Angles

Mechanical Angles

Single-turn designs normally have continuous rotation (continuous mechanical angle) without stops. Almost any optional stop can be supplied to the customer's specification at an additional cost. Certain common mechanical angles are available at a minimal cost.

To specify a special mechanical angle, the following information is needed:

• Mechanical angle and tolerance (+8°, -0° is typical)
• Electrical angle and tolerance (±2° is typical for wirewound, ±3° is typical for conductive plastic)
• Stop strength
• Relationship between electrical and mechanical angle

Unless otherwise specified, the effective electrical angle will be located within the mechanical angle.

The mechanical stop of a potentiometer is limited by the strength of the materials used in construction of the stop, typically a few dozen ounce-inches. The potentiometer cannot directly absorb the full start/stop impact of a strong motor. In such applications it may be necessary to provide a slip clutch or similar buffer device.

Multi-turn designs are always supplied with stops. Customers requesting deviations from the standard mechanical angle must designate both mechanical and electrical angle requirements.

Electrical Angles

Special electrical angles may be requested, such as 90° for a single-turn or 920° for a multi-turn potentiometer.

A practical way to achieve shorter electrical angles on single-turn conductive plastic potentiometers is by using the "shorted sections" technique. To employ this technique, the relationship of the effective electrical angle (EEA) and the continuity angle (over-travel) must be specified. See Figure 11.13.

Figure 11.13

The most cost effective custom EEA solution for a single-turn wirewound potentiometer is the use of an additional single-wire tap.

When standard electrical angles are shortened in conductive plastic elements, the power rating also decreases. This is the result of maintaining the same resistance value in a smaller area. Generally, the smaller area does not allow the same heat dissipation as a larger area. Thus, the power rating must be proportionally reduced.

In the case of wirewound potentiometers, shortened electrical angles will generally result in a decrease in resolution due to reduced turns of wire.

Resolution is equal to

1 number of turns of wire

As turns decrease, resolution percentage increases.

Linearity and output smoothness for conductive plastic and Hybritron® elements are also affected by shorter electrical angles.

Shorted Sections

If your customer requires shorted sections in a wirewound element, the type of short (voltage or current) must be specified.

Voltage shorts are achieved by adding taps shorted by a low resistance jumper wire. Current shorts are accomplished by actually shorting out individual turns of resistance wire on the element, a more costly process. Thus, voltage shorts are the more practical choice and are strictly for use in voltage divider applications.

If the wiper load is such that the output will be distorted, a current short is required. In this instance, the resistance of the short must be specified (i.e., 10 Ω maximum between the wiper and shorted area).

If your customer requires shorted sections with conductive plastic single-turn elements, the best technique utilizes a current short achieved with a band of conductive material deposited below the CP element. This is the same technique used for current taps and will satisfy both voltage and resistance shorts.