Quoting Bob Ammerman : > What is the value of your sense resistor. What shorting resistance are yo= u > happy with? Sense resistor will be in the order of 50 ohms. Official spec for =20 shorting resistance is "as little as possible" :) The idea is to =20 emulate a shorting block, but PIC controlled. Of the top of my head =20 I'd say a few milliohms would be nice. Alternatively, adding a couple =20 inches of wire (28AWG) is fine, so calculating from that, even 10 mOhm =20 should be fine. I'm sure I can get FET's with Rds-On in the order of 1-2 mOhms or =20 less, so we're well within range here. > 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. Two in series is fine. A quick search at Digikey shows me a lowest =20 RdsOn of ~2 mOhm for a P-Channel FET, which is still fine. > 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. I should, but I think I'll need a 3rd transistor (perhaps an NPN) in =20 there to switch the P-Channel FET on/off. So assuming all of the above, how "open" would this be when off? =20 Would there still be any resistance when open? I'm not familiar with =20 how Rds-Off works here. Also wondering, looking at prices of some of these very-low-Rds-On =20 FETs, I'm wondering if I'm better off with the reed relays, and =20 cushioning them to minimize any effects of vibration. Cheers, -Neil. > > -- Bob Ammerman > RAm Systems > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .