FLOATING 5 Amp Current Limit Circuit needs heatsink 60 Watt HIGH POWER 3 W minimum NPN .12 ohm c e V+ IN >----*-------+ +-----*---/\/\----*----> OUT | Q1 \_/ | Q2 / e | | +----------| NPN +-/\/\---* b b \ R1 | | c +-------------------+ This is a simple current limit circuit. Values shown will work for 5 Amp limiting. Requires one high power NPN transistor as the series pass element, and a medium power transistor that can handle at least 100 milliamps. Q1 is initially held ON via R1. When the current approaches 5 Amps the voltage dropped across the .12 ohm resistor will approach .6 volts, which is the approximate Vbe of Q2 to turn it on. As Q2 turns on, current flows through it's collector, stealing current from Q1's base. This feedback action limits the maximum curent OUT to approximately 5 Amps. The value of R1 depends on the gain of Q1. If Q1 is a darlington then you can use something on the order of 1k. If the gain of Q1 is too low, then R1 must supply more current to the base, and then Q2 has to be able to "steal" any excess current through R1. You may have to experiment a little to find the best value for R1. Fr. Thomas McGahee Technology Coordinator Archbishop Shaw High School Marrero, LA 70072 tom_mcgahee@mailsnare.com (tom_mcgahee@yahoo.com) ----- Original Message ----- From: "James Burkart" To: "Microcontroller discussion list - Public." Sent: Tuesday, March 08, 2005 4:07 AM Subject: [EE] Best method of current-limiting a PWM circuit? > Hello all, > > I'm driving a logic-level N-EMOS with the PWM output of an F877a at 20kHz > to controll a mechanical vibrator. I want to limit the current the load > can draw to 5A and I can't think of a good way of doing it other than a > big-fat resistor in series. One complication is that the coil resistance > can vary. Can anybody point me in the right direction? > > I was also thinking of using a current-sense resistor and feeding the drop > into one of the A/Ds via an op-amp but I'm sure the 877a can't respond > fast enough. > > James > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist