# Potentiometer as a rheostat

#### Gdrumm

Joined Aug 29, 2008
684
In AAC Volumn 6, Chapter 3, it talks about using a pot as a rheostat.
For a 6 volt battery and motor, a 5K ohm pot is recommended.

If I wanted to use a pot to recitfy higher DC, say 27 volts for example, is there a formula to determine the size of the pot?

Since 6 goes into 27 about 5 times, then would a 25 K ohm pot (or in that range) be sufficient?

Thanks,
Gary

#### Alec_t

Joined Sep 17, 2013
13,317
If I wanted to use a pot to recitfy higher DC
?? A pot doesn't rectify; it provides a variable resistive divider or, if the wiper is connected to one end terminal it provides a variable resistance (i.e. a rheostat).
Ohm's Law dictates how much current will flow through the pot. Be careful you don't exceed the maximum rated current or the pot will fry, particularly when set to minimum resistance.

#### crutschow

Joined Mar 14, 2008
31,483
In AAC Volumn 6, Chapter 3, it talks about using a pot as a rheostat.
For a 6 volt battery and motor, a 5K ohm pot is recommended.

If I wanted to use a pot to recitfy higher DC, say 27 volts for example, is there a formula to determine the size of the pot?

Since 6 goes into 27 about 5 times, then would a 25 K ohm pot (or in that range) be sufficient?
If by rectify you mean reduce the voltage, then a larger pot would work for a higher voltage. You can also put a resistor in series with the pot to reduce the voltage. But the power dissipated can be significant. What is the current draw of the motor?

#### Gdrumm

Joined Aug 29, 2008
684
I have a transistor diode testing meter that is running on 27 volts (three 9 volts in series). Today I got the instruction manual for the tester, and it mentions using lower voltages (4.5 volts)to test surface barrier transistors.

Instead of opening the case, and messing with replacing batteries every time, I thought perhaps I could reduce voltage output of the meter using an old potentiometer I have one from Ohmite Mfg. Co.,Chicago, that looks like a tiny Variac. It has only two wires.

Come to think of it, the meter itself has a "Calibration Knob", that is probably reducing voltage already. I'll check that out, and post back.

Thanks

#### Gdrumm

Joined Aug 29, 2008
684
Nope, the calibration knob doesn't reduce voltage at all.

So I'm back to square one.
Maybe I'll just test it out on a battery to see what I get.

Thanks

#### ScottWang

Joined Aug 23, 2012
7,326
Normally I don't like to using the pot to control the motor directly, if you want to using that way, then you can choosing as Variable Wirewound Power Resistor (BCI).

#### Gdrumm

Joined Aug 29, 2008
684
Nice,

Similar if not the same as what I referred to as a miniture Variac.

Prices are variable too, anywhere for $70.00 to about$16.00 on Ebay.

Thanks,
Gary

#### inwo

Joined Nov 7, 2013
2,419
Can you add a switch to select 27V or 4.5V?

Then use a 3-term. regulator.

If you use resistance then a pot is better that rheostat, as it adjusts the voltage not the current.

#### Brevor

Joined Apr 9, 2011
297
I have a transistor diode testing meter that is running on 27 volts (three 9 volts in series). Today I got the instruction manual for the tester, and it mentions using lower voltages (4.5 volts)to test surface barrier transistors.
There is no need to bother, surface barrier transistors came out in the early 1950's, they are no longer made. You would have a hard time finding one.

#### Gdrumm

Joined Aug 29, 2008
684
Good point, but I do test some old equipment from time to time, and thought I might be able to use this device for that at some point. (although I have yet to see one of these transistors).

It will be a project (for when the weather warms up).

Thanks,
Gary