Potentiometers are common electronics components that convert rotary or linear motion in to change of resistance. They can be found anywhere where some adjustments are needed, volume control and joysticks. You can dive in to long theory about potentiometers how they are made, what materials are used and what output characteristics they have regarding to rotation angle.
As electronics hobbyist and probably pro you usually deal with two common types of potentiometers – basic pots with knobs and trim pots. First group of potentiometers are used where user has to access potentiometer when he needs to change one or another parameter like sound volume or screen brightness. They normally are bigger in size and can be mounted vertically or horizontally to board. Also they have a panel mount with nut. This type of potentiometers needs to be mechanically stronger and resistive material more durable due to frequent use.
Trim pots in other hand are much simpler in construction as they are used rarely during system setup to fine-tune parameters. They are small in size so are convenient to mount on PCBs. They aren’t normally accessible to user – only when servicing.
Speaking of potentiometer characteristics – most common are three: Linear – when resistance changes linearly over full turn. In audio applications you can find logarithmic potentiometers and third group are known as antilogarithmic. Of course you probably can find different characteristics as well.
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Potentiometers aren’t rated for high power applications. They stand before control circuits. With standard potentiometer you cannot drive heavy loads (except rheostats). So you should keep in mind that there are limits. Depending on material these limits vary but are up to 0.5W. If you have doubts – Ohms law will help.
Last thing I want to touch about pots is resistive material. There are many material used, but most common is carbon which is cheap and good quality for most purposes. It has long life and moderate noise level when speaking of audio applications. Other materials are Cermet (ceramic and metal composite). They are better quality than carbon with lover noise level and higher stability. Commonly used in trimpots due to limited number of operations. Other materials are based on conductive plastics that give great characteristics. Mostly found in audiophile applications. And of course lets not forget wire wound potentiometers that normally are used in higher power applications. They are much lower precision with granular resistance steps due to wire windings.
If you need very precise potentiometer then one of best options is to use multiturn precision potentiometers. As their name states – you need to do several knob turns in order to go through scale. There is no big magic inside such potentiometers – usually a screw mechanism is used to slide the wiper. The more turns potentiometer needs, the more precise values you can set. You can find different multiturn resistors that reach over 25 turns for very fine tune applications.
Lets take a look at one multiturn resistor example found on Rapid electronics. As an example we can pick 5-Turn Precision Potentiometer used for panel mount. It has been manufactured by Bourns. It has been developed for both – human machine (HMI) and machine to machine interface (MMI). The case can withstand heavy rotation load while maintaining parameters within specs. This type of potentiometer is built using wire wound technology and Hybritron (conductive plastic coated wire wound). Their tolerance is about 3% and linearity 0.25%. Life-cycle is rated 1 million shaft revolutions. With five turns you can get really precise values of resistance. Say you have 1k potentiometer. So in one turn resistance change is 200 Ohms. So for one degree angle you can step about 0.555 Ohms. If you mount servo mechanism to such potentiometer you can get even even better resolution. As you can see multiturn potentiometers can be used for really finetune applications. The more turns it has, the more precise resistance can be set. Multiturn potentiometers are great for measuring angles, distances, and rotations. Other uses can be in tuning equipment of medical, industrial equipment. Also they are extensively used in scientific and measurement instruments.