relay question

[Bob C]

As posted in other questions, I'm working on my remote observatory. I
had a friend help me with the initial wiring layout last year. I
followed his advice and got the project working. What I have is a big
plywood box - approx. 3.5' x 3.5' x 3' with a 2' lid that opens on
top. I use an old garage door opener motor to open the lid. The box
has a PC inside that I have on my local network so I can control the
telescope and camera from inside the house. It all works. So there's
the background.

What he designed was to have an output from the PC, I made a pcboard
with a 82C55, that controls a 5V relay which in turn opens a 12V relay
that opens to either open or close the lid. I spent most of this
winter learning basic electronics by reading and making the pcboards.
So I have got the lid working and have a couple of questions.

First of all safety. Someone has pointed out that I had better have a
fuse on the motor in case it doesn't shut of properly. The motor is a
1/2 HP rated at 4.5A at 120V. So I bought a fused switch like is on a
furnace and put a time delay fuse on it. Currently there is a 4A fuse
in there and it hasn't blown. I also replaced the outlet with a 15A
ground fault outlet.

Next my friend had me use 12VDC 30A automotive relays for the 12V
relay. The voltage coming directly from the motor is 120V. If I'm
correct I should be using a relay that is rated 120V or greater? If I
have been reading correctly the voltage can be greater than 120V as
long as it is rated at 4.5A. So even a 240V 4.5A relay would work.
Correct?

And shouldn't I be using something greater than 28ga wire to take the
current the 10' and back from the motor to the relay? I saw a chart
that looked like I should be using 18ga or better.

I also was thinking about putting an in-line fuse on one of the lines
to the relay.

Bob

 

[Bill Vajk]

Bob C wrote:

As posted in other questions, I'm working on my remote observatory. I
had a friend help me with the initial wiring layout last year. I
followed his advice and got the project working. What I have is a big
plywood box - approx. 3.5' x 3.5' x 3' with a 2' lid that opens on
top. I use an old garage door opener motor to open the lid. The box
has a PC inside that I have on my local network so I can control the
telescope and camera from inside the house. It all works. So there's
the background.

What he designed was to have an output from the PC, I made a pcboard
with a 82C55, that controls a 5V relay which in turn opens a 12V relay
that opens to either open or close the lid. I spent most of this
winter learning basic electronics by reading and making the pcboards.
So I have got the lid working and have a couple of questions.

Neat!

First of all safety. Someone has pointed out that I had better have a
fuse on the motor in case it doesn't shut of properly. The motor is a
1/2 HP rated at 4.5A at 120V. So I bought a fused switch like is on a
furnace and put a time delay fuse on it. Currently there is a 4A fuse
in there and it hasn't blown. I also replaced the outlet with a 15A
ground fault outlet.

Test it by jamming the mechanism to stall the motor. The fuse should
blow within a few seconds. If it doesn't, replace it with a lower
value but large enough that it won't blow during normal operation.

Next my friend had me use 12VDC 30A automotive relays for the 12V
relay. The voltage coming directly from the motor is 120V. If I'm
correct I should be using a relay that is rated 120V or greater? If I
have been reading correctly the voltage can be greater than 120V as
long as it is rated at 4.5A. So even a 240V 4.5A relay would work.
Correct?

Relays have two ratings. One is the coil rating, and that's the
12 volts your friend is referring to. The 30 amps is the contact
rating he advised you to use. The contacts are rated based on
voltage AND current. If they're thorough they'll list inductive
loads as well as resistive. Look at the garage door opener and
see if you can find a rating for LRA (locked rotor amps.) That's
the minimum rating you should seek for your relay contacts, at
the appropriate voltage, in this case 120 volts or higher.

And shouldn't I be using something greater than 28ga wire to take the
current the 10' and back from the motor to the relay? I saw a chart
that looked like I should be using 18ga or better.

You're making a common mistake. Gauge numbers work opposite that
which is intuitive. Smaller numbers mean larger diameter wire.
16 gage is larger (and can safely carry more current) than 18
gauge. Generally speaking it is a good idea to have the wire
rating better than the overcurrent protection you're providing.
One assumes that the circuit you're plugging all this into is
a 15 amp house circuit. The wire size should be at least
14 gauge. If you're t a great distance form the house, it
should probably be 12 gauge.

I also was thinking about putting an in-line fuse on one of the lines
to the relay.

You already have a fuse in the circuit and protection ahead of the fuse.
Don't go crazy with fuses.

 

[CFoley1064]

Subject: relay question
From: bncarper@cs.com (Bob C)
Date: 3/15/2004 6:29 AM Central Standard Time
Message-id: <726ee98e.0403150429.523f98e4@posting.google.com

As posted in other questions, I'm working on my remote observatory. I
had a friend help me with the initial wiring layout last year. I
followed his advice and got the project working. What I have is a big
plywood box - approx. 3.5' x 3.5' x 3' with a 2' lid that opens on
top. I use an old garage door opener motor to open the lid. The box
has a PC inside that I have on my local network so I can control the
telescope and camera from inside the house. It all works. So there's
the background.

What he designed was to have an output from the PC, I made a pcboard
with a 82C55, that controls a 5V relay which in turn opens a 12V relay
that opens to either open or close the lid. I spent most of this
winter learning basic electronics by reading and making the pcboards.
So I have got the lid working and have a couple of questions.

First of all safety. Someone has pointed out that I had better have a
fuse on the motor in case it doesn't shut of properly. The motor is a
1/2 HP rated at 4.5A at 120V. So I bought a fused switch like is on a
furnace and put a time delay fuse on it. Currently there is a 4A fuse
in there and it hasn't blown. I also replaced the outlet with a 15A
ground fault outlet.

Next my friend had me use 12VDC 30A automotive relays for the 12V
relay. The voltage coming directly from the motor is 120V. If I'm
correct I should be using a relay that is rated 120V or greater? If I
have been reading correctly the voltage can be greater than 120V as
long as it is rated at 4.5A. So even a 240V 4.5A relay would work.
Correct?

And shouldn't I be using something greater than 28ga wire to take the
current the 10' and back from the motor to the relay? I saw a chart
that looked like I should be using 18ga or better.

I also was thinking about putting an in-line fuse on one of the lines
to the relay.

Bob

Hi, Bob.

1) Safety -- You're fine where you are. In addition, the motor itself probably
has a thermal cutoff built in. You're doing well on this one.

2) Relay -- Possibly a bit of a problem here. Relay contacts are specified
to handle a certain amount of current, but there are other considerations, too.
A 30A automotive relay may only be rated to switch lights or such. When you
open an inductive load, an arc will try to form across the relay contacts as
they're opening. An automotive relay usually doesn't have the contact
separation distance when it opens that you'll need to switch a 120 VAC motor
load on and off. What'll happen is you'll end up with a lingering arc across
the contacts, which will at least severely reduce their life.

You need a relay with a 12VDC coil which is rated to switch a 1/2 hp 120VAC
load. One good choice for that might be the Potter & Brumfield T92S11D22-12,
available from Newark and many other sources. It's good for 30 Amps, and is
also rated to switch up to a 1 hp 120 VAC motor load. I like 'em because
they're sealed, and they have 1/4" quick connect terminals -- perfect for
industrial-type wiring.

3) Wiring -- Your chart speaks truth. Technically, you'll get by with 18AWG,
but since you're doing line voltage wiring, I'd just go with the flow and use
14AWG. This is where the quick connects come in handy. Make sure you ground
the motor, too -- use a separate ground strap (same or lower gauge than the
line/neutral wire) if necessary.

If your DC power supply is current-limited, you don't have to worry about
fusing.

Good luck
Chris

 

[Bob C]

cfoley1064@aol.com (CFoley1064) wrote in message news:<20040315102440.20639.00001472@mb-m24.aol.com>...

Hi, Bob.

1) Safety -- You're fine where you are. In addition, the motor itself probably
has a thermal cutoff built in. You're doing well on this one.

2) Relay -- Possibly a bit of a problem here. Relay contacts are specified
to handle a certain amount of current, but there are other considerations, too.
A 30A automotive relay may only be rated to switch lights or such. When you
open an inductive load, an arc will try to form across the relay contacts as
they're opening. An automotive relay usually doesn't have the contact
separation distance when it opens that you'll need to switch a 120 VAC motor
load on and off. What'll happen is you'll end up with a lingering arc across
the contacts, which will at least severely reduce their life.

You need a relay with a 12VDC coil which is rated to switch a 1/2 hp 120VAC
load. One good choice for that might be the Potter & Brumfield T92S11D22-12,
available from Newark and many other sources. It's good for 30 Amps, and is
also rated to switch up to a 1 hp 120 VAC motor load. I like 'em because
they're sealed, and they have 1/4" quick connect terminals -- perfect for
industrial-type wiring.

3) Wiring -- Your chart speaks truth. Technically, you'll get by with 18AWG,
but since you're doing line voltage wiring, I'd just go with the flow and use
14AWG. This is where the quick connects come in handy. Make sure you ground
the motor, too -- use a separate ground strap (same or lower gauge than the
line/neutral wire) if necessary.

If your DC power supply is current-limited, you don't have to worry about
fusing.

Good luck
Chris

Thanks again for a reply Chris. Glad to hear I got some of the safety
correct after getting your e-mail. Has made me think about things in
a different light.
That e-mail made me go back through and really think what my friend
had laid out. That is why I thought the relays and wire were
incorrect. So I have the correct ones on order.

As you state in other posts, this is the basics. Here is a basic
question. I have the wires from the motor to the relay which I'll
hook up using the 14AWG wire. What is the best way to splice them
together? For that fact, splicing of any wire? I have been using
wire nuts but is butt splices or something else better? I have plenty
more electrical projects coming for astronomy so the cost of buying
crimping or other correct tools doesn't bother me.

Not sure what a ground strap is. The green ground wire is attached to
the base of the garage door opener plate? Should it be somewhere
else?

Bob