Thanks Russell > Bob's solution would be the easiest if it were not for the high current > involved. > Most devices of the type suggested are rated at an amp or less. > IF there are devices rated at 10's of amps anyone please advise - always > nice to know about. > > Some Digikey offerings > > max 1.1A. > > About 25 of these in parallel would probably work with a little design ca= re > and cost about $25 for the ICS BUT would "not be a preferred solution". > > http://www.diodes.com/_files/datasheets/ZXMS6001N3.pdf > > Turn on/off times are 40 uS max which may be slower than you need. > > I used similar in an automotive environment "quaite a while" ago (20 > years?) > Out of interest I tried connecting one directly across a car battery and > gating it on and off. > No problemo !. > I was duly impressed, even though it's easy enough to do with a little > thought, > That they HAD thought it through well enough (as they should have) was ni= ce. Yes too bad I can't find something like above for higher currents. I=20 would like to clamp each channel at 10 amps and the overall current to=20 25 amps. The unit has 5 channels. So at any time any combination of=20 those 5 channels can use up to 25 Amps. > > _________________________________ > > A more usual solution, and one which can be as fast as you wish, is to us= e > eg a MOSFET + current sense resistor + comparator. > Sub microsecond response times would be possible and probably rather slow= er > would be fine. > > One thing to watch for is inductive spikes at turn off in circuitry which > is notionally non inductive. > With care Murphy can probably target the protection FET and the LED drive= r > with an inductive spike at turnoff. > > A low Rdson protection FET will need minimal if any heat sinking ap[art > from PCB copper. > eg at 25A and 10 milli-Ohms Rdson =3D 6.25W dissipation (as above). > At 1 milliOhm Rdson its 625 mW. > > > I'd start with something like this > > *OptiMOSTMPower-Transistor,60VIPT007N06N > * > > $5.94/1 in stock Digikey. > Not as low cost as some but more suited than many. > > 60V, 300A, Rdson 0.785 milliOhm > > 62 K/W on minimum PCB footprint so COULD be run that way. >> =3D10V gate drive a good idea. (Abs max 20V so eg 12V good) > One to two orders of magnitude faster switching time than you need. > Drive with a suitably fast comparator using a current sense resistor. > Low Rsense =3D low power dissipation and lower V_Rsense so 'nicer' compar= ator > needed. > > Hall sensor could be used - watch response times. > > Suitably 'grunty' driver needed, > Gate capacitance is higher than some. Thanks for these suggestions I will look into this type of design. I=20 think it has a lot of promise. I have been looking at the Allegro hall=20 effect sensors which look quite nice. Do you think I would need a PIC to=20 do some logic ie. monitoring both current sense and voltage across VDS=20 or do you think it could all be done analog? Thanks again, Jim --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .