At 02:14 AM 3/8/2012, Forrest Christian wrote: >I have an application where I'm considering just using the voltage drop >produced by Rds(on) across a FQB22P10 fet in the place of a current >sense resistor. This is one of those applications where the exact >current draw is not really needed, but instead it is more being used as >a 'load present' detection. The load is actually around 0.5A - which >based on the just under 0.1 Ohm Rds(on) resistance, I'm right where I >need to be for my 20V/V current sense amplifier.... > >Whenever I consider something like this, I worry that I've missed some >critical piece of 'semiconductor physics' which would make this not work. > >Is there a reason I shouldn't do this, or why I wouldn't get at least >somewhat accurate results? I've been doing this for decades now - with reasonably good=20 results. RDSon is reasonably stable with temperature and the major=20 voltage variation I get with any given amount of current appears to=20 be device-to-device variation. I use a couple of different methods of dealing with the off-state=20 variation: one method is to use a small MOSFET driven by the same=20 gate signal that drives the main FET - that small FET is in series=20 with the current-sense signal. Another method uses the large OFF=20 voltage to drive a small bipolar transistor to clamp the=20 current-sense signal - I think that I first saw Bob Blick do that=20 many years ago in one of his projects. You should have no problem doing what you need to do this way. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .