# The resistance range of a potentiometer

Discussion in 'General Electronics Discussion' started by Jimbo bob, Jan 10, 2015.

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1. ### Jimbo bob

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Oct 8, 2014
Hi all ,

I am looking to get a potentiometer. However, the manufacturer offers the device in a number of different resistance ranges i.e. 5K, 10K, 20K. I want to connect its output to the auxiliary input of a engine control unit and power it from 12V car battery. My question is should I be concerned with the resistance range of a potentiometer? Why are there different resistance ranges?
What I understand is that considering the maximum wattage of a potentiometer at its maximum resistance range, there is a constant current that can flow through it and of course this current changes with maximum resistance range. Is that all there is to it?

2. ### Rory Starkweather

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Nov 13, 2014
I think you are missing something. Do you have any idea what resistance you need?

Potentiometers are kind of like water valves. They reduce electron flow by increasing resistance, analogous to closing the water valve. Electric circuits must meet Ohm's Law. V = I * R. so, if you change the potentiometer setting it will cause a change in one or possibly both of the other factors. R increases, so V increases where V is the voltage dropped across the potentiometer. If, for whatever reason, V is not allowed to change (regulated power supply?) I decreases. (Current flow ramps down.)

Basically, what you are trying to do is not simple at all, but we don't know anything about the application. If we did, we might be able to give you a better answer.

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3. ### Gryd3

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Jun 25, 2014
Potentiometers are not quire as robust as many thing they are.
If you have to ask the wattage of a potentiometer, chances are you are thinking of an incorrect use for it.

Common uses is for lower power applications. For example, adjusting volume on the pre-amplifier portion of a circuit board, or altering a reference voltage for a power supply.
If you ever want to do any heavy lifting, you need to start adding more components.
House hold light dimmers have a potentiometer in them that is used to alter the rate at which the other components switch on or off which controls the total output power. Things like electric drills use a variable resister to control a PWM driver which does the heavy lifting with the motor. Other devices use something that appears similar to a potentiometer with a multi-position switch that usually connects to a network of high power resistors, or to different windings on a transformer... Lots of options, which one is right, or good enough, depends on your application.
Give us more details on what you want to do rather than how you are thinking of doing it, and we can give you more details

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4. ### Jimbo bob

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Oct 8, 2014
First of all I truly appreciate your input. What I am trying to achieve is to find potentiometers (need a pair of them) that I can setup with the accelerator pedal of a car that I would like to work on. The voltage outputs of the potentiometers will be transmitted to an engine control unit and will be used to move a servomotor to change the position of the throttle valve and adjust air intake. I also plan to use another pair of potentiometers near the throttle valve to monitor its position.

I had a look at "SAJ2515 with "F" aluminum handle" on this webpage:

http://www.precisionsales.com/potentiometers/singleturn/spring-return.htm

I was thinking to get the 20K or 10K potentiometer just for saving Amps of current, but I am not sure whether I am on the right track or do I need to take something else into account?

5. ### duke37

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Jan 9, 2011
As you describe it, the potentiometer will be connected across 12V and the output will be taken from the slider. The output can then be varied from 0V to 12V.

There will be a voltage drop due to the resistance of the pot and the current drawn by the ECU. If the pot resistance is less than one tenth of the input resistance of the ECU then little correction will be needed.

What range of voltage can the ECU stand and still work and what voltage will damage it?

Current consumed in an application such as this will be negligible, you will probably need about 10k, consuming 15mW.

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6. ### Gryd3

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Jun 25, 2014
Just a heads up. Forum Terms prohibit the discussion of topics if they fall under one of the following categories:
This may very well fall under personal or public risk depending on the scope of your project.

I will give you some more generic details, but nothing specific to automotive until you can provide more details about what roles the modification will have. (ie, Will this be for a Go-Kart, or for a car that will be travelling on public roads?)

Potentiometer number 1, which will be connected to the pedal will, as Duke mentioned most likely be connected directly across 12V. You had asked about different ranges. Well, this is one reason. A larger value potentiometer will let less current through it, and heat up less. Picking a value of 1K or higher will be sufficient here. Please note that the 1K will constantly waste 12mA as long as it is connected to power. Simply using a 2K will drop this to 6mA. The important part here is that the middle pin of the potentiometer will be able to vary anywhere from 12V to 0V. However, it can't provide much power, so this 0-12V signal needs to be connected to another circuit to interpret it and do something with.
For example, you could use it to control a PWM signal. (Which is a square wave that can vary it's time high, verses it's time low. This relationship is what a servo uses to position itself, or how you can control average power to something like a light or motor.)
As far as the servo is concerned, depending on how you plan to connect it, you may need a beefy servo, but that is for you to determine. You need to make sure the servo will have enough torque to do the job.

One quick recap.
I had copied and pasted a portion of the terms above due to the nature of the work involved. If your sensor or circuit misbehaves or get damaged, it could cause unexpected results. One such failure could cause a 'runaway' condition, which if you are unaware is common with older diesels.
Make sure you take more than the necessary safety precautions to ensure that you remain in control of your machine before you do any modifications to it.

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7. ### davennModerator

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Sep 5, 2009
I have to agree with Gryd3's post
you need to check with the authorities in your area to confirm whether such modifications are allowed and what laws and certification are in place for safety

It is a topic that falls under the not allowed to be discussed as we don't want to be responsible for giving information, which is then not used correctly and some one is injured or killed as a result

As much as we hate to stifle creativeness ... the legalities of blame leave no other choice