, then
its output will go very close to the rails when Vin is either at 24V
or 0V. The easy way to fix that is to connect a series resistor
between Vin and the input to the working divider, but we need to
know what the source impedance of Vin looks like in order to
calculate the value of the resistance.

If you could set Vin to +24V and then then connect it to the
ungrounded ends of, first, a grounded 100k ohm resistor, second, a
grounded 10k ohm resistor, and third, a grounded 1k ohm resistor,
then measure and post the voltages read across the resistors I can
show you how to calculate the value of that series resistor if you
don't already know how.

--
John Fields
Professional Circuit Designer

 

John, I rebuilt th circuit with the spec'd comparator and it works
perfectly. A Professional Circuit, indeed!

Thanks a lot for putting in all the time to help me with this.
Especially all the time fooling around with alternative designs when
the original one was perfect.

Best,

Chris

 

[John Fields]

On 3 Sep 2005 14:15:04 -0700, crm0922@rocketmail.com wrote:

John, I rebuilt th circuit with the spec'd comparator and it works
perfectly. A Professional Circuit, indeed!

Thanks a lot for putting in all the time to help me with this.
Especially all the time fooling around with alternative designs when
the original one was perfect.

---
My pleasure!

--
John Fields
Professional Circuit Designer

 

[Jasen Betts]

In article <1125610420.718296.234890@f14g2000cwb.googlegroups.com>, crm0922@rocketmail.com wrote:

The other problem is getting the LM393. I don't think I can get that
locally. The only place I could think of has 0 on hand. Does radio
shack stock something that will work? Mouser and Digikey both take
about a week to get to me, and paying for overnight shipping seems
ridiculous for an IC chip...

what about an LM339 instead, it's nearly twice the size.
but should be dirt cheap and fairly easy to find,,,

Bye.
Jasen

 

[John Fields]

On Sun, 4 Sep 2005 22:41:15 +1200, Jasen Betts
<jasen-b@free.net.nospam.nz> wrote:

In article <1125610420.718296.234890@f14g2000cwb.googlegroups.com>, crm0922@rocketmail.com wrote:
The other problem is getting the LM393. I don't think I can get that
locally. The only place I could think of has 0 on hand. Does radio
shack stock something that will work? Mouser and Digikey both take
about a week to get to me, and paying for overnight shipping seems
ridiculous for an IC chip...

what about an LM339 instead, it's nearly twice the size.
but should be dirt cheap and fairly easy to find,,,

---
That was presented as an option several posts back.

--
John Fields
Professional Circuit Designer