# Potentiometer as a rheostat

Discussion in 'General Electronics Chat' started by Gdrumm, Feb 28, 2014.

1. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
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

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,779
1,103
?? 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.

3. ### crutschow Expert

Mar 14, 2008
13,000
3,229
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?

4. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
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

5. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
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

6. ### ScottWang Moderator

Aug 23, 2012
4,853
767
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).

7. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
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

8. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
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.

9. ### Brevor Active Member

Apr 9, 2011
279
54
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.

10. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
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