How does a potentiometer measure voltage

Potentiometer - everything regulated



Numberdescriptiondata sheet
1 Battery / voltage source 9V 
1Resistance 1.0 kOhm 
1Trimming potentiometer 10 kOhm




There are resistors with almost all possible values ​​and loads. But there are also cases where these are not enough. In some cases you need a resistor that you can adjust. I.e. whose value can be regulated. Just think of the volume control of a hi-fi system, the brightness control of a light dimmer or, for example, the speed control of a model train transformer. Potentiometers are used for this. The basic function of these potentiometers is always the same in principle. They often consist of a layer of carbon on which a grinder with an electrical tap draws the current. But types with resistance wire are also used. These are then used for higher loads.


The main difference between the potentiometers is the design. There are normal potentiometers that are equipped with an axis of 4 or 6 mm. But also potentiometers that are built into a circuit and that are no longer changed after a certain value has been set. This shape is called a trimming potentiometer or trimmer for short. Poti can also be used for potentiometers. If very precise setting options are required, the multi-turn potentiometers are used. There the control range consists of a few revolutions of the operating axis. The sliding pots have established themselves in mixing consoles etc.


Potentiometer and trim potentiometer


A (trim) potentiometer usually has 3 connections. The total resistance is over 2 of these connections. The 3 connection is the center tap. Some potentiometers have additional functions, for which additional connections are available. E.g. switches, lighting or a motor drive for motor potentiometers, which can be controlled not only by hand but also by remote control.



The potentiometer as an adjustable resistor



A potentiomer is actually a resistor with a fixed value on whose resistance layer there is a wiper. The potentiometer or better the trimming potentiometer used here has a value of 10 kOhm. With the help of the center tap it is then possible to take resistance values ​​from 0 ohms up to the maximum value.

If we measure the voltage at the load resistor R1, we find that this can also be adjusted as desired. The smallest voltage value that can be set here is approx. 0.8V and then goes up to the maximum voltage.


The rest of the voltage must then inevitably drop at the potentiometer, which can easily be proven with this measurement. With potentiometers you also have to pay attention to the load. It is to be considered that the current gets bigger and bigger, the smaller you control the potentiometer.



In order to protect the potentiometer a little, the center contact is often connected to an external contact. Since the current is still looking for a further path with this variant, it flows here over the 'unused' part.

This type of circuit also has the advantage if the wiper should no longer have any contact with the resistance layer, which can happen after very frequent use, the electrical connection does not break, which would be the case if only one external contact was used. Here the entire resistance layer would then represent the resistance and the circuit would continue to work.



The potentiometer as a voltage divider



With the last circuit it was immediately noticeable that the voltage can only be regulated from a certain value up to the total voltage. This will not change with this type of circuit either. It is possible to reduce the residual voltage with larger potentiometer values, but there is always one available.

If you now put the 3rd external contact on minus, you can regulate the potentiometer from 0V up to the total voltage. A measurement on the load resistance proves this. What value the potentiometer must have is now determined solely by the connected load.



If you now measure the voltage to the positive pole, you will find that it behaves the same way there as with the resistor. Only the relationship between voltage and control path is reversed. With this type of use it depends on the circuit how the load is connected. It makes no difference for the potentiometer.

With this type of connection, the basic current of the potentiometer is added to the actual load current. This must be taken into account when calculating the required load. Ohm's law can be used for all calculations of the potentiometer.