Could we see your schematic ? At least current measuring part and relevant= voltages ??? gus > On Feb 22, 2012, at 10:12 AM, Denny Esterline wrote: >=20 > On Wed, Feb 22, 2012 at 9:22 AM, Bob Ammerman wro= te: >=20 >>=20 >> 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. >>>=20 >>> 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. >>=20 >> What is the value of your sense resistor. What shorting resistance are y= ou >> happy with? >>=20 >> An FET will indeed pass current in both directions when turned on. >> Unfortunately it will pass current in one direction even when off! (due = to >> 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. >>=20 >> -- Bob Ammerman >> RAm Systems >>=20 >>=20 > If you choose back to back FETs, take a look at the TLP190 or it's brothe= r > TLP191. They're photo-voltaic opto-isolators and can easily drive a FET > above system voltage. They do have some drawbacks, most significantly, th= ey > 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 the= y > do solve some sticky problems... >=20 > -Denny Could we see your schematic ? At least current measuring part and relevant= voltages ??? gus --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .