Electrical Advice Please - Low-voltage fixture issues...

[lbgradwell]

Hoping someone can help me here...

I installed a low-voltage light fixture (6 @ 12V lights) in the front hallway of a friend's house that is controlled by a standard switch. If left on for more than - say - 30 minutes, it would work fine until turned off. If it is turned back on again within a minute or so, there's no problem & it continues to work fine. But if it's turned off for ~ 3 minutes, it won't turn back on and I need to wait for 10-15 minutes before it will work again!

I am aware that a LV fixture requires a special LV electronic dimmer, but this is just a normal switch...

I have a theory, but want an electrician to confirm it for me...

I tested the supply at the ceiling box and discovered the voltage there is 126.4VAC. As that is higher than I had ever seen before, at first I thought the breaker was perhaps faulty in some way, but I tested other outlets on other circuits throughout the house and found some as high as 127.3V.

I also put the meter on the 12V sockets on the fixture and they seemed to be 8.5V or so which makes me think the transformer in the light is working OK.

QUESTIONS:

Firstly, why is the voltage so high in the house?

Secondly, is there any way that the 126.4 volts might be a touch too high for the transformer in a 12V LV fixture. Is it possible that the fixture overheats due to this slightly higher-than-normal supply and needs to let the heat dissipate before it will work again?

Thirdly, if so, is there some type of voltage regulator or something else that will solve the issue?

All help is appreciated!

 

[Stuart in MN]

Nominal voltages in the US are 120V/240V, and the electric utilities generally hold that to a tolerance of +/- 5%. So, you're at the high end of the tolerance, but I don't know that it's enough to cause anything to malfunction. Has there been any problems with regular incandescent light bulbs having short life spans? That's a common symptom of voltage that's too high.

You could talk to the local utility about it. Chances are they'll say it's fine but it's also possible you can convince them to come out and put a recorder on the line for a while; if it is indeed too high, they should be able to adjust it by connecting to a different tap on the transformer out at the street.

 

[lbgradwell]

Thanks Stuart.

The 126.4V is actually just beyond that tolerance at 5.33%... I'll have to ask about the incandescent bulbs tomorrow.

At what voltage would you think problems might start for a LV fixture?

 

[MAD]

I don't think it has anything to do with the line voltage. It may be that there is auto resetting thermal protection in the 12 volt transformer that is cutting out and resetting after it cools down. Just a guess though.

 

[mrb]

is there insulation against these fixtures?

 

[lbgradwell]

I don't think it has anything to do with the line voltage. It may be that there is auto resetting thermal protection in the 12 volt transformer that is cutting out and resetting after it cools down. Just a guess though.

The thing is, Michael, I've installed many of these same fixtures before (they come from a friend's store) without incident. Yet three separate similar lights have given problems in this house so I have to believe it's somehow specific to this location. That's why I'm leaning towards the line voltage...

 

[lbgradwell]

is there insulation against these fixtures?

You mean like fibreglass pink? No, nothing like that; they aren't pot lights but rather hanging chandeliers...

 

[ishiboo]

I don't think it has anything to do with the line voltage. It may be that there is auto resetting thermal protection in the 12 volt transformer that is cutting out and resetting after it cools down. Just a guess though.

+1. Definitely the transformer. Line voltage that "high" is not uncommon at all, though usually they're low.

8.5V on the 12V is too low. If the transformer is functioning properly, it's overloaded or there is a wiring issue causing high resistance.

 

[pentavolvo]

+1. Definitely the transformer. Line voltage that "high" is not uncommon at all, though usually they're low.

8.5V on the 12V is too low. If the transformer is functioning properly, it's overloaded or there is a wiring issue causing high resistance.

agreed the 8.5 seems low to me

 

[Greatbear]

This is an odd one. I'd say the problem is thermally related with the transformer, but you'd think it would trip any thermal protection while on as well, causing the lights to go out, then back on. Some LV fixtures don't have a transformer, instead they have an electronic switching converter instead. These can cause some unusual issues when running hot, especially when overvolted as in your case. Is it indeed a transformer, or does it use a converter? I ask because not all converters allow use of a dimmer, and adding a dimmer would be an easy way to rectify the overvoltage condition and making the light more flexible.

Just some thoughts.

 

[lbgradwell]

If the transformer is functioning properly, it's overloaded or there is a wiring issue causing high resistance.

Any ideas on what might be causing this high resistance?

 

[lbgradwell]

This is an odd one. I'd say the problem is thermally related with the transformer, but you'd think it would trip any thermal protection while on as well, causing the lights to go out, then back on. Some LV fixtures don't have a transformer, instead they have an electronic switching converter instead. These can cause some unusual issues when running hot, especially when overvolted as in your case. Is it indeed a transformer, or does it use a converter? I ask because not all converters allow use of a dimmer, and adding a dimmer would be an easy way to rectify the overvoltage condition and making the light more flexible.

Just some thoughts.

And I appreciate these thoughts!

Yeah, the odd thing is that if the light is left on, it will function without problem indefinitely and that seems (to me anyway) to rule out a thermal protection issue...

I was told the fixtures have transformers, but I don't know for sure; something else to check tomorrow... I was not aware of the problems with overvolted converters, so thanks for that!

Perhaps, as you say, a dimmer is the answer...