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At a hotel I manage we purchased 34 ceiling fans with two integrated light sockets that operate on a dimmer all controlled by a remote control. Obviously using a dimmer with fluorescent bulbs won't work as desired, however we only want to use them in either the off or on position.

They work fine when on, however with the switch turned off, when we put a fluorescent energy saving light bulb in, it turns the bulb into almost a strobe light with a very low-intensity flashing. It's not enough to really bother you and you wouldn't notice it much if another light was on, however we had someone complain that it kept them up at night when in a dark room with no other lights. Measuring the other socket with a volt meter with one fluorescent bulb present measures 20V.

When we put two fluorescent bulbs in, it has the same effect, however it alternates between the two and does something like two-three flashes from one, two-three flashes from the other, and continues like that.

However, the strange thing is that when we put a regular bulb in, the voltage on the other side drops to zero. So when we have one regular bulb and one energy saving bulb, neither of them flash at all. Same is true with either one or two regular bulbs.

So basically, what could be causing this (to me) odd behavior? Is this typical for things with dimmers? Is there any way around this so that we can use two fluorescent bulbs? We already have all the bulbs (which were around $10 each) and we're just in the process of replacing the current light fixtures with the fans which are also already paid for.

Note: I'm not an electrician, so any responses will have to be written as basic as possible.

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2 Answers

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TL;DR: the dimmers aren't switching off completely: they're allowing some current to leak through, which is why you're seeing a voltage across the CFL. A different make of bulb may behave better with the leakage current that you're getting. Or perhaps a different brand of fan (if you haven't installed them all already).

I do know that operating CFLs in those sort of conditions will shorten their lives considerably, so you might actually be cheaper for you to use incandescents instead (a quick calculation says about 12 kWh per year for a 60 W bulb).

Read on for the technical explanation...


This is a circuit diagram of the innards of your fans:

Circuit diagram showing dimmer and bulbs

The voltage across the bulbs, Vb is determined by the formula:

Vb = Vin * Rbulb / (Rdimmer + Rbulb)

where:

  • Vin is the mains voltage (120Vac or 240Vac depending on country).
  • Rbulb is the resistance across the bulb or bulbs.
  • Rdimmer is the resistance across the dimmer.

The dimmer is a solid-state electronic circuit, so it has a very high effective resistance -- 10s of megohms is not unreasonable. Ditto for the control circuitry in the CFL. An incandescent bulb is a simple piece of resistive wire; a 60 W / 120 V bulb will have a resistance of 240 ohms.

Now, suppose the dimmer has a resistance of 50 MOhms and the CFL has a resistance of 10 MOhms; plugging the numbers into the equation above gives you 20 V across the bulb. OTOH, the voltage across a 60 W incandescent bulb will be about 600 microVolts, nowhere near enough to make the bulb glow.

If you have two bulbs in the light fixture, the resistance, R, of the two in parallel is given by:

R = R1*R2/(R1 + R2)

So if you have a CFL and an incandescent installed, the effective resistance is going to be very close to that of the incandescent alone:

R = 10,000,000 * 240 / (10,000,000 + 240) = 239.99 Ohms

Again, not enough to turn on either bulb.

With two incandescent bulbs, the effective resistance is half that of a single incandescent, so you have half the voltage across them.

The flickering you see with two CFLs is because the light you see is basically a high-voltage spark through the tube. The CFL contains circuitry to amplify the incoming voltage up to the point where the spark can occur. Under normal circumstances, the input voltage is enough to cause this spark 100 or 120 times per second (depending on mains frequency), which is far too frequent for the human eye to notice. With the reduced input voltage, it takes longer to reach the required voltage, so you notice the flicker. No two bulbs will be exactly identical, so they'll flicker at different rates and take different times to recover between discharges.

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+1 for the diagram and EE explanation. I really should have taken some of those courses in college. –  BMitch Jul 29 '11 at 1:10
    
That's quite the explanation. So if my basic understanding understands you well, the light's "on/off" switch is essentially just increases the resistance, but doesn't completely prevent the power from flowing, so if I were to get our maintenance guy to try to implement some hack to bypass the resistor, it would stay on all the time? –  Mike Jul 29 '11 at 2:51
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Hack and electrical are two words that don't mix well. But you're correct: if you bypass the switch, the light will be on all the time. If the remote control unit isn't completely buried inside the ceiling fan, you can also find replacements that just have an on-off for the lights and speed control for the fan. –  Niall C. Jul 29 '11 at 4:30
    
Great answer. love the math, haven't seen that since college. That is why the packaging on CFL's says very distinctly, "Do Not Use With Incandescent Dimmers" –  shirlock homes Jul 29 '11 at 23:52
    
Vaguely modern fluorescent lights will be running much faster than 100/120 Hz (they convert to DC and then back to high-frequency AC). I guess the flashes are from some kind of static build up. –  Tom Hawtin - tackline Feb 13 '13 at 3:47
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You can try using dimmable CFL bulbs to see if that works any better. But be picky about the brand since user reviews are all over the place on these.

That said, since your fan still passes some power even when turned off, it sounds like they are designed for non-cfl bulbs, which would pass this power through completely unnoticed. I've heard of this with devices that are wired to be always on (e.g. motion sensing lights), though the design never made sense to me since the device should be able to draw power separately without passing it through the bulb. When you have a normal bulb installed, the power likely drops to 0v since the bulb is now the most efficient path for electricity.

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