August 1967 Electronics World

## August 1967 Electronics World[Table of Contents] People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. All copyrights are hereby acknowledged. See allElectronics World articles. |

Here is yet another example of where the basics in electronics never changes. There are always new people entering into the realm, so even if the subject has been covered countless times already, there is always a need to print it again. Remember that at one time you were a newbie and appreciated seeing beginners' concepts explained. The old-timers of the day probably complained about being tired of seeing the simple stuff re-hashed over and over. The more things change, the more they stay the same.

The linearity of a
potentiometer can be completely changed by the position of the
wiper arm and the resistance of the load.

Most potentiometers used in communications and industrial electronic equipment are specified according to three characteristics: total resistance of the resistive material, maximum power dissipation, and the linearity of resistance as a function of shaft position. Both engineering technicians who must modify existing circuits and experimenters who are designing their own circuits face the problem of choosing the right potentiometer for the job at hand. As will be shown, this choice is not as simple as just selecting a likely looking pot from a catalogue.

Selecting the appropriate pot is somewhat more complex than many people might be led to believe. The discussion that follows points out the problems involved in selecting potentiometers for loaded voltage-divider circuits and describes how to solve the problems using a few equations and the manufacturer's specifications.

The circuit in Fig. 1 shows the conventional method of controlling the voltage across a load impedance R

When the load resistance is infinite (no load), the response of a linear pot is truly linear. As the load impedance decreases, however, the response becomes more non-linear.

In theory, it is impossible to obtain a linear response from a linear taper pot that is loaded with any impedance. In practice, though, an R

Likewise, a log taper pot will produce a truly log response only if the load impedance is infinite. As the R

Fig. 1. These curves show how the pot linearity varies with
the load.

Because of the unwanted effects of pot loading, the potentiometer resistance should be kept as low as possible with respect to the load impedance.

However, a good linear response is bought at a high price - the smaller the specified pot resistance, the greater the current through its contacts and resistive elements.

The potentiometer power dissipation specified by the manufacturer is actually a reflection of the maximum current that can pass safely through any of the pot's three connectors or any portion of its resistive element. The following equation enables the user to calculate this maximum current rating:

I

For example, a 10,000-ohm, 1-watt potentiometer can safely pass √[1/(1 x 10

The current through a voltage-divider circuit such as the one in Fig. 1 is at a maximum when the wiper arm is in the position that makes the circuit strictly parallel (α = 0).

The maximum current through any part of a loaded pot may then be determined by using the equation I

Suppose the d.c. or r.m.s, value of applied voltage (E) is 10 volts and the load impedance (R

Consider the linearity problem. The resistance of the pot should be no greater than 0.1 times R

P

The specifications for this particular pot should be 1000 ohms, 1 watt, and a linear taper.

Posted 9/9/2011