that ohm guy gets in my way alot ;-) How about a voltage divider? Or would that affect the voltage of the entire circuit? Also, my load will fluctuate alot on both the 40v and 12v circuit. -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Joe Farr Sent: Wednesday, May 15, 2002 12:42 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE] Power supply ignorance Not sure you can use a resistor because of ohms law. V=I/R Since the current drawn isn't constant, the voltage won't be either. I suppose that's why we have voltage regulators :) I have a PSU that does something similar to your requirements. To drop the 40v to 15v (which is used as input to the 12v regulator) I use a resistor and zenner diode between 40v + and - rails. Their junction connects to the base of a 2N3055. Collector goes to the 40v and emitter gives the 15v ish... Depending on the current you need, you can vary the transistor and mount it on a heat sink. -----Original Message----- From: Rex Byrns [mailto:rexb@TESTENGEER.COM] Sent: 15 May 2002 18:31 To: PICLIST@MITVMA.MIT.EDU Subject: [EE] Power supply ignorance I have a problem that has got to be a simple fix, and I am already feeling stupid. I am trying to build power supply that will give me 40v, 12v, and 5v. I have the 40v part working fine. My 12v regulator will only take 35v max, so I got a resistor, put it between the +40 output and my meter, and it shows about 30v. I installed the regulator with this same resistor btween the input and the +40v. Ground to the -40v. It does not work. The meter only shows .638 volts at the input of the regulator. Output is 0. I planned to then put a 5v regulator off of the 12 volts. Do I need to put the resistor at ground instead of + ? Maybe the regulator is bad? Thanks -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body