At 01:48 PM 7/10/02 -0400, you wrote: >Hi all. I'm looking to do some PWM on some high power LEDs. PICwise, >everything is working great. Now comes to the EE part, where I have much >less skill. I have to power 5 LEDs with a current of about 20mA each. >It's easier for me to provide higher current rather than higher voltage, >so I was thinking about paralleling them. But, I'd like to only have one >current limiting resistor for the string of 5. Whether you can get away with this depends on how close to the limit you are and how variable the LEDs are, and what their internal resistance is. Unless you've got very good data that indicates this is an okay way to go, I'd suggest adding the extra 4 resistors. This will make the current distribution much more even between the LEDs. Also, if one goes short the others will continue to work as before with only a slight 20% reduction in power (which you'll get sooner or later anyway due to aging). >According to my >calculations, it worked out to something like 40 ohms. I can't remember >the exact equation I used, I don't have the paper in front of me. You know the current, so look up the typical Vf of the LEDs at 20mA. Then subtract the power supply voltage minus the driver drop. You can use a 2N4401 with a 10mA base drive, for example, which will typically have only about 100mV drop with forced beta=10. Then you have the voltage dropped across the resistor(s). Say 5V - 100mV - 1700mV = 3.2V. Use Georg Simon Ohm's law to calculate the resistance R = E/I = 160 Ohms for 5 resistor, 32 Ohms for one. Then go back and check worst case conditions (high/low) to make sure your circuit will be reliable. http://www.fairchildsemi.com/ds/2N/2N4401.pdf >However, if I want to PWM 100mA of LEDs what would be a good value to >use? Also, what would be a good transistor to use? I was thinking about >a TIP31 just off the top of my head, but I have a feeling it's a bit of >overill. I'm PWM'ing at about 20kHz. You don't need anything like a TIP31. Not that it matters much in this application, but the switching will also be more sluggish on a part like that. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu