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Low voltage lighting transformers Andrew Duane 04-10-2007
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Posted by Andrew Duane on April 10, 2007, 9:56 am


One of my hanging ceiling lights has decided not to work any more.
It's a low voltage halogen with a small built in transformer, an LET
60 type 60-watt 12 volt unit. It looks like the transformer is gone;
it shows almost no voltage at the output with a DVM. I get about 1/4
volt that rapidly dwindles to 0.

My question is whether these transformers *need* some load to actually
product voltage, in which case my DVM is giving me a false negative.
This is much like the "floating 85V" you often get on a switched off
120V line when using a high impedance DVM to check it.

Before I drop $20 on a new transformer, I'd like to make sure it's
really the culprit. The lighting store I bought the lamp from does say
that they "do fail in service sometimes"..... hardly real evidence.


Posted by Richard J Kinch on April 10, 2007, 10:00 am


Andrew Duane writes:

> My question is whether these transformers *need* some load to actually
> product voltage, in which case my DVM is giving me a false negative.

Transformers don't. Assuming it really is just a transformer, not a charge
pump. Why not swap one of the others in to test?

Posted by Andrew Duane on April 10, 2007, 11:00 am


Well, it's a little sealed box, not necessarily a "transformer" in the
traditional sense. It could well be some funky switching supply; it's
all sealed up in a black epoxy cube. And I have no others to swap
with; this is the only fixture of the type I have at the house.

> Andrew Duane writes:
> > My question is whether these transformers *need* some load to actually
> > product voltage, in which case my DVM is giving me a false negative.
>
> Transformers don't. Assuming it really is just a transformer, not a charge
> pump. Why not swap one of the others in to test?



Posted by beecrofter on April 10, 2007, 12:36 pm


> One of my hanging ceiling lights has decided not to work any more.
> It's a low voltage halogen with a small built in transformer, an LET
> 60 type 60-watt 12 volt unit. It looks like the transformer is gone;
> it shows almost no voltage at the output with a DVM. I get about 1/4
> volt that rapidly dwindles to 0.
>
> My question is whether these transformers *need* some load to actually
> product voltage, in which case my DVM is giving me a false negative.
> This is much like the "floating 85V" you often get on a switched off
> 120V line when using a high impedance DVM to check it.
>
> Before I drop $20 on a new transformer, I'd like to make sure it's
> really the culprit. The lighting store I bought the lamp from does say
> that they "do fail in service sometimes"..... hardly real evidence.

Ok so you take and make sure the bulb is sound using the R scale on
your meter, and you verify that input power is available on one of the
ACV scales, what does that leave?
Am I missing something here?


Posted by mm on April 10, 2007, 4:01 pm


On 10 Apr 2007 06:56:13 -0700, "Andrew Duane"

>One of my hanging ceiling lights has decided not to work any more.
>It's a low voltage halogen with a small built in transformer, an LET
>60 type 60-watt 12 volt unit. It looks like the transformer is gone;
>it shows almost no voltage at the output with a DVM. I get about 1/4
>volt that rapidly dwindles to 0.

Almost every test has a matching test. Here it is to measure the
resistance of the rest of the fixture. If it's not infinite, the
lights are probably good.

Do you have a backup burglar alarm battery that you could est those
lights with. Most of them are 12 volts. Or a couple 6 volt lantern
cells connected together. Or even the car battery-- you could bring
it in, or if the fixture is off, you could take it out to the car.

In most lightbulbs connection direction doesn't matter, not sure about
these.

If you put the battery on a table, you can take 6 or 8 feet of lamp
cord and put alligator clips on each end, connect two to the battery
and two to the lights. I even have insulation piercing alligator
clips for places where there is no copper showing, but I had to buy a
box of 10.

Another thing you can do is put a hat pin or a corsage pin into the
wire and attach the alligator clip to that.



>My question is whether these transformers *need* some load to actually
>product voltage, in which case my DVM is giving me a false negative.
>This is much like the "floating 85V" you often get on a switched off
>120V line when using a high impedance DVM to check it.
>
>Before I drop $20 on a new transformer, I'd like to make sure it's

20 dollars doesn't seem like much in this situation. The alternative
is to do the tests above. and if there is more than one bulb, it's
unlikely they all failed at once or that the wires going to each one
all failed at once.

>really the culprit. The lighting store I bought the lamp from does say
>that they "do fail in service sometimes"..... hardly real evidence.


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