23 Sep
2011
23 Sep
'11
11:17 a.m.
On 23 Sep 2011 at 1:33, Eric Smith wrote:
24V at 7A is 168W. You could use an Ohmite 225W 4 ohm
wire-wound
adjustable resistor, which costs $19.16 from Mouser Electronics. You
will need forced-air cooling.
Note that the power rating of adjustable resistors is the limit when
the entire resistance is used, and must be derated when you're using
the adjustable tap.
If you're on the cheap, get a plastic pail that'll hold a couple of
gallons of water, add salt and fashion a couple of electrodes,
preferably of carbon (welding rods with do) or stainless steel.
Adjust the concentration of salt and the position of the electrodes
to get the resistance that you're after. Use in a ventilated area.
It's amazing how many KW five gallons of water will absorb before
boiling.
This is no hack--liquid rheostats have a long and distinguished
history:
http://en.wikipedia.org/wiki/Liquid_rheostat
--Chuck