dimming halogen lights

sdelloMay 17, 2009

I installed a 4 ft track lighting fixture with three GU10 spots (bulbs have pins)on it. It was a kit from a big box store. I told the guy I was going to install a dimmer, he was adamant that I needed a low voltage dimmer (it required three wires) and the setup works great.

I then went and bought another 4 ft setup with three PAR 20 halogen fixtures 50W bulbs (these screw in with what looks like a standard socket). I purchased all the components separately. I wanted a dimmer here also, but I wasn't sure if I needed the low voltage typ again or if a standard dimmer would work.

I bought a dimmer that says "Incandescent/Halogen 120V 600W"

It's a Lutron S-600PH-WH

The lights are Portfolio Track Lights. Flared Gimbal Track Lights #120700. Bulbs are Sylvania PAR20 50W floods

It doesn't have the third wire and instead connects inline just like any other dimmer/switch. It seems to work fine. Is this a proper setup or should I get a low voltage one?

Thanks in advance for any replies.

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Ron Natalie

There's nothing magic about dimming halogen bulbs in their own right. We switched to halogens decades ago in the theatre and the dimming technologies (and there are tons of them in the theatre) didn't change at all.

The issue with your first fixture is not the halogen bulbs but the low voltage power supply. To dim them both the supply and the dimmer need to be compatible.

What you've done is all that's required. Be advised, that prolonged running of halogen bulbs at lower intensities may decrease their life.

    Bookmark   May 18, 2009 at 7:52AM
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Be advised, that prolonged running of halogen bulbs at lower intensities may decrease their life.

Can you provide a citation for this statement?

    Bookmark   May 18, 2009 at 8:29AM
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Found the answer:

Yes, conventional incandescent dimmers will work to dim halogen lamps. However, the effectiveness of the halogen cycle to keep the lamp walls clean and give longer life may well be affected. This cycle depends upon correct lamp operating temperatures, which of course will be changed when the lamp is dimmed. Therefore, using a dimmer may not extend the life of your halogen lamp as much as a dimmer typically extends the life of a standard incandescent lamp.
The halogen lamp is designed to prevent the tungsten from depositing on the inside of the bulb wall and darkening it. Because the halogen action stops working when the bulb wall temperature falls below 260 degrees Centigrade, which may happen when the dimmer lowers the voltage, the halogen lamp blackens and its life is not prolonged as much as an incandescent lamp on a dimmer. Eventually a severely dimmed halogen lamp can become blackened and fail.
The wall blackening can be partially reversed if the halogen lamp is operated at full power, non-dimmed, periodically to allow the halogen cycle to remove some of the deposited tungsten.

I guess the reason I had never seen a reduced lifetime is that we tend to change the brightness frequently (sometimes it's full bright, sometime very dim, etc) and the full brightness usage counteracts some of the problem related to dimming.

    Bookmark   May 18, 2009 at 11:52AM
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Ron Natalie

The description is mostly right. The bulbs don't fail because the envelope darkens, that's just the side effect. The bulb fails because the boiled off pieces of the tungsten are not redeposited on the filament. You gotta pretty severely dim the things to disrupt the halogen cycle, but if you leave things "moody" for a long time it can happen.

It is correct, that if you don't let things go too far, getting the bulb hot enough again will recover it.

    Bookmark   May 18, 2009 at 12:47PM
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I've heard the same about running them periodically at full.

Something about the original post jumped out at me - if the fixture is GU10 bulbs, then they're 120v, right? Therefore there is no transformer/switchmode supply, and there's no requirement for a 'special' dimmer. Some do say they're suitable for halogens, and some halogen fixtures say they're dimmable, but from what I can see, a standard one ought to work.

    Bookmark   May 18, 2009 at 4:52PM
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Hopefully, you have figured out by now that your GU10 spots didn't require a LV dimmer. Regular old dimer is fine. Several years ago a contractor installed a LV halogen (mr-16)fixture in my house and used a 120V dimmer. It works fine, but if you do some research, you will find that there are some issues with using a 120v dimmer with 12/25V lights. I've never heard of using a LV dimmer on 120V fixtures. Apparently, from what you say, it must work. However, I'm not sure if there are any issues what that. Perhaps the experts can chime in.


    Bookmark   May 18, 2009 at 8:11PM
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thanks for all the good info. The low voltage dimmer on the first setup seems to work just fine.

The "traditional" dimmer that I put on the second setup seems to work just fine also. Of course as I said previously it was marked "incandescent/halogen" right on the package.

so far I'm in business. Thanks again.

    Bookmark   May 18, 2009 at 11:04PM
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Ron Natalie

All dimmers I've seen that are rated for magnetic transformer loads are also rated for incandescent / resistive loads as well. It's just overkill.

    Bookmark   May 19, 2009 at 8:46AM
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Since the op's happy with the answer, hopefully you don't mind if I hijack the thread:

What is the advantage, if any, of true lv over mains (120)v halogens?

When I lived in Australia, virtually all gu-sized halogens (integrated reflector/glass filter) were 12v, presumably because it's hard to get 240v to work in such small bulbs.

Here in BC I've bought a remodel kit with gu-10s and have had one or two blow in the fitting I've used so far, although I'm assuming it may be because of the cheapie bulbs they tend to supply with fittings. I know both are dimmable, so what's the advantage?

At the moment, it'll be a remodel situation in a basement ceiling, so no insulation. Later I will be installing upstairs so will have to contend with insulation in the ceiling, whether it's 12v or 120 I know I'll need enclosures of some sort.

    Bookmark   May 20, 2009 at 2:18PM
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Lower voltage bulbs can have a more rugged filament.

A 12 V filament can have a tighter winding spacing, and even be double would, ti increase resistance to mechanical vibration.

At 120 V (and 240 V) the windings (if present) must have a large enough gap from turn to turn to prevent arcing over.

I have had a number of customers pay to have 120 V under cabinet pucks removed and replaced with 12 V pucks.

They had bulbs blowing on a weekly basis from the normal vibration if putting things into the cabinets and removing them.

If you do not have to contend with vibration there is no real issue.

    Bookmark   May 20, 2009 at 3:35PM
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Halogen PAR20s are line-voltage 120v bulbs, and thus can be dimmed with basic incandescent-type dimmers including the Lutron you bought.

I've never used GU10s and they confuse me - they're based on MR16 bulbs, which are normally low-voltage 12v, but GU10s are often (always?) configured for 120v power. Not sure if you need a low-voltage dimmer with these off the top of my head.

    Bookmark   May 23, 2009 at 12:19AM
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Be careful when dimming low-voltage halogen lights. They use a transformer to convert the mains to 12V. Some of these transformers, mainly the heavy ones with a coil, are "inductive" loads. Others, mainly small electronic ones, are capacitive loads (but not always!). Your dimmer should behave correspondingly, otherwise you risk to blow up either your dimmer or your transformer.

    Bookmark   May 23, 2009 at 7:06AM
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