The variable powersupply - about the pot

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Hi.

Why do i have to short two pins on the potentiometer?
What good does that do?

[phono]

it makes it a variable resistor rather than a potentiometer

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Excuse my ignorance but i don't know the difference.

[phono]

http://sound.westhost.com/pots.htm

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Thanks, i've been googling like an idiot for an explanation.

Now - to understand it too!

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It's me again

I do now understand the difference between a variable resistor and potentiometer but i still fail to see the difference between
having the third leg shorted to the wiper and not in the case of variable resistor.

If the last leg is kept hanging the resistance over the variable resistor is the same as if that leg is shorted to the wiper?

[guest]

if you do not connect the third leg
and use it as a variable resistor
it will still work the same
but ive always seen it done that way
perhaps its lower noise for some reason

[Brassteacher]

if you do not connect the third leg
and use it as a variable resistor
it will still work the same
but ive always seen it done that way
perhaps its lower noise for some reason

It's a practice that has been held over from the days of vacuum tube gear. Pots were less reliable back then, and there would be the possibility of the wiper itself lifting off the resistive element. A vacuum tube uses what synth people would call a positive control voltage on the grid, i.e., the more positive the control voltage the more current flows. However, like most JFETs, the slope of that control voltage starts on the "negative side", meaning that to turn the tube or JFET off, the voltage has to be made increasingly negative on the gate. Unlike most JFETs that are fully off at -3V or so, some vacuum tubes don't turn off until -50V or more. So, if for some reason you had a variable resistor feeding the grid of a tube (analogous to the gate on a JFET) and the third leg was not connected to the wiper, the grid of the tube would go to 0V. This doesn't sound like a big deal until you realize that turns the tube fully on, and I mean ON. The amount of smoke emitted from our theoretical device would be directly proportional to the amount of time the wiper stayed lifted!

With the third leg connected to the wiper, if the wiper lifted the full resistance of the pot/variable resistor would be seen by the grid of the tube, which was usually chosen to be a value which would turn the tube completely off, allowing the smoke to be effectively contained.

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Amazing, thanks a lot!