Chapter 3 - Resistor types

What is a trimpot?

A trimpot or trimmer potentiometer is a small potentiometer which is used for adjustment, tuning and calibration in circuits. When they are used as a variable resistance (wired as a rheostat) they are called preset resistors. Trimpots or presets are normally mounted on printed circuit boards and adjusted by using a screwdriver. The material they use as a resistive track is varying, but the most common is either carbon composition or cermet. Trimpots are designed for occasional adjustment and can often achieve a high resolution when using multi-turn setting screws. When trimmer potentiometers are used as a replacement for normal potentiometers, care should be taken as their designed lifespan is often only 200 cycles.

Trimpot definition

Trimmer potentiometers and preset resistors are small variable resistors which are used in circuits for tuning and (re)calibration.

Types of trimpots

Several different versions of trimpots are available, using different mounting methods (through hole, smd) and adjusting orientations (top, side) as well as single and multi-turn variations.

Single turn

Single turn trimmers/presets are very common and used where a resolution of one turn is sufficient. They are the most cost effective variable resistors available.

An open trimpot with a carbon resistive trackOpen, carbon track Single turn enclosed trimpotEnclosed, side adjust SMT mounted variable resistorEnclosed, SMD

Multi turn

For higher adjustment resolutions, multi-turn trimpots are used. The amount of turns varies between roughly 5-25, but 5, 12 or 25 turns are quite common. They are often constructed using a worm-gear (rotary track) or leadscrew (linear track) mechanism to achieve the high resolution. Because of their more complex construction and manufacturing, they are more costly than single turn preset resistors. The lead screw packages can have a higher power rating because of their increased surface area.

Top adjustable multi-turn trimmer potentiometerTop adjust, worm-gear Side adjustable multi-turn preset resistorSide adjust, worm-gear Linear trimpot, side-adjustableSide adjust, lead screw

Trimpot symbols

The following IEC symbols are used for trimpots and preset resistors. Although this are the official symbols for occasionally adjusted resistors, the standard symbols for a potentiometer or rheostat are often used.

Trimpot symbolTrimpot symbolIEC standard Preset resistor symbolPreset resistor symbolIEC standard
Potentiometer symbolPotentiometer symbolIEC standard Rheostat symbolRheostat symbolIEC standard