> -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of Bob Ammerman > Sent: 22 February 2012 16:23 > To: Microcontroller discussion list - Public. > Subject: Re: [EE] Bipolar, FET or ??? for shorting-switch? >=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. > > > > 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 yo= u > 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 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. Could he not use P channel MOSFETs? You can get some very low RDS(on) devi= ces these days. Regards Mike =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .