Thursday, 11 September 2014

How to use a trimpot (or any three-legged potentiometer)?

First time I saw a trimpot (trim-pot as trimmer potentiometer, 10KΩ in my case) I immediaty started wondering why the heck does it have 3 pins. Would 2 not be just enough? I started googling and, surprisingly, the answer was not that easy to find. I'll share what I found out, but first, let me answer the title question: how to use a trimpot?
  1. First of all, if you flip the trimpot over, you will see 3 pins, just as on the diagram I clumsily prepared:

    That is the back of your trimpot. Well, you get the idea.

  2. Let's describe what's on the diagram. The two horizontally aligned pins (let's call them A and B) are your reference points and the third pin is called wiper.
  3. Now, the idea behind the trimpot is *brutally* simple:
    • Resistance between A and B is always constant (in my case, 10KΩ)
    • Restistance A-wiper and B-wiper varies depending on the trimpot's knob (or screw, whatever you have) so that if A-wiper is 10% of trimpot's nominal value, B-wiper will be (100% - 10%) of trimpot's nominal value.
And that's it, there's nothing more when it comes to trimpots. However, if you want some examples or if you're genuinely interested in what I have to say, please read on!

To follow up, let me list a couple of interesting facts about trimpots. Great majority of them has 3 pins, but there actually are trimpots with just two pins, like this one:

Why 3 and not 2 then? From what I've read, two most popular reasons are:
  • To keep soldered trimpots in place while adjusting their values. A lot of people simply ignore either pin A or B.
  • To utilise the fact that when the resistance on one side increases (e.g. A-wiper), the other one decreases (e.g. B-wiper), and vice-versa.
Additionally, as it turns out, trimpots are not really designed for adjusting their values too frequently - they are expected to work following the adjust-seal-forget formula. If you expect the resistance to change often, potentiometers are probably a better choice.

To illustrate how trimpots exactly work, I ran a couple of simple measurements (top: wiper, left: A, right: B). Before I start, though, please keep in mind that neighter my multimeter nor the trimpot are perfect, so expect some measurement imperfections. Also, the multimeter measures resistance in .

Trimpot's knob goes all the way down the B-wiper side. Whole resistance is allocated between A and wiper.
A-wiper measurement B-wiper measurement

Trimpot's knob goes all the way down the A-wiper side. Whole resistance is allocated between B and wiper.
A-wiper measurement B-wiper measurement

Trimpot's knob is somewhere in the middle of the range. Whole resistance is shared equally between A-wiper and B-wiper.
A-wiper measurement. I expected something around 5KΩ, but 5.00KΩ exactly is pure concidence. B-wiper measurement. 5.07KΩ - that's more like it!

Trimpot's knob is somewhere on the B-wiper side which means this side's resistance will be lower compared to the A-wiper side. Additionally, 7.00KΩ + 3.06KΩ = ~10KΩ, which is the nominal value of the trimpot.
A-wiper measurement. B-wiper measurement.

And finally, even though the trimpot's knob goes all the way down the B-wiper side, A-B resistance is always constant.
A-B measurement.

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