 2000 Microchip Technology Inc.

Preliminary

DS00219A-page 1

AN219

INTRODUCTION

Resistor potentiometers can be found in electronic cir-

cuits across a wide spectrum of applications. Most typ-

ically, they function in a voltage divider configuration in

order to execute various types of tasks, such as offset

or gain adjust. The two types of potentiometers com-

pared in this application note are the mechanical poten-

tiometer (also called a trimmer potentiometer) and the

digital potentiometer. The physical descriptions and cir-

cuit models of these two devices are shown in Figure 1.

Basics of Mechanical Potentiometers

The first type of potentiometer on the market was

mechanical in nature. This type of potentiometer is still

available and adjustments of the wiper are imple-

mented by twisting a knob, moving a slider, or using a

screw driver. Although this method seems awkward,

given the advent of the digital potentiometer, mechani-

cal potentiometers still find their way into various elec-

tronic circuits.

Earlier mechanical potentiometers were built by wrap-

ping a resistive wire around a cylinder. With this con-

struction, the wiper moves from one winding to the

next. As the wiper is moved across the element, there

are discrete steps in resistance. Following this style of

fabrication, the mechanical potentiometer was built

using a resistive thick film that was screened onto a

ceramic substrate. With this construction, the change in

resistance across the element is continuous.

There are a variety of resistive materials that are used

by mechanical potentiometer manufacturers. They

include molded conductive plastic, conductive plastic

film, screened conductive plastic, and cermet. Each

resistive material has its own set of performance char-

acteristics. In this application note the digital potentiom-

eter will only be compared to the more popular cermet

potentiometer. Cermet is a thick film resistive material

that is a mixture of fine particles of ceramic or glass and

precision metals such as silver, platinum, rhodium, or

gold. The wiper of the mechanical potentiometer slides

along the distance on the resistive material providing

an analog resistive output that has an infinite number of

positions across the span of the element.

Figure 1:

The mechanical potentiometer is constructed so that the user can easily adjust the position of

the wiper (PW) by hand or with a screw driver. The digital potentiometer is manufactured so that the

position of the wiper is adjusted by means of a serial digital code. The circuit representation of the digital

potentiometer and the mechanical potentiometer is fundamentally the same.

Author:

Bonnie C. Baker,

Microchip Technology Inc.

Mechanical

Potentiometer

Model

Digital

PA

PB

PW

Potentiometer

Model

An example of PCB

mountable Mechanical

potentiometers

wiper

PA

PB

PW

MCP41010

MCP42010

contact

resistance

resistance

Comparing Digital Potentiometers to Mechanical

Potentiometers