[VAC] Re: Variable resistor for brake controller?

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

As I recall the resistor board on my last truck, it probably used iron
wire for the resistors, had several different lengths between assorted
studs so that a suitable value could be connected. Like someone said,
looked like a fat screen door spring.

I recall making a 1 ohm resistor on a Sunday to test a client's computer
power supply when there were no electronics stores open and I was 40
miles from home. I used 18 gauge solid iron wire. Took about 21 feet. I
didn't coil it up but that would coil neatly around a rod (then pull the
rod). It could be tapped with the smallest split bolt, or made in
sections. Connections probably should not depend on solder, it might be
melted or vaporized. Iron wire for this application probably should be
galvanized to slow its rusting though if it ever heats up enough to glow
the zinc will be gone. One could also use resistance wire the size used
in some electric clothes dryers or use some screw in 660 watt electric
heaters (in parallel 21 ohms each, or could have the resistance wire
shortened to an ohm or less). With the screw in heaters one could adjust
the length of the resistance wire for each application and screw in only
the one needed for that particular load. That is if those screw in
heaters can still be bought.

Otherwise I'd seriously consider buying a couple 1 ohm, 50 watt
adjustable resistors from Mouser (www.mouser.com). They're catalog
number 588-D50K-1.0 $11.81 each. They would handle 7 amps continuously.
Or one 1 ohm fixed and one adjustable. The 1 ohm fixed is cheaper.
558-L50J-1.0 for $6.72. It appears that mounting brackets may not come
with the resistors, though the last power resistors I recall buying in
individual boxes did include mounting brackets. Just in case, they are
catalog 588-9 for 27 cents each. Takes two per resistor.

They do have a 1 ohm 50 watt rheostat, but its out of reason for this
application, 588-RJS-1.0 for $30.11. It doesn't come with a shaft lock
so one would probably need to be added. There isn't one in the Mouser
catalog. Though mounted on the tow vehicle dash (or fire wall with an
extension shaft) it would be handier for adjusting while on the road.

There are other places such as Mendelson Electronics (www.meci.com) or
Fair Radio Sales or Gateway Electronics or numerous other surplus
electronics stores that MIGHT have suitable resistors on hand. Fair
Radio and Gateway do have web pages. Perhaps even Hosfelt, but I'm
unhappy with them. They charged me $7.38 shipping for three LEDs last
month. I placed a phone order with Mouser on Tuesday and it was at my
door in Iowa on Friday morning. Major national electronics distributors
such as Newark and Allied should also have those same power resistors as
Mouser but they don't want to deal with individuals or even small
companies, definitely not with small orders. Mouser DOES want to deal
with individuals and small orders.

Gerald J.