# latching relay for my lights

Thread Starter

#### reaman4ever

Joined Sep 2, 2007
16
Hello, I would like to electronically control my ceiling lights and lamps in my house, and I am trying to figure out which relays would work for me.

Based on wikipedia, I want a latching relay, so that the control signal only has to be applied for a short time, and then the lamp should stay in that state until I send another signal.

I am not sure what the relay resistance should be (ideally zero, but I don't know what would be considered tolerable). I don't know how much current will run through. I live in America so I have 120VAC wall sockets.

My control signal will be probably in the 5V range, and preferably battery operated.

So what kind of relays can I get? What should I estimate my current at? What is the maximum resistance I should tolerate? And if you have any other advice I would appreciate it.
thanks

*********added explanation below
I would like to use latched relays to control the lighting in my house. I don't want any control wires coming out of my light switches (I want to be able to control individual lights at the switch, not rooms at the circiut breaker) so I want it to be radio activated. I want to use latched relays so that I don't have to continuesly transmit an "on" signal (I realize I could use a solid state flipflop) and so that the switch wont take any power except while switching.

I would like the switch and its radio receiver circuit to be controled by a 9V battery, though I am considering building a AC to DC converter into the circuit to take the 120VAC to something like 5VDC.

Does that controll of the relay need a specific signal or can I just hit it with a 5V pulse?

#### Voltboy

Joined Jan 10, 2007
197
You need to be more specific on the power consumption of the lights and the amps, etc. Without that its hard to get the resistance.

#### thingmaker3

Joined May 16, 2005
5,084
Yoda is correct.

Power (Watts) is equal to the square of the current (Amps) multiplied by the electromotive force (Volts).

Using a little of the ol' high-school algebra,
$$\sqrt{Watts/120}$$ = Amps

So, for a 60 watt lamp, the contacts would have to be rated for not less than $$\sqrt{60/120}$$ = 0.7 A = 700 mA

For a 100 watt lamp, they would have to be rated at not less than 0.92 A = 920 mA.

I'll bet my left-over turkey against your left-over sweet potatoes that any relay you've looked at will handle an order of magnitude more current than this.

Note about the battery: I strongly suggest a 24V transformer power supply - they don't ever run low in the middle of the night during snowstorms.

As for resistance - the resistance of the contacts will be negligible. Resistance of the coils will be the load of the 24V supply.

One manufacturer's introduction to such systems can be found here:
http://www.douglaslightingcontrol.com/pdfs/lowvoltage/design.pdf

Another one here:
http://www.nexlight.com/images/SpecPDFs/Pn0023.pdf

Thread Starter

#### reaman4ever

Joined Sep 2, 2007
16
Power (Watts) is equal to the square of the current (Amps) multiplied by the electromotive force (Volts).
Isnt P=IV ?
so I = P/V

#### eeboy

Joined Sep 27, 2007
90
Indeed it is... P=VI

Perhaps he was confusing it with a similar equation for power which is P=I^2 * R or P=V^2 / R.

Regardless, he is correct. You will need to properly size the relay for the expected voltage and current.

Be aware that there are additional precautions if you are switching an inductive load such as a motor.

#### thingmaker3

Joined May 16, 2005
5,084
Perhaps I had too much wine and confused I^2*R with I*E to achieve P = I*R = I^2*E

I'll go have another glass and refrain from posting any more until the morrow. There's still some of that P=I*E left, too...

#### thingmaker3

Joined May 16, 2005
5,084
Brain works much better on coffee and omelettes than it does on tryptophan and viognier...

500 milliAmps are drawn by a 60 Watt bulb. Most relays developed for LV control of residential lighting et.al. can handle 20 Amps.

#### GS3

Joined Sep 21, 2007
408
reaman4ever, do you have a good reason to prefer relays over solid state devices? You are not the first one to want to do this. This problem has been solved long time ago and there are standard commercial products which will be simpler and more reliable (and safer!) than anything you could put together. Can you give us more details?

Thread Starter

#### reaman4ever

Joined Sep 2, 2007
16
reaman4ever, do you have a good reason to prefer relays over solid state devices? You are not the first one to want to do this. This problem has been solved long time ago and there are standard commercial products which will be simpler and more reliable (and safer!) than anything you could put together. Can you give us more details?
I assumed relays would be better because I could used a latched relay, and it would only use power when switching. I should probably give much more detail about my idea. I will post a moire detailed explanation here, and add it to my original message.

I would like to use latched relays to control the lighting in my house. I don't want any control wires coming out of my light switches (I want to be able to controll individual lights at the switch, not rooms at the circiut breaker) so I want it to be radio activated. I want to use latched relays so that I don't have to continuesly transmit an "on" signal (I realize I could use a solid state flipflop) and so that the switch wont take any power except while switching.

I would like the switch and its radio receiver circuit to be controled by a 9V battery, though I am considering building a AC to DC converter into the circuit to take the 120VAC to something like 5VDC.

Does that controll of the relay need a specific signal or can I just hit it with a 5V pulse?

thank you

#### thingmaker3

Joined May 16, 2005
5,084
Again, many companies make UL listed components for what you want. Wireless LV control of residential power and lighting is already on the market. Some are even "plug and play," installing in minutes.

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