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I have an over-the-stove microwave/ hood vent combo with 2 adjustable-brightness lights. The lights appear to be 12V lamps with a G4-style connector, and they burn out all the time... like every 4 or 5 months. The bulbs themselves are like $8 a piece, and you have to take apart the hood with a special tool to get at them so this was getting to be a frustrating experience. I decided to go with LED to reduce the maintenance.

I found some 12V LED lights with the G4 style base, and bought them. When they arrived, I replaced my burnt-out bulbs with these LEDs.

They didn't work. There looked like just a little bit of light were coming out of them, but nothing near my expectations. I tried reversing the polarity with no difference in result.

I figured that the power to the light sockets must be AC, or the LED's did not support different levels of voltages (I got them cheap from China). So, credit card in hand, I went looking for LEDs that could handle AC and were dimmable. I found some, ordered them, and received them.

I tested it out after I replaced the first bulb. Eureka! The bulb was bright, and dimmed properly. All done. So I replaced the second bulb.

All of a sudden, both bulbs no longer work: very dim, not dimming, just like my first LED experience. Scratching my head, I experimented:

1 LED installed, 1 incandescent: bright, dimmable light

2 LED lights installed: dim, barely visible

1 LED installed, other socket empty: dim, barely visible

2 incandescents installed: bright, dimmable

I'm trying to figure out how the sockets could be wired that would give me this result, and what I can do about it.

share|improve this question
Technical links to the LEDs might help - I don't mean pretty brochure pages but proper techy stuff light voltage ratings. Also, do you know for sure what your current circuit outputs in terms of AC amps and volts and frequency? – Andy aka Mar 28 '14 at 10:47
@Andyaka It's 12V, I assume AC. The amps are strong enough to drive the 10W bulbs (the LEDs are 3w) other than that I really don't have much additional information, sorry. – Jeremy Holovacs Mar 28 '14 at 13:23
Any chance the two sockets are wired in series? I can imagine that even AC-compatible LEDs wouldn't fare well in a series circuit fed from a standard dimmer. – user39432 Mar 28 '14 at 16:10
It's worth noting that even when LEDs say "dimmable", the degree to which they are compatible with specific kinds of dimmers -- and the actual range of useful levels -- does vary, presumably because the power supply circuits inside the bulb vary. I've found that going for the cheapest LED bulb available is rarely a good bet. – keshlam Mar 28 '14 at 17:02
up vote 16 down vote accepted

Perhaps the dimmer circuit won't work without a resistive load. Some thyristor dimmer circuits might not see enough holding current from a rectifier-capacitor load to stay on for the half cycle. You may be able to wire a fat resistor in parallel with the LED lamps and have it work, however it may require some experimentation to determine an appropriate value.

Here's a typical small triac datasheet. The holding current is specified as maximum 10mA or 20mA, depending on type. If a phase-control trigger circuit provides a short pulse, the MT2 current must reach the holding current during the trigger pulse or the triac will not stay on for the remainder of the cycle. (bottom line below).

enter image description here

Here (from Wikipedia) is what the output of a typical phase control looks like. The gate trigger pulse would take where the vertical red chevrons are located. (a) is the power waveform. (b) is the output with triggering near the beginning of the cycle (bright) and (c) is the output with triggering near the end of the cycle (dim).

enter image description here

I would try about a 150 ohm 2W resistor -- if you get a few you can parallel several and go up in ~1W increments. You may find that too high a resistance causes half-wave triggering (intermediate brightness), since triacs have different holding currents in the positive and negative directions.

share|improve this answer
Hmmm... so if I understand correctly, the reason the bulbs aren't working... is because they are too efficient? That seems awfully ironic for some reason. – Jeremy Holovacs Mar 28 '14 at 13:28
It is ironic, but that doesn't make it any less real. Expect more of this (e.g., automotive turn signals). Sometimes they put a ballast resistor in the lamp just to make the application work. – gbarry Mar 28 '14 at 15:46
I can confirm the need for resistive load -- I have dimmers throughout my house that are unreliable (i.e. lights flash or won't turn on) without a resistive load. Adding one incandescent bulb to the otherwise LED load completely solves the problem. – alx9r Mar 28 '14 at 17:15
This looks like it's the answer. I guess a visit to my local Radio Shack is in order for a mini circuit board and some 2W 150Ohm resistors. Wish me luck! – Jeremy Holovacs Mar 28 '14 at 18:16
@jeremyholovacs yep. Too effecient. Same thing with car led bulbs. Even the dumb resistor ballast ones are so effecient, the car thinks there is an open circuit. Its hilarious when you think about it. – Passerby Mar 28 '14 at 18:49

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