I'd attack it as follows. a) Have a very careful look for a ratings plate or part number. b) make a very rough guess as to the power of the motor based on size & by comparing it with other (similar if possible) motors of known power. c) measure the DC resistance. Multiply this by 10 for an estimate of running load. Work out voltage from this & power estimated above. (eg if power is 20W and DC resistance is 2 ohms, then the running "resistance" is estimated at 20 ohms, the current will be 1A for 20W and the voltage 20V. d) connect to a power supply and wind the voltage up slowly until reasonable rpm is acheived. Note: if motor is series wound you may be advisedt to fit a fan or other load to it before winding the volts too high. e) run for a while at your best bet voltage & check temperature rise is not excessive. Just a suggestion though - try at your own risk :-). Richard P "Lindy Mayfield" r.sas.com> cc: Sent by: Subject: [EE] Finding out voltage of a DC motor piclist-bounces@mi t.edu 15/04/05 11:09 Please respond to "Microcontroller discussion list - Public." How can I find out the proper voltage of unknown DC motors, like ones that I've scavenged from somewhere? I tried connecting the wires to my multimeter that was set to AC and spinning the shaft, but I didn't get far with that. (-: If I run one with the wrong voltage I can burn it up, right? Thanks! Lindy -- 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