LED Dimmer - What should be used for Switching?

I have been asked by someone to provide an LED Dimming circuit. Currently there are 21 LEDs @ 1Watt (12V) each. The LEDs currently are just being switched on or off. Since I have not done anything like this at such wattage before, I would like to request some help in deciding how to drive them.

I am considering driving it using 16F877A. I will use HPWM output for this, but I would like to ask

1. What frequency should be used?
2. What should I use to drive the LEDs, a powerful Mosfet or something else is there in the market for this purpose?

I will keep updating my questions here as I will go along. If mosfets can be used then I would like to ask which one is recommended. Searching the internet I found some mosfets which are capable of 60Amp to 80Amp. Will these be a wise choice? Will heatsink be required?
Thanks

This link might give you some ideas.
http://www.opensourcepartners.nl/~costar/leddimmer/

If it will be for one LED and low current, you may get away with a MosFet or similar.

But for 21 LEDs, I think you should consider the power loss in heat.

I am currently experimening with devices from ZMDI that are sold by MOUSER.

I test the boost current regulator ZSLS7025 that I need to power 25 watt LED at 720mA and about 36 volt from a 12-24 volt dc input and with Dimming capability.

This company makes such devices either for boost or buck. Have a look at this link:

http://www.zmdi.com/products/led-drivers/

Ioannis

If you put them in series, it will make 21 LEDs x 12V = 252Volts, correct?

If yes, and you are in Euro zone with 220VAC line, then convert 220VAC to DC, and drive them with a single mosfet only with 12F683 pwm output dimmed.

You will get some heat due to voltage difference but "I think" it can be taken away from the mosfet with a small size cooler.

220x1,4=308Volt

308-252=56Volt

56volt*0.084mA=5Watts. This must be dissipated not on the mosfet only but on a resistor too. Otherwise LEDs may get overloaded.

Not much, but not efficient either.

Also there will be live part if this is of concern.

Ioannis

Quote:

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Originally Posted by Ioannis

220x1,4=308Volt

308-252=56Volt

56volt*0.084mA=5Watts. This must be dissipated not on the mosfet only but on a resistor too. Otherwise LEDs may get overloaded.

Not much, but not efficient either.

Also there will be live part if this is of concern.

Ioannis

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220VAC * 1.27 = 279.4 Volt DC

279-252 = 27

27 * 0.084 = 2.26 watts

not bad with your approach.

Sayzer, what is the 1.27?

Ioannis

I know what it is but not sure if I can explain it correctly.


May be, Alain can give a hand here and explain it in technical and understandable terms.

You considered 308VDC using full bridge rectifier (and elec-cap), right?
Have you actually measured it?

http://en.wikipedia.org/wiki/Root_mean_square

So, I insist on sqr(2), meaning x1,41 of the rms voltage.

Ioannis

ok.

First, we multiply AC voltage by 0.9.
then, we multiply the result by 1.41.

There, you have the result for DC voltage.

In our case here,
we muliply 220VAC by 0.9 = 198
then we multiply 198 by 1.41 = 279VDC.

This calculation is for full bridge rectifier with an elec-cap.

Quote:

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Originally Posted by sayzer

First, we multiply AC voltage by 0.9.

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Why?? .....