Question for EE folks: low voltage lighting

[arcas]

I have two sets of low voltage lights. One is a run-of-the-mill outdoor setup and the other are little halogen under-cabinet lights. Both of these guys consist of a power supply, some amount of zipcord for main cable and a number of light fixtures that clamp onto said cable. Both manuals list minimum and maximum distances that lights may be from the transformer. The maximum distance I can understand. However, I don't understand the minimum distance.

For example, let's assume it's a 60W 12VAC transformer supplying a 16ga main cable and assume the manual says "Lights must be at least 3 feet from the power supply". Why? Electrically, what's the difference between a setup with lights at 2ft, 5ft and 7ft and a setup with lights at 3ft, 5ft and 7ft?

A 16ga cable's DC resistance should be negligible at those distances. I would have thought the 60hz impedance would be negligible, too? Is there something else going on? Capacitance maybe?

 

[darthsidious]

do the things produce a lot of heat - maybe it's to make sure the transformer doesnt affect the bulb or something (though that seems unlikely)

Maybe they need some cap on the output for filtering, and they depend on the wire for that?

I really have no idea, these are just guesses.

 

[PottedMeat]

So these are just 120VAC->12VAC transformers parallel connected to 12V halogen bulbs? Strange, you could solder the bulb directly to the transformer and the bulb would work fine. I guess it's maybe to dissipate the heat over a larger area.

 

[slpaulson]

My only thought is that since they are halogen lights, they can get very hot, and a longer wire will dissipate more heat.

 

[91TTZ]

Originally posted by: slpaulson
My only thought is that since they are halogen lights, they can get very hot, and a longer wire will dissipate more heat.

But the wire isn't what dissipates the heat when the bulbs get hot. Most of it is radiated.

 

[91TTZ]

The voltage is probably rated to be within the range of the bulb at a certain distance from the transformer. Too close and the voltage is too high, too far away and it'll be dim.

Remember, with only 12V you're going to have a lot more current loss in the wire for the same wattage bulb compared to 110V.

 

[PottedMeat]

Originally posted by: 91TTZ

Originally posted by: slpaulson
My only thought is that since they are halogen lights, they can get very hot, and a longer wire will dissipate more heat.

But the wire isn't what dissipates the heat when the bulbs get hot. Most of it is radiated.

He probably means heat generating components are spaced further apart is all.

 

[91TTZ]

Originally posted by: PottedMeat

Originally posted by: 91TTZ

Originally posted by: slpaulson
My only thought is that since they are halogen lights, they can get very hot, and a longer wire will dissipate more heat.

But the wire isn't what dissipates the heat when the bulbs get hot. Most of it is radiated.

He probably means heat generating components are spaced further apart is all.

That could be it, too.

 

[LTC8K6]

Too close to the transformer will mean too high a voltage/wattage for that bulb, iirc. This will take too much power from the rest of the bulbs, iirc.

 

[91TTZ]

Originally posted by: LTC8K6
Too close to the transformer will mean too high a voltage/wattage for that bulb, iirc. This will take too much power from the rest of the bulbs, iirc.

I agree with you on the voltage/wattage point, but as far as taking too much from the other bulbs, they're wired in parallel so that shouldn't be an issue.

 

[BigJ]

Originally posted by: 91TTZ

Originally posted by: LTC8K6
Too close to the transformer will mean too high a voltage/wattage for that bulb, iirc. This will take too much power from the rest of the bulbs, iirc.

I agree with you on the voltage/wattage point, but as far as taking too much from the other bulbs, they're wired in parallel so that shouldn't be an issue.

With conventional lighting, when houses are closer to the transformer, they'll see higher line voltage (sometimes in the 125v range). People have to use 130v bulbs in there because normal bulbs blow.

Could this be a similar case?

 

[arcas]

I thought about that but it seemed to me that the resistance and impedance would be rather negligible at such short lengths so that there wouldn't be much voltage drop due to the wire itself.

To whit, a quick google suggests that 16ga lamp cord (which appears to be basically what this cable is) has about 4 ohm resistance per 1000 feet. So a 2ft length of wire would have a resistance of about 0.008 ohm and a 3ft length would be 0.012 ohm. So with 60W worth of lights at 12V, the corresponding voltage drops due to the wire's resistance would appear to be 0.0398V and 0.0597V, respectively. So there's about a 0.02V difference in the two positions.

So I guess it's possible that the transformer is really putting out 12.06V or thereabouts but I wouldn't think that 0.06V of overvoltage (which you'd get if you basically attached the light to the transformer itself) would have much effect on a light's lifespan.

Could be a heat issue like some of the earlier posters wrote...