# How to give specific range of resistances?

• posted

Hi there - I need some odd valued potentiometers. I have two different setups I need:

one that ranges from 178.2 - 207.1 ohms

and

one that ranges from 96.3 to 168.6 ohms

So - my first inclination was to just use a single resistor for the smaller value and a potentiometer to cover the range. So for the first I'd need a 178.2 ohm resistor, and a 28.9 ohm pot. However - 28.9 ohm pots seem to be slightly less common than I would like. Same goes for the second setup and the desired 72.3 ohm pot. Now - I can fudge around on these values a bit - but not too much. These values are setting the output voltage of a light, and if that voltage goes too high I suspect the magic smoke of the device will be released.

So - any suggestions? I don't need very linear operation from the potentiometer, if that matters. I don't think many pots have mechanical stops to stop you from setting the value too high. What do I do?

Thanks!

-Michael

• posted

Why do you need such precise resistances? What does turning the pot control?

You can do a series-parallel pad on a pot to force the limits, but it's mathematically a pain.

John

• posted

It doesn't need that level of precision - but it needs to be fairly close (+-10%).

What is this series parallel pad you're referring to? Would that just be putting a resistor in parallel with the pot?

-Michael

• posted

Close. The pad is in parallel with the rheostat and decreases its Rmax to delta R.

Better? Hard to tell as I am still not sure what you would like to accomplish.

• posted

50 ohm rheostat shunted by a 69 ohm resistor gives an Rmax of 29 ohms. Rmin is obvious. Add a series 178 ohm resistor and you'll have 178 to 207!
• posted

------+--------r1------------+-------- | | | | | | +-----r2----pot--------+

As I said, mathematically a nuisance. If it's an opamp circuit, there are usually cleaner ways to do this.

John

• posted

o | | +----+---+ | | | .-. .-. | Rp | | | |

• posted

The range is the specification of a two-terminal device (like a potentiometer with wiper connected to one fixed terminal), i.e. a rheostat.

Consider a parallel combination of fixed resistor R1 and (series-combination R2 and Rvariable);

R1//R2 = 178.2 Ohms, R1//(R2 + Rvariable) = 207.1 Ohms

is a pair of equations in two unknowns (R1 and R2) and Rvariable :== 100 Ohms is a free chosen quantity. That freedom is required because pots aren't available in odd values.

Solve for R1 and R2 and if they aren't negative, that's a good solution. Otherwise, go to the next standard Rvariable value.

As others have noted, protection against overvoltage/overcurrent usually uses other techniques- a current limit could be a LM317 with program resistor, or a voltage limit could be a TL431 with program potentiometer. Rheostats are kinda... old-school. Sometimes that's good.

• posted

As an alternative to adding a shunt (parallel) resistor, just put mechanical stops on the pot.

If your tolerances are 10%, it's a bit silly to be giving resistor values to 0.1% --

One might suspect there's a better way to do things. Can you tell us more about the application?

One thing to beware of with potentiometers is that the wipers sometimes fail to make good contact with the element. In that case, the resistance goes to "infinity" or at least some large value. It can be worthwhile when you want a rheostat function (a variable single resistance) and you have a potentiometer (a resistance with a variable tap point) to tie the wiper and one end together. Then the resistance will normally only jump to the end-to-end value if the wiper fails to make contact properly.

Beware about DC current through the wiper. Some pots handle that gracefully, and others do not. Wire-wound pots are probably OK.

Cheers, Tom

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