Peculiar electrical problem.

Stephen Newell · Mar 7, 2014

I have a customer that has one of these dusk to dawn outdoor lights that they have problems burning out light bulbs. I put my volt meter on it and the voltage was scrolling from 3 volts to 140 volts and back over and over never stopping on any certain voltage. What could cause this? It’s not the meter as it works fine elsewhere.

Steve@Advance · Mar 7, 2014

More info would help. Is this a mercury vapor, metal halide, sodium?

Where are you testing the voltage? Incoming power? In the socket?

The basics are to be sure the lamp type and wattage is correct for the ballast. Look inside for damaged wiring, these run hot, sometimes the insulation cooks, wire nuts crumble, etc.

If nothing is obviously wrong, best option is to replace the fixture. Ballasts typically cost as much as the whole fixture.

MarkB_MI · Mar 8, 2014

As with all electrical problems, go back to the source and work forward. I assume you're monitoring the voltage under load, meaning there's a good bulb in the light.

Stephen Newell · Mar 8, 2014

Actually I removed the fixture and put the meter on the wire. It was not under a load. Funny though, I put the fixture back on and put a new bulb in and the light didn't fluctuate.

MarkB_MI · Mar 8, 2014

Assuming you're using a digital meter, you need some sort of load on the circuit or you'll just be measuring noise. The 10 megohm input impedance of a typical DMM is essentially an open circuit.

Stephen Newell · Mar 8, 2014

I don't know one meter from another but it was digital. It's one that harbor freight gives away free. Now having said that I checked a different line with the same meter in another part of the house and it quickly settled on 125 volts.

MarkB_MI · Mar 8, 2014

Here's the deal, Stephen:

Because of the high input impedance of your meter, the test leads are effectively antennae, picking up all kinds of noise. You've probably noticed that when you connect only one lead, the display drifts all over the place and probably gives you some false reading. You need a low-impedance load across the input to short out that noise. Any normal load, such as a light bulb, will short out the noise.

Checking voltage with a load also ensures you have good connections and don't have a marginal switch. It's possible to have a solid 120 volts coming out of the wall with no load, but have that go to zero as soon as you add a load.

My guess is your light has a bad switch or other connection. The problem is gone for now, but it will be back.

Peculiar electrical problem.

Stephen Newell

Well-known Member

Steve@Advance

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MarkB_MI

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Stephen Newell

Well-known Member

MarkB_MI

Well-known Member

Stephen Newell

Well-known Member

MarkB_MI

Well-known Member

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