Two more great ideas - there is an electronics surplus place in town - I was thinking of going there to see if they had a suitable ammeter so I'll pick up some big wirewound resistors as well. and the second idea is to put a current sense resistor on it - even though I don't need it for the rough measurements I'm making now later on I could add a microcontroller as a feedback mechanism. Then of course using a system similar to Mario's (digital potentiometer?) make it programmable. Today, however, get the basics done. Thanks all, keep the suggestions coming! DougM On Sat, Oct 1, 2011 at 6:23 AM, 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 resist= ors. > I still haven't found the time, but I'd like to make a programmable sink > with them, some mosfets and a PIC. > > Basicly I'd first select the resistors to get the best precision possible= , > then I'd group some in power of 2's, and (through mosfets) toggle them > with the PIC. > > i.e. like PIC bit 0 turns on mosfet 0 which puts to ground a 400 ohm > resistor (just 4 of them in series). PIC bit 1 turns on mosfet 1 which > puts to ground 200 ohm of power resitors. PIC bit 2 turns on mosfet 2 > which puts to ground 100 ohm. PIC bit 3 turns mosfet 3 which puts to > ground 50 ohm (2 100 ohm resistors in parallel), on And so on, for say > 12 bits. > > Then the PIC would measure current, and activate the power resistors > network accordingly to cause the wanted current or power sink. > > Greets, > Mario > > > At 01.00 2011.10.01, you wrote: >>> I like the idea of nichrome wire on a very large heatsink with a >>> series of FETS - I've got a bunch of IRF530's in stock - I'll drive >>> those against the nichrome on the same large heatsink (I've got a >>> stack of old CPU sinks around) >> >>If you don't mind having mean current equal to target current =A0rather >>than steady current you can use PWM with a resistor load. That plus a >>suitable FET gives very little dissipation in the FET. >> >>You can use air wound nichrome wire as the load and so need no >>heatsinking at all for the resistor. >> >>If you must have steady current, add a suitable series inductor and a >>catch diode and you have a buck converter. >> >>I have extensively =A0used PWM + an air wound =A0Nichrome resistor + >>MOSFET up to 5000 Watts with good effect. This was the load for an >>alternator that was DC rectified and then had suitable filter >>capacitors added so the capacitors saw the PWM ripple while the >>alternator saw the relatively steady capacitor voltage as load. Worked >>very well. >> >>Nichrome is normally encountered as either tape in toaster elements or >> as a relatively thin wire for AC mains use. This is usable at low >>voltage with suitable numbers of strands in parallel. However, much >>thicker wire is available from electrical suppliers allowing the >>construction of robust high power load resistors at lower voltages. >>Wire can be wound in a spiral and supported occasionally to make an >>open wire resistor. >> >> >> =A0 =A0 =A0 =A0 =A0 =A0Russell McMahon >>-- >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist > > -- > 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 .