FAQ's (2)

Q: Homebrew rotors and the Potentiometer election

A: Most antenna rotors include a POT. This POT provides a Voltage feedback when it's powered and it's used by the Control Unit (CU) for displaying the Antenna Position. This POT uses to have 500 Ohms. This resistance value provides a low current, when it's powered at low voltages.
Example when it's powered at 5V

I (Amp)  = V / R = 5V / 500 Ohms = 10mA

The power it must supports is as follows:

Pwr (watts)  = R x I² = 500 x (0.01)² = 0.05 Watts

When we have a homebrew rotor we must add a Potentiometer that will provide at our radio shack the antenna position. A good value choice could a POT from 500 to 2K Ohms. 1K Ohm POT uses to be a value that can be easily found at any electronic shop. 
We'll use this POT as a Voltage feedback so, we'll read the Voltage that is returned from the POT. 

As most POTs use to have a 300º mechanical rotation, if it's connected directly into an azimuth antenna rotor (360º) axis without a reducer, it will be damaged soon. Sometimes to add a reducer it could be a complex task, or can add some error (mechanical tolerance). So a good election it's to use a Multi Turn Potentiometer. It's very easy to find a 3-Turns Pot - 1K Ohms.
One of the more important factor for a good accuracy and precision it's the LINEARITY. Don't worry if the resistor tolerance it's not too good, we'll measure the Voltage Feedback, so it's essential a good linearity. We could get a 3 Turns Pot - 1K better than +/- 0.25% from SPECTROL, BOURNS, etc. Those are available on-line at: 

• www.farnell.com
• www.rs-components.com

As mosts ADC uses to operate from 0V to 5V, if we power the multi turn POT @ 12Vcc, it means 1 turn will genrerate a Voltage feedback of 4Vcc (12V / 3turns). However in this way, we are making that the POT left limit (CCW) will be also the antenna rotor limit (left or CCW) It can causes that if at any time, we turns some few degrees the rotor into the left, the POT will crash. We can avoid this if the POT is powered using +V and -V. In other words:
If we power a 3 turns POT using:  +12Vcc and -12Vcc, the feedback voltage equal to ZERO is when the POT is at the middle position or at 1.5 turns. The formula for convert V into the Anglee is as follows:

Vx = ((V2 - V1) * Ax / TOT_A) + V1

Where:
V2 -----> it's the voltage Max @ Pot
V1 -----> it's the voltage Min @ Pot
Ax -----> Anglee at position X
TOT_A --> Total anglee of the Pot

Example 1 - Vx @ 1.5 turns:

V2	+12Vcc
V1	-12Vcc
Ax	1.5 turns = (360º + 180º) = 540º
TOT_A	1080º

Vx = ((12 - (-12)) * 540 / 1080) + (-12) =  (24 * 540 / 1080) - 12 = 0V

It means at middle scale (1.5 turns) the V will be 0V

Example 2 - Vx @ 1.5 turns + 90º:

V2	+12Vcc
V1	-12Vcc
Ax	540º + 90º = 630º
TOT_A	1080º

Vx = ((12 - (-12)) * 630 / 1080) + (-12) =  (24 * 630 / 1080) - 12 = 2V

Example 3 - Vx @ 1.5 turns + 360º:

V2	+12Vcc
V1	-12Vcc
Ax	540º + 360º = 900º
TOT_A	1080º

Vx = ((12 - (-12)) * 900 / 1080) + (-12) =  (24 * 900 / 1080) - 12 = 8V

CONCLUSION

If we assume that the 1.5 turn it's the "0" reference, we have a secure margin at both sides:

• To the left (CCW) movement (1.5 turns)
• To the right (CW) movement (0.5 turns)

For elevation we could use any standard POT (300º mechanical rotation) and powered similar, but with different values. Example:

Example 1 - Ax? for Vx = 0V:

V2	+12Vcc
V1	-6Vcc
Vx	0V
TOT_A	300º

Vx = ((V2 - V1) * Ax / TOT_A) + V1 so

0 = ((12+6) * Ax / 300) - 6

Ax = 6 * 300 / 18 = 100º degrees

So you should mount the Pot at 100 degrees for the Horizontal position, so the V feedback will be 0V

If 100º will be the Horizontal limit, the Vx @ 180º from this point will be:

V2	+12Vcc
V1	-6Vcc
Ax	100º + 180º = 280º
TOT_A	300º

Vx =((V2 - V1)*Ax / TOT_A) + V1 = (18 * 280 / 300) - 6 =  16.8 - 6 = 10.8 V

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