Why aren't many / most LED light bulbs dimmable?

[clare]

I refuse to argue with an idiot

[clare]

The voltage drop across a diode is relatively constant, so the voltage across the resistor has to also be relatively constant, with only the current being changed by the change of resistance. This is not 100% accurate, but for this discussion I believe it is close enough.

Not agueing with krw - just agreeing (to a point) with gfretwell. I say "to a point" because it is not totally linear. Much more linear than some would have you believe in a DC circuit.

[krw]

As usual, when you're shown to be wrong you run away with fingers in your ears rather than admitting it and learning something. You *are* an idiot.

[gfretwell]

I guess the point is LEDs are so efficient to start with and dimming them will cut the draw even more, why make things hard on yourself to squeeze out an extra few pennies a year.

If you are replacing an incandescent any thing you do has to be better.

[krw]

It is *NOT* "relatively constant" when you multiply the change times the number of LEDs in a 120V string and compare that to the voltage across the balast resistor. If you string a few together and have a large balast resistor it matters less but you're simply wasting that much more power, losing gains you made by using LEDs in the forst place.

Of course not. You could *NEVER* admit that you're wrong.

Of course you would be WRONG, as usual.

[krw]

I could say the same thing about replacing incandescents in the first place. Why, for a few mennies a year. In fact, that's what I do.

Not necessarily true. You still haven't come up with a good scheme to dim them.

[gfretwell]

OK Here you go

These are garden variety indicator LEDs (what I had handy) There is a 270 ohm in series with 4 LEDs and a 1k pot wired as a rheostat. With the pot set to 0 ohms get a tad over 15M/A and the LEDs are as bright as you can expect from these, may even be overdriven. I al dropping 4.11v over the 270 ohm resistor. Turning the pot down toward 1k ohms dims the LEDs quite smoothly At 1K ohms plus the 270 I get 3.7 MA or so and I am dropping 4.8 volts across the resistors.

I agree it is not exactly linear but I also do not understand how this small variance makes any real world difference since most of the voltage is still being dropped across the LED string. You certainly would have a hard time seeing it on your electric bill.

[G. Morgan]

How does the voltage change across a fixed circuit? I think everyone here is talking about an Xmas tree lighting situation (now), in a series arrangement.

[gfretwell]

The difference between an LED and an incandescent is far more than pennies and when you dim the LED you are still reducing the power you use. Garden variety triac dimmers are not that efficient either but we still say they save money. Have you ever looked at the lumens per applied power number on a dimmed incandescent? They are horrible. Bear in mind they are still drawing power when they are virtually too dim to see.

The application I am making right now will be low voltage so my little experiment is really a prototype. I will just need to make the necessary adjustments for the high intensity LEDs I bought.

[Art Todesco]

Yes the string (I assume your are talking about a Christmas string) is a rectifier, but it is only a 1/2 wave rectifier. Adding a full wave rectifier, as others (and me too) have done, makes the string brighter and eliminates much of the annoying blinking. Yes it still blinks, but full wave rectification makes it a whole lot better.

[Ralph Mowery]

The circuit is not fixed. The conversation is that you change the resistor value to change the brightness of the LEDs. There is very little change in the voltage across the LEDs, but the big change is the voltage drop across the series resistor as the value of it is changed to change the current that changes the brightness of the LEDs.

[mike]

There are a bunch of issues. Yes, LED's are VERY dimmable. The questions are: Can you build a reliable, affordable, LED lamp that works with the EXISTING dimmer mounted on your wall. Can you build an affordable dimmer that works with CFL, LED and incandescent lamps. If you're buying a dimmer to dim a 20W LED, will you pay for the extra cost to dim a 300W incandescent load. Not much of a premium for old-school dimmers. For one that works with LED, the premium can be significant. Vendors don't want to warranty replace a dimmer you blew up by plugging in an incandescent.

And dimming is not the only problem. Lots of the X10 home automation stuff uses SCR circuits that are not CFL or LED friendly. Ditto for motion sensor lights...but that's for another thread.

Everybody reading this probably feels comfortable opening the box on the wall and changing the dimmer. But Suzie homemaker may be faced with this option. A)Go buy a 57W incandescent for 25-cents on sale and screw it in. B)Go buy a $40 LED lamp and a $40 dimmer and PAY somebody to install it. The stuff doesn't exist (at reasonable prices) because the demand isn't there. The demand isn't there because the cost is too high because the demand isn't there and one might argue that the technology isn't there either.

Basing your decision on the STATED lifetime numbers is ill-advised. My budget can survive a failed 25-cent incandescent. I'm much more sensitive to a failed $40 LED whatever the failure mode. I have NEVER had a CFL fail because the bulb quit. I've had many fail because the electronics exploded. Don't think I ever got 8000 hours out of one.

The dimmer on your wall doesn't work because it requires a load to charge the cap that provides the phase shift that delays the turn-on and dims the incandescent. Sure you can design one better, but

1) That one ain't on your wall 2) It will initially cost so much that you won't put one on your wall.

As for the LED lamps. You've got a 160V peak or twice that depending on where you live coming out of the wire. Your LED is 3V or so. It was mentioned that you can stack them up to 95% of the volts. Not so. There are voltage variations between LEDs and temperature variations. When you stack a bunch of them up, that variation can figure significantly into the means you use to limit current.

Putting 40 of them in series drops the reliability by a factor of 40 or so. Yes, I expect some statistician will take issue, but the factor ain't small.

If you use a resistor to limit the current with only small headroom, the current becomes very voltage sensitive. You don't want your lights to dim every time the line voltage sags a volt. And there's also the reliability issue. You don't want your bulbs to burn out every time your air conditioner compressor kicks off and spikes the line. And you don't have room or $$ for a line filter in each bulb.

Another way to limit current is to use a series capacitor. It's much more efficient, but is critically dependent on having a sinewave input. Line transients can pop your lamps. And the dimmer on your wall puts a HUGE transient into the lamp every half cycle. And the caps aren't cheap.

Then there are the MUCH MORE EXPENSIVE active ways to manage the current.

Unless you're designing a new lighting system, you don't have control of both ends of the system. For currently installed systems, Suzie homemaker is gonna go to home depot and buy whatever is on sale and works with what's currently installed.

At today's prices, the only rational compromise is CFL and stick with incandescent where you absolutely must have dimming. YMMV

As an experiment, I did buy some of the lights-of-america 1.5W 20-led lights for cheap at a garage sale. Make great night lights, but after a year, they're getting dimmer. Wish I'd measured the initial light output. And they waste over half of that 1.5W in the dropping resistor. So much for efficiency.

[AZ Nomad]

Standards light dimmers use PWM, not variable voltage. They should work great with LEDs if not for the fact that LED's have a power supply that converts AC power to low voltage DC and which will result in the same low voltage DC based on the peak voltage in, the same for

1% as for 100%.

[AZ Nomad]

bullshit. LEDs have a voltage drop of 1.7V. Silicon diodes by the way have a voltage drop of 0.7V.

Put a meter across a lit LED some time if you don't believe me.

[krw]

He's talking about dimming LEDs, as in under-cabinet lighting.

[Jim Yanik]

" [e-mail]" wrote in news: [e-mail]:

the high power Cree XR-E LEDs I used for my homemade bike light are spec'd at 228 lumens at 1 amp,but emit half that(~114 lumens) at only .35amps. So,they are more efficient at the lower current.

[krw]

Sure they're efficient. They have a fairly well fixed 1.4V across them, times the duty cycle of the brightness setting. At half-power that's well less than a watt. Your rheostat will easily produce that much heat.

[krw]

Now, scale that to your 120V application.

It will *not* be a small variance when you add another 30 or so in series and a smaller ballast resistor. Add in the sine wave excitation and you're going to find your results don't match your experiment very well.

[Jim Yanik]

[e-mail] wrote in news: [e-mail]:

E = IxR Ohm's Law. it's simple math. if you provide less current to the LED's because the resistor is a higher value,the voltage dropped across the resistor INCREASES.(and the power it dissipates) the resistor converts the additional power to heat.

[krw]

The manufacturers could put an anti-parallel LED in the same package with the same effect. The power dissipation is the same (application dependent) so the package wouldn't need to be changed, just another junction on the same die.