Discussion in 'General Electronics Chat' started by s57aw, Feb 12, 2014.

1. ### s57aw Thread Starter New Member

Dec 18, 2013
12
0
Hello,

I would like to mesure reverse voltage difference from linear potenciometer with 40uA Ammeter. I get 10.70v DC from pot and that would be full scale on ammeter and 9.50v need to be 0 on ammeter, so I need to measure 1.20 v difference and reflect it to 40uA scale.

Would like to use that as indicator for antenna direction.

I need to add shunt resistor and probably some resistors / trimmer in series to get just a right value.

I am not a professional and I would be happy to get some help on this values

Regards,

Robert

2. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
Just so I understand the question.... You want a 40uA meter movement to indicate zero when there is 9.5v and full scale when there is 10.7v. Is that correct?

Again, just so I understand the conditions.

What is the voltage source feeding the potentiometer?

3. ### s57aw Thread Starter New Member

Dec 18, 2013
12
0
Yes, that is correct..

Zero @ 9.5v and full scale @ 10.7v on 40uA meter

Source feeding voltage to the pot is 12.05v

Robert

4. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
I would start by building a voltage divider for one side of the meter (- side) to put 9.5 v on the negative terminal. I would then place a resistor (50K trim pot) in series with the + terminal of the meter and the wiper of the pot. The The voltage divide could be another trim pot , but would stay with a fairly low value with it, such as a 5k. With the antenna pointed to where there is 9.5V, adjust the pot used as a voltage divider for zero on the meter. Move the antenna to get the 10.7 volts and adjust the 50K series pot for full scale. Verify both ends and adjust until no further adjustment is needed.

5. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
Here is a drawing of what I suggested.

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6. ### s57aw Thread Starter New Member

Dec 18, 2013
12
0
Thanks a lot, it looks like a great starting point

Robert

7. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
As a suggestion to help protect your meter, use a volt meter to set the wiper of the ZERO pot to match the voltage from the antenna pot. Also, set the Full Scale pot to the full end of range with the highest resistance. After that, it is just move-adjust, move-adjust, etc, etc.

8. ### crutschow Expert

Mar 14, 2008
13,019
3,235
You may want to protect it from overvoltage and excess current. Depending upon the meter resistance a diode or two across its terminals may provide such protection.

9. ### s57aw Thread Starter New Member

Dec 18, 2013
12
0
Great support ... It works as suggested, R2 about 2.5k and R3 about 25k did the job.

About the protection diodes over meter terminals... do you suggest diodes in both directions?

Thanks again

Robert

10. ### crutschow Expert

Mar 14, 2008
13,019
3,235
If the voltage polarity can only be one direction then you only need diode(s) in one direction but for general protection, diodes in both direction are better.