Trim pot calculator? - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: November 18, 2017, 02:54:13 pm
content: (sorry for 3 posts in a row)Does anyone know a trim pot calculator online or simple(r) rearranged equations to calculate them?Basically today I needed two trim pots to tune a 555 to as close to 50% duty cycle and 50Hz as possible. I found it tricky to calculate the correct values for the two trim pots to make them fairly precise.The inputs would be:Minimum resistanceMaximum resistanceTrim Pot resistanceI figured out I need to put a resistor in series with the trim pot to set it minimum value or it would create a sub 1 ohm short. Then I had to use a parallel resistance calculator to get an example pot position resistance.In my example I wanted a 14.254KOhm resistance but I wanted a full sweep of the pot to cover about 200Ohms. There was much faffing with online calculators.If there isn't one I suppose I could write one in python, but sure this is a commonly solved problem so either there will be online calculators or the maths is easier than I thought.

author: matseng
author_rank: Frequent Contributor
timestamp: November 18, 2017, 03:54:50 pm
content: Are you using like 0.001% super precision capacitors considering your desired resistance is specced down to a ridiculous precision. A normal cap got 5-20% tolerance so you need to be able to trimpot away that much error if you are aiming for a specific frequency like 50hz.Use a 10-turn trimmer if you want do to something precise. But on the other hand most caps have a rather bad tempco so the 50.xxx Hz will end up at some other value after a while anyways.Dividing down a crystal oscillator would probably be the simplest way to achieve any kind of semi-precise and semi-stable frequency.But it all boils down to how precise your needs really are. Driving an (alarm)clock with the 50Hz or just a 12v->230volt inverter for powering a TV or something similar are two completely different things....

author: paulca
author_rank: Super Contributor
timestamp: November 18, 2017, 08:34:40 pm
content: There was no need to be that precise, I just decided to try and do the maths and get it as close as I could.In the end I got 50.0xxHz on the oscilloscope.The question about trim pots is more general as they seem to be quite common for anything you want precision trimming on.As I understand it you can set the minimum and maximum of the circuit via parallel and series calculations. It's that bit that is a bit of a faff to work out, the only way I have found (as I'm loosy with anything but basic algebra) is trial and error in a parallel / series online calculator.+------R220--------+-| |-+-R220---T0-5K---+Gives you from 220/220 in parallel to 220/5220 in parallel. 110 Ohms - 211 Ohms across the full sweep of the trim pot, instead of 5K which is useless for trimming.I'm pretty sure a calculator that would take the 110 Ohms and 211 Ohms and result in one or more possible combinations would be extremely useful. Would be easier if you specify the trim pot size.Note it could be something like 4K - 5.4K to trim a 4k7 by +-500Ohms. Doesn't have to be for precision.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2017, 09:28:52 pm
content: Most of us simply use a series resistor + a trimpot so that Rs+Rtt/2=Rnom and choose the trimpot value (usually you can get 1, 2 and 5 in any decade) to get a suitable trim range. If you need low tempco limited range precision trimming the circuits get a lot more complex and the easiest option is to SPICE it with a list of values to step through.