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astetson 01-13-2012 06:55 PM

power surge with 120v relay
I work at a movie studio and went crazy with my office decorations - it looks a bit like Rainforest Cafe, except in the old west.

Well, my lights are LED modules plugged into one outlet and controlled with data from an Arduino microcontroller. In a completely separate outlet (though likely on the same wall-circuit) is a strobe light which is controlled via a 120v relay which receives a 5v signal from a totally separate Arduino microcontroller. Then the two Arduinos are hooked up for serial communication.

Still - whenever the relay comes on (regardless of how long it stays on), a huge power surge floods the lights which eventually fries the boards that they are attached to at $10 a pop!

I have a surge protector on my strobe light - no good.
I have tried unplugging the LED signal from the Arduino - no good.
I've tried plugging something other than a strobe light into the relay (my laptop charger) - no good.

The way I have constructed my relay is that it interrupts the power line and leaves the ground untouched.

* I have no idea how a power surge even occurs when the relay is turned on, other than perhaps theorizing that the potential (and therefore voltage) between a completely empty circuit and one that is happily full is higher.
* I have no idea how the surge finds my lights when there is literally no physical connection between the relay and my lights other than the houses wall circuit
* Not sure why the surge protector isn't working (except if an increase in voltage does not register as a surge?)
* Above all, NOT SURE what the H to do about it!

(astetson cries in his cereal)

Any thoughts?


Missouri Bound 01-13-2012 07:19 PM

I'll bet if you checked the voltage where the led's are plugged in when you hit relay you would see a voltage surge. I'm thinking you have mwbc in play and there is a loose or faulty wire.

Billy_Bob 01-13-2012 08:26 PM

You will need to post your wiring diagram and/or pictures for anyone to be able to see anything wrong...

dmxtothemax 01-13-2012 09:17 PM

So if I interperate correcttly what you say ?
The relay controls the strobe ?
Does the strobe work properly when it is on ?
The strobe controler somehow causes the other lights
the led ones to do weird things ?
Is this correct ?
Do the leds work ok when strobe not on ?

astetson 01-16-2012 08:59 PM

Yes that's right, the relay controls the strobe.

The strobe functions perfectly without the relay, yes - and it turns out that it's not just the strobe that causes the surge. *Any* 120v load on the relay will cause the surge when it turns on.

Yup - the LED lights work perfectly without the relay. And you are correct, when the relay switches on, the LED lights get super bright momentarily while their voltage soars (I have yet to use a meter to get a reading on it, though).

I wonder if a DPDT relay would help (I'm using a SPST relay on the hot wire of the circuit currently).

I'm tremendously curious to solve this issue, but I should tell you that I devised a solution to my needs over the weekend: I'm using two battery-powered strobes with transistors soldered onto the battery leads. I use the Arduino to fully saturate the transistor (at 5v), and tada - it acts just like a relay switch. :)

Still, if you have any insight, I'd love to hear it.

AandPDan 01-16-2012 09:19 PM

Your relay is frying the boards. You need a protection relay.

The coil on the relay is an inductor. When you apply power a magnetic field is created. When you turn off the relay, the field collapses, generating a voltage spike.

Modern electronics don't like spikes.

astetson 01-16-2012 09:24 PM

Does that mean that these simple relays are generally useless? Or that they would require their own dedicated circuit in your home/office?

It just bugs me that the relay is plugged into a socket across the room from the socket that the lights are plugged in. But I guess electrons don't care about 13 feet, huh. :wink:

AandPDan 01-16-2012 09:37 PM

No, they're not useless. Lots of devices use relays to control higher voltage/current loads.

All you need to do is put a diode across the terminals on the 5 volt sides. The diode is wired so it is opposite the normal flow of power to the coil (otherwise it would cause a short). In this way when the power is cut, the diode will conduct the spike through the coil, effectively dissipating it.

astetson 01-16-2012 10:46 PM

AandPDan, I'll post a pic tomorrow, but I actually have a diode on the 5v side. And the power surge on the when the relay opens makes total sense. I think the biggest mystery, though, is that my voltage spike happens when the relay closes (ie - when the 120v device powers on).

I'm using the relay kit from Sparkfun:
That's the PCB, and I have all the parts on there soldered in place.

Anyway, don't lose any sleep over it since the transistors/battery strobes will be working for me with no problem (I tested 'em this morning). Thank you tons for your input, though!!

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