At 01:04 PM 4/5/01 -0500, Eisermann, Phil [Ridg/CO] wrote: > The resistor *is* your current to voltage converter. Not sure how >you're controlling the motor (PWM? FET/BJT? simple on/off?), but if one side >of the resistor is tied to ground, you can use an op-amp (non-inverting >configuration) to scale the small voltage drop across the resistor into 0-5V >for A/D conversion. Beware, though. The motor current will have a lot of >noise. Some low-pass filtering before the A/D wouldn't hurt. > > I like the SenseFET for this application. International Rectifier >IRCZ44 comes to mind, although you might not get much accuracy because >that's a 50A device. It's a 5 pin MOSFET. The extra two pins are an >integrated 'current tap' This current tap reflects a fraction of the >drain-source current. You convert this scaled down current with a resistor. >The advantage is that you don't have the current-sensing resistor wasting >power. You only have the on-state resistance of the FET to worry about. For >the IRZC44, that's 0.028 Ohm. Only drawback is that since the motor is 2.5A >and the FET is 50A, you might not get much accuracy because the load is such >a small fraction. You'd need a large gain on the op-amp, and possibly >correlate some actual current readings with output to get good results. Or >you could use a different version of this FET (one with less range). DigiKey >now carries these types of FET's, so you might start there to see what you >can easily get. Exactly right! So use the IRCZ24 (much lower current rating). We use a lot of them . . . The exact on resistance is not critical since it is a current ratio arrangement (assuming you use IR's recommended amplifier circuit which, unfortunately, requires a negative supply). dwayne Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 17 years of Engineering Innovation (1984 - 2001) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu