Variable resistor: why tie wiper to out lug?

[Taylor]

Most of the time, when you see a pot used as a variable resistor, the wiper is tied to one outer lug. Why is this? I've seen it mentioned in passing that it's a good idea to do that rather than leave one lug unconnected, but I can't think of why it would matter. Is it a fail-safe in case the wiper goes open?

[Mugshot]

Most of the time, when you see a pot used as a variable resistor, the wiper is tied to one outer lug. Why is this? I've seen it mentioned in passing that it's a good idea to do that rather than leave one lug unconnected, but I can't think of why it would matter. Is it a fail-safe in case the wiper goes open?

from what ive read, i think that pretty much answers your question. it has been my practice to tie the other outer lug to the wiper when a pot is wired as a variable resistor.

[aziltz]

I say, find a way to use the other half of that POT.  turn something down when you're turning something else up, etc.  its much easier when the wiper is going to ground or Vref though.

[John Lyons]

It's an old convention as a fail safe in case the wiper went open.
With some resistance the circuit would still remain stable.
Say if a 100k pot went open it would go to 100K resistance
instead of "infinity" and possible oscillation etc etc.

[MoltenVoltage]

I was wondering the same thing.  That makes sense.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

grammar police

when its wired to change current (as a variable resistor), its a rheostat

when its wired to change voltage (as a voltage divider), its a potentiometer
(voltage = potential)

[Taylor]

I was wondering the same thing.  That makes sense.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

grammar police

when its wired to change current (as a variable resistor), its a rheostat

when its wired to change voltage (as a voltage divider), its a potentiometer
(voltage = potential)

grammar police

its=the property of it
it's=it is



But I do appreciate that correction. I always thought "rheostat" referred only to a variable resistor with only 2 connections, that is, a rheostat is a specific piece of hardware. I didn't realize that a pot becomes a rheostat when changing current.

[aziltz]

i see nothing wrong with the wording "variable resistor", but i never really knew where rheostat came from and why it was different so thanks!

[PRR]

> a rheostat is a specific piece of hardware.

Yes; but the cost difference from 2-lug to 3-lug is so small, and 3-lug can be used as 2-lug, so the general market small knob-units are only sold as Potentiometers.

If you go back in history, or old catalogs, you find a few situations where the "only" use for a specific value was as a rheostat, and the standard part was 2-lug. When tubes used filaments from battery, filament and battery life was were problematic, we had filament rheostats to reduce filament brightness, radio gain, volume, and filament damage. Large pre-SCR light dimmers were often special-taper rheostats. But the half-buck "pot" has almost always been built as 3-lug, because 90% of applications were potentiometric (volume controls, squelch reference) and the other 10% of applications were cheaper 3-as-2 connection than to stock and distribute a parallel 2-lug part.

[jacobyjd]

I was wondering the same thing.  That makes sense.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

grammar police

when its wired to change current (as a variable resistor), its a rheostat

when its wired to change voltage (as a voltage divider), its a potentiometer
(voltage = potential)

haha nice

Skitt's Law
Expressed as "any post correcting an error in another post will contain at least one error itself" or "the likelihood of an error in a post is directly proportional to the embarrassment it will cause the poster."