At 07:23 AM 10/1/2011, Electron wrote: >I think it's easy at surplus locations to find batches of power resistors. >E.g. I got one (almost for free) containing one thousand 5W 100ohm resisto= rs. >I still haven't found the time, but I'd like to make a programmable sink >with them, some mosfets and a PIC. Metal Oxide 5W resistors work out nicely - they range from completely=20 non-inductive to somewhat (barely) inductive. I tend to choose lots of higher-value resistors, then put them in=20 parallel as needed. Putting several of a resistor in series, then=20 putting that series string in parallel with a smaller number of=20 resistors limits the power dissipation to the weakest element, which=20 can be as low as a single resistor. For example, choosing 100R resistors would limit my load rating to=20 about 22Vdc - not high enough for me. A better choice would be=20 470R. Or, in the case given by Electron (above), make each resistor=20 out of several 100R resistors in series. 5- 100R resistors in series=20 would make 500R @ 25W. That's good up to about 113V continuous -=20 usable for most anything that I might need. Using all of those resistors in that fashion (200 strings of 5) would=20 give you almost a full 8 bits of resolution, all the way from about=20 2R5 through 500R with a maximum working voltage of about 113V=20 (continuous) and a maximum power rating of 5kW at the lowest=20 resistance setting. Eminently reasonable, actually. One of the many advantages of choosing Metal Oxide resistors is their=20 extremely high tolerance to severe overload without failing or=20 changing resistance. I've run them red hot for several hours just to=20 see how they behaved and was pleasantly surprised at their stability. 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 .