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I have 5 spot light bulbs in my bathroom. All use MR16 connection. I replaced them all with 5w LEDs and when I turn on the switch they flash once then go off. If I remove one so there's only 4 then the same thing happens. However, if I put one of the old halogen 20w bulbs in then all the LED bulbs stay on and everything is fine.

Why is this happening? And what can I do to get rid of that last halogen and replace it with an LED?

  • Are these on a dimmer? some dimmers will not work with LED's. What happens if you put 1 halogen in place of one of the LED's – Ed Beal Apr 27 '16 at 21:56
  • Its a normal switch. Like stated in my question, if I put 1 halogen back in then the 4 LED bulbs hold their light. Seems like a halogen needs to be in there to draw enough power to the circuit or something? – nutman Apr 28 '16 at 8:15
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    Can you check the voltage on your halogens. There are some light fixtures that run off a lower voltage (a ballast or transformer may be in the fixture). – Ed Beal Apr 28 '16 at 13:06
  • @EdBeal I would reword to "check the voltage in the sockets" just to be clear :-) . – Carl Witthoft Apr 28 '16 at 17:33
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    I would check both the voltage of the lamp that works and the voltage supplied. There are 120V lamps if this style and 12 or 24v. if the lamp has 12v or 24v stamped on it there should be a transformer. – Ed Beal Apr 28 '16 at 18:53
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This symptom is impossible without silicon electronics being involved in the delivery of power. Passive wires and windings are not capable of causing this. Common problems are common, and my first take is usually correct - a series connected old-style smart device.

But Carl Witthoft has an interesting theory: these are probably low voltage bulbs. We know voltage is right because the LEDs do work properly when power is available. That's not surprising, since many LED products use constant current power supplies in switch mode, which can easily accommodate a range of voltage.

Most low voltage power supplies are passive, or at least well behaved. So I normally don't think about them. It would certainly explain everything if you had one that was "too smart for its own good". The five LEDs should draw power comparable to one halogen (at least on half the AC cycle... since the D stands for "diode"). So there's clearly something a little more sophisticated going on. I could see some power supply designer thinking himself oh, so clever for adding a protective circuit that shuts the supply off if it doesn't detect any halogens. To be fair, LED lighting is a very new concept.

For reference, the usual cause - and my first guess - and the one to apply if this proves to be 120VAC circuit -- relates to dimmers, sensors and other smart devices designed to substitute for an old switch in a "switch loop", where there is no neutral. These devices place themselves in series with the bulb, and power themselves by letting some current flow through the circuit when the light is "off". This depends on particular characteristics of incandescent (and halogen) bulbs. It often doesn't play well with fluorescents and LEDs. This is why Code now requires neutral in switch loops - so a new generation of these devices can power themselves directly off hot and neutral. For this, the answer can be to roll back to a plain switch - unless neutral happens to be in the box, in which case use a modern device.

  • It is already a human operated switch. Not a dimmer. just a standard switch. – nutman Apr 28 '16 at 9:12
  • Bizarre! There's got to be something. LEDs don't do that by themselves. Longshot it's some sort of interaction between LEDs, what happens if only a single LED is plugged in? – Harper Apr 28 '16 at 9:21
  • the same thing happens. It seems as if the circuit requires a halogen in there to draw enough power.... as if a minimum amount is required. e.g. 1 - 5 LEDs - flashes once. 1 - 4 LED & 1 halogen - both light up and stay lit – nutman Apr 28 '16 at 10:07
  • not accurate or true. – Carl Witthoft Apr 28 '16 at 17:34
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According to any number of help pages, e.g. this one, MR16 sockets and bulbs are designed for 5 to 24 V operation. It's highly likely that's what you've got here. So the question becomes: what is your current operating voltage, and what is the voltage requirement for the LED bulbs you bought? If there's a mismatch, that's why things don't work.

  • Nope. The LEDs work fine when the upstream electronics decide to turn the power on. Ergo voltage is correct and may be 120V. This symptom is impossible without electronics involved somehow, and your low-voltage theory does explain how that might be. And this... is how the hivemind works. We both have a chunk of the problem and are both part right and part wrong. – Harper Apr 28 '16 at 18:20
  • I have a very similar problem with 12VAC halogen track fixtures on a 120VAC track. Replacing the halogen bulbs with LED ones, results in the LED flashing at power-on and then going dark. The behaviour is the same regardless of whether one or all of the halogen bulbs are replaced with LEDs. I disassembled one of the track fixtures and found a 120VAC to 12VAC step down coil feeding the bulb. The coil and at least one capacitor are sunk in resin inside the fixture. So I think I either need a smarter fixture, or to bypass the analogue circuit w/my own digital one. – RoUS May 12 at 20:20

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