Can I dim a switching power supply?

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Can I dim a switching power supply?

Post by ian » Wed Oct 24, 2007 6:40 am

O.K. I know you're laughing. Now stop and think about it.
Specifically I was asked by a customer that has a dimming control
if they could use it to control a LED lamp. The LED lamp has a switching
power supply.
Are they completely incompatable?
Or is it even possible the dimmer will shut off the switcher when it
chops the sine wave resulting in dimming?
Or is this just a bad idea that won't work?


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Post by haklesup » Wed Oct 24, 2007 10:28 am

Many switching supplies have a trimmer adjustment for output voltage but it wouldn't have the range to act as a dimmer. They usually just tweek the output by a few volts and are often hard wired but you can find it by looking at the silkscreen on the PCB.

You should just use the power supply to power a dimming circuit for the LED. I've noticed many new ICs for dimming LED and other related lighting control.

Maybe I misunderstood, They want to use an AC incandescent lamp dimmer to control input voltage to the switcher. If it worked at all, the switcher would probably compensate and maintain output voltage until it was too low and then shut down. Ultimately if it runs at all depends on the switcher design but I do not think you would ever get a variable DC voltage (maybe variable wattage)

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Post by dacflyer » Thu Oct 25, 2007 5:34 am

a dimmer would work fine if it wasn't a switcher type of supply,,but all in all..sounds like your out of luck..:(

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Post by ecerfoglio » Thu Oct 25, 2007 6:50 am

Some "electronic transformers" made for low voltage incandescent lamps (ie 12 V 50 W ) are dimmable (not all of them)

An "electronic transformer" is just another name for a switching power supply :grin:

If the supply has a fixed duty cycle oscilator it should work,

BUT if there is any feedback to regulate the output voltage it will ... :sad: regulate it :sad: ... and will not dimm the light
E. Cerfoglio
Buenos Aires

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Post by Joseph » Thu Oct 25, 2007 6:59 am

Your best approach in that that type of case would be to have a switching supply which can be regulated by variation of the duty cycle it feeds to its transformer. In that case, you would have to adjust the duty cycle.

Alternately, you could use a transformer circuit designed to have adjustable power output at 60hz. I have some small transformers which I can use for this type of project, and actually plan to, just to power LEDs for lighting. If I find pre-made dimmers at a good price, I may go with it being dimable. Otherwise, I plan to use some diacs and triacs I have already and probably omit the adjustable potentiometer.

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Post by ian » Sat Oct 27, 2007 2:54 pm

You know, I'm amazed by this. I've been working on electronic long enough to know you
can never be sure of ANYTHING. But I was ABSOLUTELY SURE you couldn't dim a switching power
supply, but because of my experience in NEVER being sure of anything when it comes to electronics
I threw it out here for comments. From the comments here, and more research on the web
I've found that indeed, some switching power supplies CAN be dimmed and Lutron actually
sells a line of dimmers specifically for "electronic transformers".
I guess this is why we make the BIG BUCKS.


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