On Wed, Feb 22, 2012 at 9:22 AM, Bob Ammerman wrote= : > > From: "PICdude" > > So I have a current sensor with it's output going to a PIC. I'd like > > the PIC to control a "short" across the current-sense resistor and > > wondering what's the best way to do this. In this case, best is > > most-effective (lowest short resistance), though small size and cost > > is always a factor. > > > > The signal being current-sensed is bi-directional, up to 10mA, and the > > common-mode voltage is in the 24V range. The PIC will be powered from > > 5V. I'm confused about using a single FET to do this due to the > > bi-directional signal, and thinking I could use an electronic switch. > > What is the value of your sense resistor. What shorting resistance are yo= u > happy with? > > An FET will indeed pass current in both directions when turned on. > Unfortunately it will pass current in one direction even when off! (due t= o > the intrinsic body diode of the device). You can wire two FETs in series = to > block in both directions when off. Your problem may be getting it them > turned on. Do you have any voltage in the system at least several volts > greater than the high side of the sense resistor? You would need to have > (or > create) such a voltage to turn on the FETs. > > -- Bob Ammerman > RAm Systems > > If you choose back to back FETs, take a look at the TLP190 or it's brother TLP191. They're photo-voltaic opto-isolators and can easily drive a FET above system voltage. They do have some drawbacks, most significantly, they don't produce much current, so depending on the gate capacitance of your FET, they can be fairly slow - a few mS switching time is common. But they do solve some sticky problems... -Denny --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .