Setting max limit of a potentiometer knob?

[rcx132]

I've finished the alternator upgrade on my buggy. The regulator is external type and has an external potentiometer that allows the driver to set the voltage. I thought this could be useful for controlling the charge current after winching.

The regulator can dial up to 20V, which will damage the battery and ECU if accidentally set that high.

I've done some reading, and apparently I can add a resistor in series with the red pot wire to limit the maximum voltage the driver can dial in.

When the regulator is set to 15.0V (the maximum I want to allow), I measured the following resistances after removing the pot:
521 ohms between black and red
198 ohms between black and white
330 ohms between white and red

Am I right in saying that I need to fit a 330-ohm resistor?

 

[Seadog]

That's what I'd do, then turn the pot fully clockwise, of course.

 

[voipio]

It's probably not as straightforward as that, though 330 Ohms would be a useful starting point.

Were the potentiometer resistances, given in the original posting, measured with the pot out of circuit? If not, then there are likely to be other circuit elements, to which the pot is connected, which will influence the measured values. 521 Ohms between black and red is not a standard potentiometer resistor track value. Potentiometer track resistance values tend to be nominally 470, 680, 1000, 1500, 2200, and 3300 Ohms, being part of the so-called E6 resistor series. There could be exceptions, though, and the tolerances tend not to be as tight as they are on typical fixed value resistors.

 

[Agroshield]

My electronics is not good, but I was under the impression that if you want to set a maximum voltage on something, you can use a zener diode. That might be either an alternative or a safety backup to the potentiometer.

 

[voipio]

The zener diode would need to have considerable power handling capability (many 100s of watts), if it's going to serve as a "clamp type" limit for an alternator regulator output.

 

[Pete.]

Ive done this using two potentiometers to adjust the max output voltage of a phase angle chip. I'll see if I can find the schematic.

 

[6ply]

I've finished the alternator upgrade on my buggy. The regulator is external type and has an external potentiometer that allows the driver to set the voltage. I thought this could be useful for controlling the charge current after winching.

The regulator can dial up to 20V, which will damage the battery and ECU if accidentally set that high.

I've done some reading, and apparently I can add a resistor in series with the red pot wire to limit the maximum voltage the driver can dial in.

When the regulator is set to 15.0V (the maximum I want to allow), I measured the following resistances after removing the pot:
521 ohms between black and red
198 ohms between black and white
330 ohms between white and red

Am I right in saying that I need to fit a 330-ohm resistor?

View attachment 473795 View attachment 473796

So you want a reference voltage adjustable 0 - 15V output? Can we assume this goes into a high impedance input, so does not load the potential divider?

 

[Maker]

When the regulator is set to 15.0V (the maximum I want to allow), I measured the following resistances after removing the pot:
521 ohms between black and red
198 ohms between black and white
330 ohms between white and red

Am I right in saying that I need to fit a 330-ohm resistor?

Assuming it really is a 521 ohm pot (Which is unusual), you want a 173 ohm resistor (A third of the total pot resistance) in series at the "high" (Presumably red) end.

 

[6ply]

This is one way you could do it.

View the graph and see how the output voltage is adjustable between 0 - 15 V.

The Zener diode has a minimum current to reach the Zener voltage. Typically, this is about 5 - 10 mA.

The voltage across R1 = input voltage - Zener voltage

R1 = voltage across R1 / current through R1

If the "load" has any significant current, this also has to be considered in the above calculations.

 

[Misterg]

I've done some reading, and apparently I can add a resistor in series with the red pot wire to limit the maximum voltage the driver can dial in.

When the regulator is set to 15.0V (the maximum I want to allow), I measured the following resistances after removing the pot:
521 ohms between black and red
198 ohms between black and white
330 ohms between white and red

Taking that at face value, and assuming that it's just acting as a voltage divider, I reckon* you'd need an 880 ohm resistor in series with the red wire to give the same ratio as you have now, but with the pot fully clockwise. 820 Ohm is the nearest common value:

You currently have a voltage divider ratio of 198/(198+330) = 0.375.

You're trying to get the same ratio, but with the full value of the pot where you're measuring 198 ohms currently. The pot resistance will make up 0.375 of the total, so the total resistance must be (198+330)/0.375 = 1408 ohm. Take away the resistance of the pot (198+330) and you're left with the resistor being 880 ohm. (820 ohm nearest available value)

(I've used 198+330 = 528 ohms for the value of the pot, but you could equally use the 521 ohms you measured and get a similar result.)


[* I might not be right, though!]

 

[richie00boy]

It will be a 500 ohm pot, the 521 ohms you measured is within it's manufacture tolerance.

I'm an electronics engineer. Full marks go to Maker however the preferred value you will be able to buy will be 180 ohms, which will mean you don't quite hit 15V but is probably safer than blowing things. It will need to have a power rating of at least 0.25W.

The zener idea isn't practical with your pot value so forget that.

 

[voipio]

If the potentiometer is being used as a simple potential divider, with negligible load and, viewed from above, the fully clockwise position gives the maximum output voltage of 20V, then I would agree with @Misterg 's series resistor calculations. However, the wiper position at the 15V setting, suggests either the potentiometer position with respect to output voltage is non linear, or the minimum setting doesn't give 0V output.

@richie00boy is right about the likely nominal pot value, since the most commonly available value progression is in decade multiples of 1, 2, 5. I was dealing with some 220k and 470k pots the other day, which kind of threw me.

 

[Misterg]

However, the wiper position at the 15V setting, suggests either the potentiometer position with respect to output voltage is non linear, or the minimum setting doesn't give 0V output.

Given its purpose, I suspect it is something like 12-20V range (the measured values suggest an 8V adjustment range, anyway: (20V-15V)/(1-0.375) = 8V).

...always assuming it is working as supposed.

 

[voipio]

If, as @Misterg suggests, the top end of the pot (red wire) represents 20V at the output, and the bottom end of the pot (black wire) represents 12V then, by my calculation, the bottom end of the pot will be connected internally to the 0V supply rail via 792 Ohms (12 x 528 / 8), giving a total for the resistor chain of 792 + 198 + 330 = 1320 Ohms. For the top end of the pot to be at the equivalent of 15V, the series resistor connected to it will need to be 5 x 1320 / 15 = 440 Ohms.