Electronics > Projects, Designs, and Technical Stuff
Do sub 0.1 ohm variable resistors exist?
e100:
At that level are mechanical wipers still used?
I could imagine in the old days of no heath and safety you would have electrodes dipping into mercury in a trough and you would vary the distance between the electrodes.
Looking for a way to vary the current through a 0.38 ohm load at 14 amps fed by recified AC.
ArthurDent:
If you're using rectified A.C. then I assume there is a transformer connected to line voltage with about 6+ volt output to work with that load. I would use a variac on the primary side. That would give you far better control of the load current and use parts you can easily find.
Years ago they would use large carbon pile resistors to adjust low voltage/high current. A number of about 2" square slabs of carbon were pinched together in what amounted to a vise and the more pressure, the lower the resistance. The speed pedal on some sewing machines also worked this way.
Another way that is far more complicated would be to wire a high value pot across the .1 ohm resistor and have the voltage on the wiper go to an opamp that could control a circuit similar to a lamp dimmer on the transformer primary.
coppercone2:
custom job
with a glass seal and a magnetically coupled motor, there is no reason you can't do the mercury thing safely so long you follow the part
hagster:
Maybe this mercury alternative will do.
https://en.m.wikipedia.org/wiki/Galinstan
ejeffrey:
I agree that a variac is probably the easiest and best option. A PWM controller or a linear transistor regulation would be my second choices.
I've never seen a pot that handles that sort of current/resistance range but the resistance element can easily be made from a suitable gauge nichrome heater wire. Getting a wiper to run along it with relatively constant contact resistance is more of a challenge and depends on the application. How many cycles does it need, how accurately do you need to set it?
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