Cutting Open a Trimming Potentiometer

The previous page showed photos and part numbers of breadboard-compatible trimpots. How do they work?

Most electronic parts are solid state -- that is, they don’t have moving parts. Obvious exceptions are motors, switches, and potentiometers. You can readily find cutaway diagrams for motors and switches, but trimmers often get overlooked.

Carefully cutting away the plastic casing of a 3/8-inch trimpot reveals a simple, elegant, reliable design. The bottommost layer consists of a ceramic plate with three riveted wire leads, three conductive traces, and a dark circular segment of resistive film. The film is usually cermet: a combination of ceramic and metal.

Cermet trimpot substrate

Cermet trimpot substrate.

Looking at the base, it is obvious why connecting the first and third wires to an ohmmeter measures the full resistance of the trimpot. Regardless of the position of the dial, electricity flows through the entire resistive element just as though a single fixed resistor were connected between the wires.

The dial is a single piece of molded white plastic that rests with its center on the middle electrical contact.

Inside single turn potentiometer

Inside a single-turn potentiometer. Left: Dial in place. Middle: Dial removed. Right: Back of removed dial.

Underneath the dial is a metal wiper that applies mechanical spring pressure to keep firm contact between the center lead and the outer resistive element. This allows electricity to flow from either the first or third pin, through a certain amount of resistance film, through the metal wiper, and into the center pin. Depending on the position of dial, the distance that the electricity has to endure the resistor is increased or decreased.

The wiper is similar to a brush in a motor.

Trimpot brush wiper consisting of tiny metal segments close up Noodly appendages

Trimpot brush wiper consisting of tiny metal segments.

A series of small bent wires provide multiple contact points to ensure that good electrical contact is maintained. This reduces or eliminates electrical noise, sudden shifts in value, or complete disconnection that might occur if there was only a single wire rubbing over the resistive element or center trace.

Multiturn

Multiple-turn trimpots are very similar to single-turn trimpots, except for the addition of gearing to provide finer adjustment of the dial.

Inside multiturn trimpot with worm drive

Inside a multiturn trimpot with worm drive. Left: Gear in place. Middle: Gear removed. Right: Back of removed gear.

The white plastic piece containing the wiper has gear teeth molded on it, similar to a spur gear, but called a worm gear or worm wheel. A long brass rod has been machined into a screw shape called a worm. As the brass rod is turned, it slowly rotates the center plastic piece with the metal wiper, to slowly adjust the resistance.

Manufacturers can alter the size or coarseness of the worm to alter how many turns it takes to rotate the worm gear from the start of the resistive film to the end.

Notice that the gear has bunch of teeth missing. This gap prevents the gear from rotating beyond the track of resistive film. As such, an overly ambitious user won’t damage the trimpot by turning the screw too far in one direction.

Cool huh? I guess trimpots really are much more complicated to produce than a chip or fixed resistor.