Multi-turn Potentiometer Alternative

[Ranger14]

I am new to electronics and I have several projects the need multi turn pots.. What would be a cheaper alternative to using multi turns pots??

[Marinated]

It depends on the project. If you're following a design that specifies multi-turn pots but you don't personally need the accuracy, use a single-turn pot. Alternatively, you can combine two single-turn pots for fine and coarse adjustment. If you only need the accuracy in part of the range - for example a higher precision current limit for lower values and coarser limiting of higher values - use a log pot. Otherwise, you're likely to use a rotary encoder.  These are digital devices that require simple programming. To replace pots with them, you'll need an analog output. This can be achieved with a DAC, with filtered PWM, or with a digital potentiometer. For a more specific answer you'll need to ask a more specific question.

[FlyingHacker]

The two pot method mentioned above is the most common alternative. Note that you can get some multi turn pots off eBay from China for very little money. They are almost definitely counterfeit, but they will work for experiments.

[Marinated]

If you decide to use the fine/coarse method, there are some tricks you can use for better performance. Try connecting lowish equal-valued pots in parallel between your power rail and ground, and connect the wipers in parallel through different value resistors, say a decade apart. This gives you a much tighter range of output impedance.

[mariush]

Buy just one value multiturn pot, but take advantage of volume prices.
For example, pick 100 ohm or 470 ohm or 1000 ohm ... 100 would be excellent for fine tuning, 470 would be just right for fine control of 100k potentiometers or something like that, 1k is just very common and may be cheaper.

Big knob style potentiometers would be 10$ and up if you buy one piece, as you can see here http://www.digikey.com/short/3rn22n

But the trimmer style potentiometers can be close to 1$ each when you buy just one : http://www.digikey.com/short/3rn2wt  but the price goes down a bit if you order 10 or 25 or more ... i'd say maybe 10% off when you buy 10, 20-25% off when you buy 25 ... here's same page for 25 pcs minimum order : http://www.digikey.com/short/3rn2w1

So when you need some potentiometer for a project, but you don't have the actual value you need, you can put in series a resistor and a potentiometer , or a regular potentiometer and your fine tuning potentiometer

[David Hess]

I prefer fine/coarse controls where a 10 turn control is not absolutely necessary.  I have at least one instrument though, a Tektronix 7A13 differential comparator, which uses a 10 turn coarse control and a 1 turn fine control and that is what is needed for 4 digit resolution; a 10 turn control by itself is not sufficient.

[FlyingHacker]

I prefer fine/coarse controls where a 10 turn control is not absolutely necessary.  I have at least one instrument though, a Tektronix 7A13 differential comparator, which uses a 10 turn coarse control and a 1 turn fine control and that is what is needed for 4 digit resolution; a 10 turn control by itself is not sufficient.

No doubt. The multi-turn pots have more of a tendency to have noise and drift. The dual pot method also allows must faster adjustment, and then fine tuning.

[vk3yedotcom]

If your fine pot is 1k and the coarse pot is 10k then the fine will be 10 times easier to adjust.  Or you might want a narrower range. 

You can do things like series and parallel resistors to limit the range of the fine pot.

Always use a linear pot for your fine pot. The coarse can be log or linear depending on what you're adjusting.

You can alter the taper of pots (ie make a linear non-linear) with resistors as explained below.



An alternative (a bit old fashioned but the clunks may be satisfying) is to have a rotary switch and resistors as your coarse and use the pot as your fine.

So let's say you want 0 - 10 k range and you have a 1k pot. You might use a 10 position switch with resistors of 0, 1, 2, 3k etc up to 9k.  Use 1% resistors and maybe series or parallel depending on how accurate you want (you wouldn't want excessive overlaps or missed coverage).

[David Hess]

I prefer fine/coarse controls where a 10 turn control is not absolutely necessary.  I have at least one instrument though, a Tektronix 7A13 differential comparator, which uses a 10 turn coarse control and a 1 turn fine control and that is what is needed for 4 digit resolution; a 10 turn control by itself is not sufficient.

No doubt. The multi-turn pots have more of a tendency to have noise and drift. The dual pot method also allows must faster adjustment, and then fine tuning.

Drift and noise are better in 10 turn potentiometers if they are wirewound which is almost always the case.  In the case I mentioned above with the 7A13, the problem is that you can see the individual wirewound steps under certain conditions so a fine control is needed.

Another option which is uncommon these days and expensive is to use a mechanical reduction between the knob and potentiometer.

[timb]

You can always use an inexpensive rotary encoder and digital pot IC (or current DAC with external reference input wired backwards). That's of course assuming you already have an MCU on board and the voltage/current going through the pot isn't very high.

[Assafl]

Are you sure they need multi-turn pots and not trimmers? A ten turn precision wirewound can be pretty expensive (albeit NOS show up on ebay sometimes in good condition), but a 15 turn trimmer is a dollar or less (cermet).

BTW - this is a good resource about pots. https://www.bourns.com/pdfs/onlinepotentiometerhandbook.pdf

It has everything more than you'll ever want to know about pots.

[FlyingHacker]

You can get Bourns (likely counterfeit) ten turn pots for around a dollar or two apiece on eBay, especially when you buy five or ten at a time.