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 resistor=
s.
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  rather
>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  used PWM + an air wound  Nichrome 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.
>
>
>            Russell McMahon
>--=20
>http://www.piclist.com PIC/SX FAQ & list archive
>View/change your membership options at
>http://mailman.mit.edu/mailman/listinfo/piclist

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