LED Dimmer - What should be used for Switching?

[FromTheCockpit]

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

What frequency should be used?
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

[dhouston]

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

[Ioannis]

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

[sayzer]

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.

[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

[sayzer]

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

220VAC * 1.27 = 279.4 Volt DC

279-252 = 27

27 * 0.084 = 2.26 watts

not bad with your approach.

[Ioannis]

Sayzer, what is the 1.27?

Ioannis

[sayzer]

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?

[mackrackit]

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

[Ioannis]

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

Ioannis

[sayzer]

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.

[rmteo]

First, we multiply AC voltage by 0.9.


Why?? .....