POSITION TRANSMITTERS
CALIBRATION
A - 3
The resistive change of the potentiometer is approximately 9.9
ohms (nominal) per degree of cam rotation, with a total resistance
of 2000 ohms (nominal).
Application of the potentiometric option must consider the possibil-
ity of end resistance wrap-around.
POTENTIOMETRIC APPLICATION EXAMPLE
A customer has a Type AV positioner with a 90-degree, direct act-
ing cam, potentiometric position transmitter, along with a 24-VDC
supply. The customer wishes to output a five-VDC signal when the
Type AV positioner output is 80 percent.
1. Determine the angular rotation of 80% of 90°.
0.80 x 90° = 72°
72° of cam rotation is 80% of 90°.
2. Determine the resistive change of the potentiometer through
72° of rotation. For every degree of rotation the resistive change is
9.9Ω (nominally):
72° x 9.9 Ω = 712.8 Ω
The change of resistance for this rotation is 712.8 Ω, and appears
between TB1-1 and TB1-2.
3. To guard against wrap-around, the minimum resistance at the
potentiometer wiper at zero percent cam rotation is set to 200 Ω.
Therefore, the resistance between TB1-1 and TB1-2 should vary
between 200 and 912.8 (712.8 + 200 = 912.8) Ω for a cam rotation
of zero to 80%.
4. Determine the required current in the potentiometer that
develops 5 VDC at the wiper at 80%:
5 VDC ÷ 912.8 Ω = 5.48 mA
5. Determine the nominal voltage to produce 5.48 mA across the
potentiometer:
5.48 mA x 2000 Ω = 10.96 VDC
6. Determine the dropping resistor value that reduces the supply
voltage to 10.96 VDC:
Connect a 2380-ohm variable resistor in series with the power
supply. A variable resistor is required to account for the 20-percent
24.00 10.96–() VDC
5.48 mA
---------------------------------------------------- 2380Ω=
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