Low voltage light flicker problem

Discussion in 'Electrical Forum discussion & Blog' started by jvstevens, Jun 13, 2009.

  1. jvstevens

    jvstevens Member

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    Earlier this week I installed a single low voltage can light (electronic transformer) operated by a standard incandescent dimmer switch. The transformer specs say it works with standard dimmers. The can is a 4" and has a max bulb rating of 35W. When I turned it on, the light flickers when dimmed, but stays steady when fully on. After doing a little research, I've more or less come to the conclusion that the 35W load is too small for the dimmer to operate. Most dimmer specs I've seen require about a 40W load minimum. Another reason why I think this is the problem is because later on that day I installed another can light circuit, but with two 35W cans (70W total) and it works fine (same type of dimmer and transformer).

    I'm now trying to figure out the best way to fix the situation. I can't increase the bulb wattage, I don't want to add another can light to the circuit, and I'd rather not change the dimmer (assuming I could even find one that goes down to 35W). So, I'm thinking of adding a passive resistive load to the low voltage circuit to bring the total power load above the minimum dimmer spec level. Yes, I know its kind of a waste of energy. Are their devices made specifically for this problem (say in Lutron or Leviton's product line), or do I have to conjure up my own solution using power resistor(s). Any guidance here would be greatly appreciated!
  2. Thatguy

    Thatguy Homeowner

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    www.hosfelt.com has pretty good prices on power resistors, if they have what you want.
    You may need forced air cooling or a heat sink, which means you need to figure thermal resistance (in °C/W). Wiki has an article on this.

    Say hi to Ward and June.
    Last edited: Jun 13, 2009
  3. jvstevens

    jvstevens Member

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    Ward and June say hi back, and said for me to tell you to say hi to That Girl.

    Thanks for the input. As it turns out, my local Fry's Electronics store down the street sells 25 watt power resistors for 99 cents each. Cheap! So I got a 2.2 ohm and a 2.7 ohm power resistor that I will put in series to give 4.9 ohms total. According to my calculations, that presents a 29.4 watt load on a 12 volt supply, which brings the total power load (including the 35 watt halogen light) to 35 + 29.4 = 64.4 watts. That should be enough to get the dimmer working properly, right? I decided to break up the load in to two resistors so as to minimize heat. The 2.2 ohm will dissipate 13.2 watts while the 2.7 ohm will dissipate 16.2 watts...all well within the 25 watt rating.

    Anyone care to double check my calculations?

    So, this seems like it should work, but it still somewhat troubles me that I have to rig up my own set up to solve this prolem. Surely this has come up before. Is this what others do to solve the problem?
  4. Thatguy

    Thatguy Homeowner

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    Fry's? You seem to be pretty far west of me.

    I've never heard of anyone else having this problem. I guess they just put in more lamps.
    These resistors might get too hot to touch even running them at half rated power, depending on how much surface area they have.

    You may want to check them with your "digital probe."

    burn in 30 sec at 54C
    5 sec at 60C
    1 sec at 71C

    If the thermal resistance is 3.3°C/W these resistors would be at ~50C above amb. temp. but that's for a 10x10x5 mm package.
    Last edited: Jun 13, 2009
  5. jimbo

    jimbo Plumber

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    Location:
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    Well, what type of BULB are we dealing with? If it is fluorescent OR led, then dimming may not be possible.
  6. jvstevens

    jvstevens Member

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    46
    Halogen, MR16

    By the way, here's the resistor I'm planning on using. Its a wirewound power resistor. Whaddaya think, heat wise?

    25W resistor
    Last edited by a moderator: Jun 13, 2009
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Is the low-voltage can an 'all-in-one' thing or is the transformer separate? If you feed in 120vac, and it changes all that to low-voltage for the bulb, are you sure the transformer won't burn out when you nearly double its load? A separate transformer is more likely to support more current, one special built for a specific fixture won't.

    If it's an all-in-one, just bite the bullet and switch the fixture to accomodate a 50-60W 120vac bulb, and be done with it, or a bigger bulb. No kludge that might burn the house down. Or, find a place you can put a second fixture on that circuit to bring the current up so it will work right.
  8. Thatguy

    Thatguy Homeowner

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    I think That Girl was too squeaky clean for me; I had to dump her. :(

    2.5 x .5 x .5 = 2.5 + .5 = 5.5 sq. inches surface area for your NTE part.
    A 15w bulb is too hot to touch and has a comparable (?) surface area.
    Alternately you might find a datasheet for a similar size resistor that does specify the thermal resistance. They're out there somewhere in Cyberspace.

    It might not work, anyway.
    Incand. bulbs have a cold resistance 10x to 15x the hot resistance but this resistor's value is independent of the current through it.
    I don't know what the dimmer will do, but for 99 cents you could try it.
    Last edited: Jun 14, 2009
  9. jvstevens

    jvstevens Member

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    The transformer is separate and is rated for 75 watts output. Right now it just has the 35 watt load. So if I add on the resistive load, it puts it at about 64 watts, according to my calculations.

    One reason I'm kind of stuck using the existing can (Elco E400) is that it is the lowest profile can I could find. There is very little clearance above the can due to an important wood structural element located above it. Standard remodel cans would not fit in that location.

    There is no good place to put in a second can light to beef up the load. I briefly entertained the thought of putting one up in the attic where it can shine uselessly, but its too much of a pain to go up there and change the bulb when it burns out. Plus, that idea just seems too hokey to me.
  10. jvstevens

    jvstevens Member

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    I'm not quite sure why you are concerned about how "hot to touch" it is. My plan was to just put the resistors up in the attic out of the way, so no one will ever touch them, and they can radiate their heat in peace. Not sure how I will mount them yet, though. Probably put them in a big metal J-box, and remove several of the slugs?
  11. Thatguy

    Thatguy Homeowner

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    Yeah, I think that'll work but UL and the NEC may not agree.
    Check your flammables against this list
    http://www.tcforensic.com.au/docs/article10.html
    That way, no matter what happens, you can show Due Diligence and Duty of Care.
  12. PEW

    PEW DIY Senior Member

    Messages:
    487
    If you have a 2 wire dimmer, I would just go with a 3 wire dimmer and be done with it.
    Last edited: Jun 15, 2009
  13. jvstevens

    jvstevens Member

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    I'm not sure I understand how this would help my situation.
  14. jvstevens

    jvstevens Member

    Messages:
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    Well, today I had a chance to troubleshoot the situation, and sure enough, my theory was correct. It was flickering because the electrical load (35W) wasn't large enough for the dimmer to function properly. And when I put in my two power resistors that I talked about earlier, the flickering went away. So, I was about to go with the power resistors as my final solution, when I said, "Ah, what the heck, let me try one of my old used dimmer switches that's been laying around for years, and see if it works". Sure enough, when I put in a different dimmer, it worked great...no flickering! I guess the old dimmer had an especially high operating power threshold. At any rate, I decided to just go with the different dimmer and scrap the resistor idea, as the dimmer solution is a bit more conventional and standard than those crazy resistors. Should save more energy too.

    Thanks for everyone's help!
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