Using a Light Dimmer on LEDs through an AC Adapter

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Imager
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Using a Light Dimmer on LEDs through an AC Adapter

Post by Imager » Wed Nov 29, 2006 11:59 am

Wondering if it is possible to dim LEDs which are currently being powered through an AC adapter (to provide 12v DC) from 110v?

I have heard that you require a specific type of dimmer control for devices which have a transformer in them?

Any feedback would be appreciated.

Thanks...

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haklesup
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Post by haklesup » Wed Nov 29, 2006 2:40 pm

You must be thinking of a Variac autotransformer which is actually an adjustable (with a knob) transformer (variac is actually a brand name). These are much heavier and more expensive than a lamp dimmer.

A normal lamp dimmer does not reduce the voltage at the output, it simply chops a piece of the sine wave off thereby reducing the power available to the load. While this will probably result in some dimming of a LED powered by a transformer and recitfier, it would be a narrow range with a sharp dropoff.

I think a fan/motor controller (which look just like dimmers and are generally sold in the same stores) would be a better option if you insist on going that way. I couldn't easily find a reference which shows the difference (between lamp dimmers and cieling fan speed controllers), maybe someone else knows where to find that.

However, I would (and I think many here would) not use a variable control on the AC voltage but instead use a variable output DC control. This can be done with a 3 terminal voltage regulator IC or a simple voltage divider if you don't mind wasting a little power. In either case, you would have variable power to the LED from a potentiometer knob.

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Chris Smith
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Post by Chris Smith » Wed Nov 29, 2006 3:02 pm

Yes it works but not with all brand dimmers.

Some require a certain load just to start, others wont go below the 50% voltage mark.

A variac is the expensive way to go but with some brand "trial and error" you should be able to dim the leds.

Also the transformer can matter if its too small on some models.

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dacflyer
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Post by dacflyer » Wed Nov 29, 2006 4:13 pm

its leds,,,you should be able to use a rheostat ? or ?

the leds are in series i am sure.

or better yet.. i think you said they was operating off of 12 volts.
why not goto a junk yard and get one of them dimmer moduals for dash lights.
i have used them here a lot..just 3 wire connection typically..
pos , neg , output.. try it.. i am sure you can get one for little or nothing. and they handle a lot of current

good luck

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jollyrgr
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Post by jollyrgr » Wed Nov 29, 2006 4:34 pm

Try using a ceiling fan speed control type dimmer. Or a dimmer made for transformer based track lighting. They make these and this should work for you.
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Robert Reed
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Post by Robert Reed » Wed Nov 29, 2006 5:14 pm

Jolly

"Try using a ceiling fan speed control type dimmer. Or a dimmer made for transformer based track lighting. They make these and this should work for you."

Not sure if these would work. I added 3 fans with speed controls last summer. Naturally, my curiosity got the better of me and I had to open them up. They used large value series capacitors (3 - 10 MFD) in series with the line, one value for each speed step, for control. Seemed weird but it works OK for the fans.

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Chris Smith
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Post by Chris Smith » Wed Nov 29, 2006 6:57 pm

Light controllers using "ZERO CROSSOVER" are the types that work.
Others are questionable.

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MrAl
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Post by MrAl » Thu Nov 30, 2006 2:21 am

Hi there,

If i were doing it i would definitely go with dimming on the 12v side.
If the LEDs are the small 20ma type you may get away with a pot.
If not, use the pot to adjust the voltage regulator someone already
mentioned. Simple and safe too.
LEDs vs Bulbs, LEDs are winning.

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dacflyer
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Post by dacflyer » Thu Nov 30, 2006 2:05 pm

i still like the dashboard light controller, its small and easy to use.and handles a lot of current.. not that you'd have that problem.
and you can get one at junk yard for little or nothing...

let us know what you decide.

Imager
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Post by Imager » Fri Dec 01, 2006 6:04 am

The feedback looks great.

To clarify a bit further. What I am trying to control is a LED lighting solution with an integrated power supply (the AC adapter) from an existing wall switch controlling an existing wall receptacle - and dimming at the device is really not an option.

Looks like a good quality dimmer (speed controller?) for a ceiling fan might be the solution I need - although the cost may end up being more than the purchase price of the lamp!

Thanks..

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dacflyer
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Post by dacflyer » Fri Dec 01, 2006 6:15 am

the reason i mentioned the auto dashlight dimmer was because you said you wanted to control the leds from the 12 side of your supply,

a dimmer / speed controller will not work on the 12v side. only from the 120v side.

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Chris Smith
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Post by Chris Smith » Fri Dec 01, 2006 8:28 am

You can find cheap dimmers, not just specific fan speed controllers. I purchase them whole sale at times for 2 bucks apiece and use them in the shop, fan controlling, and dozens of other uses.

Some don’t go below the 50% level but they are still good for other uses.

I have a cheap dimmer on the ceiling fan right now that has lasted for more than fours years now with no noise or hum and several in the shop for power supply controllers.

I also take over heated ones and replace and step up the SCR to handle more that the 600 watt rating. Add in a cooling fin and they work for ever.

Don’t start expensive.

Ill look at the brand that controls my fan but Im sure all of them are made in china any way.

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dacflyer
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Post by dacflyer » Fri Dec 01, 2006 1:50 pm

hey chris,,there is a difference in the dimmers and fan speed controllers.
they both look alike and wire up the same way ( 2 wires in series)
but i have found that the dimmers do not work so good with moter driven stuff. theres like flat spots in the operation..and in some instances they will make the motor humm excessively.

just my 2 cents worth.

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Chris Smith
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Post by Chris Smith » Fri Dec 01, 2006 1:59 pm

Dac

I know, those are the cheap or bad types.

I have one right now on the motor fan with no [extreamly low] hum, buzz, or flat spots.

Power Controller Corporation
San Antonio Texas
103-763-3 ... 192 or 19S

I cant hear the motor unless I turn everything off in the room, and then its barely audible at low speeds. More like strain than buzz.

And It will go all the way to shut off and all the way back to full speed.

A few years back I picked up a lot of them from All Electronics and they are the better type.

Not all are equal, this much I have learned.

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dacflyer
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Post by dacflyer » Sat Dec 02, 2006 3:30 pm

oh, ok,, ;)

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