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Old 11-03-2011, 09:00 AM   #1
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Relay Switch Question


I'm trying to use a relay to control an instrument that uses a 120VAC source. I need to control the relay (basically the coil) using a 5V signal. So it would have to be on a normally closed circuit and when I send the 5V signal it would break the circuit and turn the instrument off. So my question is, what relay would I need ? do I need to use 2 relays in series, as in a smaller relay to handle the 5V signal and and larger one with a higher contact rating for the 120VAC circuit?

I am limited to using whatever is available at radioshack...

Thanks,
Amir.

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Old 11-05-2011, 07:48 AM   #2
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Relay Switch Question


common relays available at Radio Shack are going to have 12V or 24V coils most likely. Plus you may run into issues with the 120V contacts handling the load you have in mind. What exactly are you up to?

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Old 11-05-2011, 09:10 AM   #3
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Relay Switch Question


I don't believe you will find anything useful at Radio Shack, you should be looking at someplace like Newark Electronics www.newark.com.

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Old 11-05-2011, 10:33 AM   #4
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Relay Switch Question


Before you spec a relay you need to know:

120 VAC load in amps or watts

The available 5 volt signal output in milliamps (you probable will have to use a coil rated at 6 volts). Whether the 5 volts is AC or DC.

You should find a single relay or contactor that will meet your specs.

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Old 11-05-2011, 11:07 AM   #5
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Relay Switch Question


if it is 5VDC then just get a power pack that is 5VDC/115V and break the 115V to the transformer depending on what your looking to run so tie the instrument 115 to the TR and break it with a switch..might want to have it CLOSE/5VDC POWER during usage N.O go to N.C... and break NO POWER in the off instead it sits in open with power on the coil.
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Old 11-05-2011, 05:28 PM   #6
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Relay Switch Question


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Originally Posted by biggles View Post
if it is 5VDC then just get a power pack that is 5VDC/115V and break the 115V to the transformer depending on what your looking to run so tie the instrument 115 to the TR and break it with a switch..might want to have it CLOSE/5VDC POWER during usage N.O go to N.C... and break NO POWER in the off instead it sits in open with power on the coil.
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Old 11-05-2011, 08:39 PM   #7
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Relay Switch Question


Quote:
Originally Posted by aalansari View Post
I'm trying to use a relay to control an instrument that uses a 120VAC source. I need to control the relay (basically the coil) using a 5V signal. So it would have to be on a normally closed circuit and when I send the 5V signal it would break the circuit and turn the instrument off. So my question is, what relay would I need ? do I need to use 2 relays in series, as in a smaller relay to handle the 5V signal and and larger one with a higher contact rating for the 120VAC circuit?

I am limited to using whatever is available at radioshack...

Thanks,
Amir.
Remote control units for controlling lamps and fans are available. Any chance one of these would do the job that you want.
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Old 11-05-2011, 09:41 PM   #8
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Relay Switch Question


Quote:
Originally Posted by aalansari View Post
I'm trying to use a relay to control an instrument that uses a 120VAC source. I need to control the relay (basically the coil) using a 5V signal. So it would have to be on a normally closed circuit and when I send the 5V signal it would break the circuit and turn the instrument off. So my question is, what relay would I need ? do I need to use 2 relays in series, as in a smaller relay to handle the 5V signal and and larger one with a higher contact rating for the 120VAC circuit?

I am limited to using whatever is available at radioshack...

Thanks,
Amir.
Relays.. gotta dust off the cobwebs for that one. A regular single pole double throw with contacts rated at 115V ??A and coil rated for 5V?C ??A would work. Just wire the N.C. to the load. I think you will have a difficult time finding a relay with a 5 volt coil that isn't a circuit board mount.
An idea of the project would help. The way you have it described is the load would be re-energized whenever you lost the 5 Volts for whatever reason. I thought most stuff like that is done with solid state these days. Radio Shack has been pretty much a joke the last decade or so unless you wanted some high end audio or video stuff.
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Last edited by jschaben; 11-05-2011 at 09:44 PM.
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Old 11-06-2011, 07:05 AM   #9
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Relay Switch Question


Radio Shack does offer up a 5VDC/120VAC reed relay. But it is quite limited on the load it will handle.
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Old 11-06-2011, 09:44 AM   #10
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Relay Switch Question


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Originally Posted by PoleCat View Post
Radio Shack does offer up a 5VDC/120VAC reed relay. But it is quite limited on the load it will handle.
But you can use that relay to drive another that can handle the load.

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