led driving problem..

[dokuyucu]

hi all,

in my circuit i need to drive and switch 100 (if it is not enough, may be 1000)led. and i need 20-30 mA for each led. i think that i can drive them serial, parallel or serial-parallel mixed with power transistors. and my driving signal is PWM signal. because of this, i must not only drive the leds but also switch them. but i ever design such a circuit before. so i need help about led driving circuit.

thanks for sharing your time..

Best Regards,

Oguz Dokuyucu
od@postaci.com

 

[buzzp]

Do you have to control them seperately or are they all switched at the same time? Each LED will drop around 1.5V (check your data sheet for Vf). If they are hooked in series then you need 1.5 * #LEDs= voltage supply. For 100 this would be 150 volts for 1000 this would be 1500V. I have never used PWM with LEDs but if you are the amplitude of the pulse will be higher than 150 or 1500 Volts. The only practical way to do this is hook them in parallel with current limiting resistors in series with each LED. The voltage will basically be the forward voltage drop of one LED plus the voltage drop of one current limiting resistor. The supply should be capable of sourcing the current of all 150 LEDs. So if the current you pick is 25mA then for 100 LEDs you need to supply 2.5 A for 1000 LEDs you need to source 25A. This number will change some since your using PWM. You may not need transistors depending on what your using to drive the LED's (micros can generally source 25mA) so if all are on at once, transistor is in order, if seperately you better mux them or get a micro with mucho I/O available, not sure they have any with 100 I/O so a mux would be in order.
I remember a similar question in here before so try a search.

 

[sdmays]

From what voltage will you be supplying current to these LED's? Dictates what you do regarding driving the LED's.

What is the forward voltage spec of your LED's? Also dictates what you do regarding driving your LED's.

At what frequency were you planning to switch your LED's on and off?

What kind of power transistor were you planning to use: MOSFET, BJT, etc...? You might find a number of ordinary BJT's is sufficient, depending on the current for which each transistor is responsible.

Do you have access to an oscilloscope? An oscilloscope is always handy for determining what is REALLY going on.

 

[dokuyucu]

hi.

i can use three different powerfull switching equipment. These are TIP122 NPN Darlington Transistor(100V 5A), IRF830 TMOS Power FET(500V 4.5A) and TIP3055 Power Transistor(100V 15A). i will connect 10 led serially. and then connect 10 parallel group (each group exist 10 serial led). what kind of circiut i can use to switch transistor. i think that i drive darlington tr. at first and then fet with this. i am waiting for your advices..

Oguz Dokuyucu
od@postaci.com