Pool Light Malfunction

Mar 8, 2008
Here are my posts on a Yahoo pool group concerning a problem with my pool light. It's not resolved yet. I'd appreciate any ideas.

I have one more post to the group which I'll bring here as soon as it's published and I can copy it. Basically, it concludes that neither the light itself or the thermal/overload protector on the transformer is the problem and moves the focus to the transformer itself.


Mar. 3, 2008

My pool light (12v 300w) turns on fine but within a minute it shuts down. Then
in a minute
or so, its back on but just for a few seconds. Then it's off for 30 seconds or
so, on for five
seconds and off again.

I've replaced the bulb thinking it was an intermittent filament break. New bulb
does the
same. There was no water in the bulb compartment.

Can this be a problem caused by a faulty low water sensor in the light assembly?

Any ideas will be appreciated.


Mar. 10, 2008

Re: Pool light malfunction

Thanks for the comments so far. I think Mike is on to something when he
suggests the
transformer may have an issue.

I measured the amperage with a clamp on ammeter on both the primary and
sides of the transformer. When the light is on, the primary side shows about
three amps-
-about what one would expect with a 300 watt bulb. The secondary peaks at about
amps initially but in a few tenths of a second is down to 17 amps as the bulb
heats up.
Again, about what one would expect. But after 30 seconds or so everything shuts
and current goes to zero.

The light is connected to the 12-volt tap on the transformer. I tried the
13-volt tap but it
just made the on/off cycle quicker so I went back to the 12-volt tap.

The transformer is cool to the touch during all this.

Then I disconnected the light and measured the current on the primary. It was
between .1
and .2 amps and stayed on forever. It didn't cut out. I figure .1 to .2 amps is
about normal
for just energizing the primary so no big revelation here.

Next I connected a spare 12v/300w bulb to the secondary. The pool light and
wiring was out of the circuit during this test.

The primary and secondary amperages were the same as for the pool light---and
after 30
seconds or so, the light shut down. Now, as Mike suggested, it became pretty
clear that
the overload protector in the transformer was cutting out. In fact, I think I
can hear the
mechanical snap as it does so. I suspect it's a bimetal switch. But again, the
itself was cool to the touch. There's absolutely no indication of overheating
and the
current draw certainly does not seem excessive.

So, I think Mikes right. There's a problem with the overload protector.

I'm calling the manufacturer on Monday and see what they have to say. If
they're no help
I'm going to remove the transformer and see if I can find the protector. If I
can find it and
if I can physically isolate it from the transformer while still keeping it wired
in, I should be
able to determine if heat is the issue. I suspect it has a fault that causes it
to be intolerant
of normal current draw.

Thanks again for the ideas. I'll keep you posted.


Mar. 9, 2008

Re: Pool light malfunction

Well, here's the latest in my pool light saga.

After testing current flow and verifying that the pool light itself was not the
problem, I
concluded, with Mike's help, that the problem lay in the transformer. Further
testing there
strongly implicated the current overload protection device in the transformer

So today I called Intermatic, the light manufacturer aand learned that they have
serviceable parts for the transformer. Sending the entire assembly to them for
repair was
the only option and they candidly said that would likely cost more than the
purchase price
of a new unit ($125). I pleaded with the customer rep that there must be
another option
and she referred me to a technical guy who clearly understood the product and my
problem. He referred me to the company that makes the transformer for them.

A call to to that company (I'll privately provide the name if anyone needs it)
got me to
another customer rep who listened politely but informed me they were a
wholesaler and
couldn't sell direct to a consumer. I asked if she could make an exception and
referred me to her boss. He actually seemed willing to help and suggested I
talk to their
engineer in charge of that product. NOW I was getting somewhere!

The engineer listened to my problem and asked a bunch of questions about the
I had done. We talked for nearly 10 minutes. He ultimately agreed that the
protector was the likely problem and offered to send me a replacement from his
lab stock.
No charge. Now that's customer service.

If all goes well and I get the part as he offered, and if it is indeed the
problem I'll be back
in business for no cost. Alternatively, if, for some reason I don't get the
protector, I intend
to cut the existing one out of the circuit and bypass it. It's there only to
protect the
transformer. It has nothing to do with pool safety. But just to be on the safe
side, I'll
protect the entire circuit with a GFI. That should protect the transformer and
give a good
margin of personal safety as well.

I'll keep you posted. I hope to have the overload protector by late this week.

Mar 8, 2008
re: Pool light malfunction

Here's the latest post to the Yahoo Pool group and the current status of my search for the problem with my pool light:

MR 14, 2008
My poollight issue continues. I got the replacement thermal/overload protector for the
transformer today. An engineer at the manufacturer sent it gratis.

I installed the new protector, took a few photos to share the process, and turned on the
light. I wish I could say the problem is resolved but, alas, the light blinked out exactly as
it had with the original protector in the circuit.

I redid the diagnostic process by taking the pool light out of the circuit and instead wiring
in a replacement bulb directly at the transformer. Within 30 seconds, that light turned off
too, thus confirming that the pool light is not at fault. I then took the thermal/overload
protector out of the circuit and tested against the light with just the bare transformer in
the circuit. Again the light went out after a few moments.

That leaves the transformer itself as the only remaining element in the circuit that has not
been replaced. I'm now guessing there must be some intermittent problem causing the
wiring in the transformer to open after warming up under current draw. I'll need to think
a while about how to go about diagnosing the problem.

Any thoughts would be welcome.




LifeTime Supporter
Platinum Supporter
TFP Expert
May 7, 2007
Silver Spring, MD
Welcome to TFP!

It sounds like you have been very through and methodical.

I have seen something a little like what you describe, but it was much more intermittent. The light would work for a while, fail for a while, then work again at random intervals. It turned out to be a broken wire that was usually held in contact by the insulation, but now and then the wind would cause the cable to bend just a tiny bit and pull the two ends apart enough to break the circuit. It all became obvious when I found the right section of conduit to push on and the light started blinking on and off.

What you describe sounds much more like an auto reset protection switch of some kind. I'm surprised that it wasn't the thermal overload protector, it is such a perfect match for the symptoms.
Mar 8, 2008
My latest thoughts (and efforts):

Yesterday I had concluded that the pool light transformer itself must have an intermittent open in the winding. But I needed more time to think on further diagnosis. I've often found if I'm faced with a tough or incomprehensible issue, if I just walk away from it for a while, something will pop into my head.

Well, this morning it occurred to me that the transformer wasn't the last remaining unchecked component in the circuit as I had mistakenly assumed yesterday. There is the circuit breaker in the main power distribution box. What if that was bad? Could it be faulty in some way so it would disconnect the circuit under load and then pop back on when things had "cooled down"?

The pool light circuit in the main box is protected by a twenty amp circuit breaker. Among others there are also two 30 amp breakers for the pumps. I disconnected the pump from one of its 30 amp breaker and connected the light circuit to it. I flipped the switch hoping the light would stay on proving that the original light circuit breaker was faulty. Thirty seconds later the light went out, just as it has with every other component substitution.

Now I'm virtually certain there are no remaining components to check and I'm back to the transformer as the source of the fault.

It's back to thinking time. Walk away and let the subconscious, and maybe someone else who reads this stuff, come up with ideas. I suppose in the end, I'll have to spring for the $125 option of replacing the entire transformer assembly. I'd sure like to come up with some way to get a substitute transformer in the circuit first, just to make sure it's the bad element before I spend the money. Or, if I can't find a substitute, find some way to observe the faulty area in the transformer so I truly know it's bad.

Any ideas?



LifeTime Supporter
Oct 5, 2007
Valrico, FL
If you have a volt meter connect it to the secondary of the transformer where you connect the light to. See if the voltage drops when the ligh turns off. If so then there is something in the light that is openning up. Also most pool lights are designed to only work while in water. If you are using them out of the water there might be a thermal switch openning on them.
Mar 8, 2008

Thanks for the suggestion.

Do you mean that there may be a thermal switch built into the bulb itself? I had suspected that there was a thermal switch in the light housing but it never occurred to me that there may be a thermal switch within the bulb. If that were so, it would sure change the way I am approaching this problem.