# Potentiometer as a Variable Resistor

Discussion in 'General Electronics Discussion' started by MrClamperSir, Feb 4, 2016.

1. ### MrClamperSir

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Feb 3, 2016
Hello everyone!! I'm new to electronics and am very passionate about making useful circuits.

Currently I am using a 12VDC battery to power my tools. I need a variable supply so I have been using a 25Ω 1A rheostat to deliver this need. My machines run on 3V-8V depending on the need.

So far this has been working ok. However I find myself at the top of the rheostat (knob turned most of the way up) while working and I am looking to replace that with something that will provide me more power at a lower setting.

Any suggestions on what Ω and amp values I'd be looking at to accomplish this?

Also an brief overview on why I need to increase/decrease the ohms/amps

Thank you

Last edited: Feb 4, 2016
2. ### MrClamperSir

72
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Feb 3, 2016
Hello!! I recently tried to replace my 25Ω 1A rheostat with a 5K potentiometer. I wired it using two terminal pins instead of three (as was suggested by a book on wiring circuits).

At first I was happy to discover my multimeter read out was varying from 0VDC-12VDC just like I was hoping. However when I went to power my tools with the same setup, NOTHING!! No power at all.

I checked back on my multimeter and it was still wired the same with the same readout but I was unable to draw any load.

Anybody know why this happened and how I can rectify it? I'm assuming that the 5K is the wrong values to allow enough current through but that's just a guess on my part

Thank you

3. ### Harald KappModeratorModerator

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Nov 17, 2011
Do you know Ohms law? One form of it is I=V/R. With R=5kΩ and V=12V, the resulting current is much too low to drive a tool. There's a reson the old rheostat had 25Ω only.

You multimeter shows the change in voltage as expected because the current drawn by the multimeter is negligible in this setup (or equivalently the multimeter's resistance is high 1MΩ vs. 5kΩ of the potentiometer).

Why do you want to exchange the 25Ω rheostat at all? What is the purpose?
If you need more control over the power or speed of your tools, go for an electronic speed controller (ask your favorite search engine or go to the nearest hobby shop).

4. ### MrClamperSir

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Feb 3, 2016
I am aware of Ohms law but my grasp of it isn't firm.

I want to replace it because I'd like a little better range at the bottom end of the instrument and the rheostat is a coarse adjustment. I'd like to be able to fine tune the current.

I've seen the electronic speed controllers before but what's the output look like? The reason I use a battery vs a variable power supply is because the power is cleaner. my tools are sensitive to ripple like in the switched mode supplys.

5. ### MrClamperSir

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Feb 3, 2016
Is there a way to use a pot as a variable resistor while getting the current I need? I would hate to burn it up by using the wrong value but like I said my grasp of Ohms law isn't strong. I've tried to learn it from books but with the real world examples, it escapes me how to use it when dealing with variables.

6. ### Harald KappModeratorModerator

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Nov 17, 2011
Typically pulse width modulation - a strongly pulsed signal.
That excludes electronic speed controllers.

In this case use a regulated linear power supply.Controlling ~1A with a variable resistor of whatever kind is not a state of the art solution. Or build one yourself.

7. ### MrClamperSir

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Feb 3, 2016
Thank you Herald!

I actually have a linear supply I'm working on but as you can see, I'm having issues with that too!

8. ### MrClamperSir

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Feb 3, 2016
When you say "state of the art solution" are you talking about a circuit of some kind? My end goal is to deliver the most clean, the easiest setup power from battery to machine.

9. ### Colin Mitchell

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Aug 31, 2014
If you want to replace a 25 ohm rheostat with a 5k pot you need a transistor connected to the pot.
The pot should 500R

10. ### Harald KappModeratorModerator

10,079
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Nov 17, 2011
I take it then that you run your tatoo tools from this power supply, right?

A modern switch mode supply with a high switching frequency should be no issue at all with that kind of tool. These tools seem to use inductive drives (motors or coils to move the tatoo needles - excuse me if I don't use the expert designations) which inherently act as low pass filters. The residual ripple on a well designed switch mode regulator should be well under 100mV (much less) which at 3V nominal is less than 3%.

I truly doubt that thsi will affect the operation of your tools. That's an opionion, not sound knowledge, but know that even audio equipment today uses switch mode regulators and I bet the ears are much more susceptible to noise than the skin to an ever so slight variation in the speed of a tatoo machine

11. ### Colin Mitchell

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Aug 31, 2014
You really need PWM

12. ### MrClamperSir

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Feb 3, 2016
I would've that that as well but just like you can't get the same tones from todays amps and mp3 players compared to the tube amps or listening to vinyl, the machines (well designed and thoughtfully engineered ones) run completely different from one supply to the next. There are a few decent switched mode supply on the market but I can't get behind them for some reason. They linear supply's or the 12V battery seems to be really consistent and reliable every time.

13. ### MrClamperSir

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Feb 3, 2016
If I use the schematic you provided for the linear supply, could I use the LM338 in place of the LM317?

14. ### MrClamperSir

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Feb 3, 2016
I'm not 100% sure what that is but I believe it's a digital signal of some sort? That would defeat the purpose of using the 12V battery to supply my power. I need the current to be as clean as possible on the output.

15. ### dorke

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Jun 20, 2015
What you actually need is an adjustable step-down DC to DC converter 12V to (3-8)V at 1A or more
(they are dirt cheap this days).
Like this one. or a one with a digital readout like this one.

If the number of tools you are using is small,
you can consider attaching a dedicated converter to each tool, adjusting the voltage once and leaving it that way (tool+converter).

If however, you need constantly adjusting the voltage of the tool as you work with it .
You will need to replace the trimmer-pot on the converter board with a " shaft potentiometer" and
wire it to a panel/box.

The briefest explanation to your question is this:
Ohm's Law

16. ### MrClamperSir

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Feb 3, 2016
What does the output look like on a unit like that?

17. ### Harald KappModeratorModerator

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Nov 17, 2011
Possibly. You may have to adapt component values, though. When you're not certain, better head for the LM317.

18. ### MrClamperSir

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Feb 3, 2016
I'm not looking at all for anything digital, buck, switched. I'm looking to produce simple, linear, straight forward control for the battery. Is this possible?

19. ### dorke

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Jun 20, 2015
It is possible,
use a linear variable regulator IC(LM317 and the like) or module.
What is the problem with using "anything digital, buck, switched"?

20. ### Harald KappModeratorModerator

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Nov 17, 2011
The sequence of above posts is a bit confusing to to my merging two threads on the saem topic. I beg to excuse that.