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Old 11-26-2009, 04:08 PM   #1
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Low Voltage Relays


Way back in my "Industrial Electricity" course days I recall the use of low voltage relays. I still see them now and again in school gyms and other halls, etc, but never see them in home applications. I guess the premise is to remove the 120v from the switch, and have the high voltage/current only at the light (correct me if I am wrong on this assumption). I am looking at renovating an attached garage into a rec room and thought they would be kinda "neat". Does anyone have any advice to give on using these devices in this manner ? Any pros, cons, etc?

Thanks,

- Joe
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Old 11-26-2009, 04:18 PM   #2
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If you use a short-circuit-proof bell transformer to drive the relay coil then you probably don't have to worry much about the NEC for the low-voltage wiring.

Then pick a relay whose coil is sensitive enough to be driven by the above-mentioned relay and whose contacts can handle the 120v load.
The current gain of a relay/contactor is at least 20, so a 10A 120v/240v load could be switched by a relay whose coil carries 0.5A or less, @ 24vac.

Try hosfelt, digi-key, jameco, mouser, marlin p jones or allelectronics for the relay.

Last edited by Yoyizit; 11-26-2009 at 04:23 PM.
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Old 11-26-2009, 11:11 PM   #3
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Service electrician here. Have seen (but not worked on) low voltage/relay based switching systems in a few houses.

One was an older (1970s? 1960s?) era house. Impossible to get replacement switches, impossible to really determine how the system was supposed to work due to many years of jury-rigging changes to it. The relay layout was spread out over two walls in the "mechanical room".

Second one that comes to mind had a dedicated lighting control panel with programmable switches and lighting scenes. The home owner hated it due to its complexity. Sometimes they just wanted to be able to turn the lights on and off.

I didn't like it because if I need to add a separately switched light to a room, I have to take wiring all the way back to the control panel which was in a finished laundry room in the center of the basement. No easy path to get there. Otherwise, could have gone to the attic, dropped a new switch leg to the switch box and taken it directly to the new light.

I think I'm from the school of thought that thinks that wiring every thing for 120v is the way to go. There are some very nice programmable systems based on 120v that you can control with a computer but leaves you flexible if you want to change in the future.
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Old 11-27-2009, 06:45 AM   #4
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Don't use relays where normal wiring will do. Don't add complexity if you don't have to. The use of relays, as I see it, is to be in situations that require long and/or multiple switch legs or where normal high voltage wiring is difficult or impossible to be accomodated. However, if there's no other way, with good design and quality components they are virually troublefree. I've used relays, e.g., in controlling large light arrays which were controlled from several locations.

Last edited by oilseal; 11-27-2009 at 06:51 AM.
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Old 11-27-2009, 07:35 AM   #5
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Thanks Guys...


Sounds like the KISS (keep-it-simple-stupid) method applies here! I'll take the advice and stick with a conventional approach.

Thanks,

- Joe
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Old 11-27-2009, 09:10 AM   #6
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Depending on the project, using a low voltage relay can be cost-effective.

Click here for one such relay assembly.
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Life is uncertain -- eat dessert first!!
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Old 11-27-2009, 03:05 PM   #7
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Quote:
Originally Posted by kbsparky View Post
At 10VDC max. and 10 milliamps max. they probably are below the radar for just about every NEC, etc. rule.

Last edited by Yoyizit; 11-27-2009 at 03:49 PM.
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Old 11-27-2009, 07:59 PM   #8
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i forget the brand name but u can still get the relays and latching switches not only is there the safety of low voltage but three way and four way switches are eliminated my suppler in maryland is ed supply in salisbury md hope this helps is a really easy system to install when you think it out how the system works
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Old 11-28-2009, 12:01 AM   #9
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There is the other style of relay still on market what we called GE RR-7 relay it still avabile thru electrical supply centre { not all places will have this in stock } but if you have mulit switching location sometime you can justify the cost depending on the layout.

Merci,Marc
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