Rheostat "flat end zone"

Is it possible to arrange a potentiometer wired as a rheostat so that instead of going down to zero, it remains at a certain resistance for the final part of it's travel? I don't want to add a resistor in series as it's part of a switched resistance and it would skew the scale. So for a 100k pot, you turn the knob, when it gets down to about 7k, you can keep turning the knob but it doesn't get any lower in value. It's the R in a two-inverter RC oscillator, I just want to avoid it stopping oscillating at the very bottom of the scale.

Probably just a theory exercise as it's a very cheap circuit! This comes under "nice to have".

So you want 7K minimum and 100K maximum? You can do that with two external resistors. Add a 7K in series with the pot, and a shunt the whole string with a 1.5M resistor. I guess that would still skew the scale, but not badly.

throbscottle said:

it remains at a certain resistance for the final part of it's travel? I don't want to add a resistor in series as it's part of a switched resistance and it would skew the scale.

Click to expand...

Add an external mechanical stop to prevent it reaching zero?

There is no other way of making a short circuit (with the pot wiper at the end of the track) a higher resistance except adding a series resistor, or adding in an offset or bias later in the circuit.

Some commercial motor controllers have a low value preset connected to a terminal for the low end of an external pot to connect to, as an alternative to 0V, so the pot "zero" can be adjusted to some small non-zero value & eliminate any dead area.

throbscottle said:

Is it possible to arrange a potentiometer wired as a rheostat so that instead of going down to zero, it remains at a certain resistance for the final part of it's travel? I don't want to add a resistor in series as it's part of a switched resistance and it would skew the scale. So for a 100k pot, you turn the knob, when it gets down to about 7k, you can keep turning the knob but it doesn't get any lower in value. It's the R in a two-inverter RC oscillator, I just want to avoid it stopping oscillating at the very bottom of the scale.

Probably just a theory exercise as it's a very cheap circuit! This comes under "nice to have".

Click to expand...

Just add a series resistor, there's no 'down side' as adding a 7K resistor in series with 100K is still well within the tolerance of the pot. As for the scale, you simply scale it accordingly.

Hi Nigel - it puts a 1 to 2 second difference on a timer where the pot varies the upper 20(ish) seconds of a switched stage - so it's 0-20, 20-40, 40-60, 60-80, 80-120. It's mostly cosmetic I suppose, but I don't want all my ranges to start at 2 and I don't want the first range to be 2-off from the rest. It's not meant to be an accurate timer (it's allowed to be seconds off overall), but I'd like it to be consistent!

I think Yli might have hit on the best answer, though it's probably just better to live with the annoying knowledge that it stops working at under 2 seconds.

rjenkinsgb - interesting you mentioned that. I had thought of adding a lever to the pot's shaft which can operate a micro-switch, but then I decided it's overkill for what it is.

Unless you get a pot manufacturer to custom make something for you, or add a mechanical limit, then adding external resistance is about the only way to limit the resistance range.

Be aware though, that the tolerance of most pot's is fairly poor. +/- 20% is typical. And +/- 30% is common. If you need tighter values you may need to hand select the pots. Or individually adjust the parallel and series resistors.

Get a dual concentric shaft one. Use one for position. When it's lower than some value put in a fixed resistor.

You could use a vernier dial and adjust the pot centering plus fine trimming the limits with resistors or presets to match it, so you get a repeatable value through the limits of the dial scaling?

You may have to try a few different pots to get one that works well; different makes tend to have slightly different rotational angles and sizes of dead spots at the ends of the track.

eg. **broken link removed**

(Don't confuse those with "turns counter" dials for multi-turn pots - the reduction is internal to the dial mechanism with this style).


Or just switch the low end "offset" resistor in only when the device is on the 0-20 scale?

You could open the back of the potentiometer, cut a slit from about 7000 ohm region to the end of the rotation (zero ohm) in the resistive disk and then use conductive adhesive to add a 6800 ohm SMD to bridge the gap.
However, even this solution seems like it is more work than it's worth but the only workable solution to your problem and your stated constraints.

There is/was such things, I saw one a while back, I think it was marked something like 47k - 4k7 stop, mijn resistance in one direction was 4k7.