Coarse / Fine adjustment for SMPS output

Thread Starter

dumpystig

Joined Apr 10, 2013
23
I have a Farnell G24-5S PSU which I'm installing in an insulated enclosure so it can be used for powering the multitude of battery operated toys that my 6-year-old son has. The binding posts will be removed and fitted on the top of the enclosure together with a combined 7-seg LED volt / amp meter so the output can be easily adjusted and monitored.

In the attachment you can see that a 25K pot across two front-panel terminals will enable the output to be controlled from approx. 1 ~ 25V. I already have a 1W 25K Linear pot connected and it does adjust the output as intended. But it's only a 270deg pot and it's a real struggle getting to the actual voltage I'm after for a particular toy.

A 3- or 5-turn pot would make adjustment so much easier and more accurate but I'm having no luck finding one at 25K. So I've been looking at coarse / fine adjustment using 2 pots in series (there are more values available that add up to ~25K) but after researching what I initially thought would be a pretty simple solution I am now stumped; there are quite a few different methods that may or may not be suitable for my project and I'm unsure which way to go. Also I can't ascertain what would be an ideal ratio between the two pots i.e. 22K + 3.3/2.7K, or 20K + 5K1...

Has anyone already accomplished what I'm trying to do, or have any pertinent suggestions?
 

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Thread Starter

dumpystig

Joined Apr 10, 2013
23
Too easy! ;)

One of the first things I thought of but I read that it wasn't a good compromise, although I can't remember exactly why.
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
Trimpot wouldn't give me the (tool-less) ease of adjustment which is an important factor as this project is another method of introducing my son to electronics so he will learn what voltage is needed and how to set it, observe the volt / amp display etc..

I'm still doing a lot of research and am amazed at how many different ways and combinations of connecting pots / resistors there are. Ideally a 25K Lin 3- or 5-turn pot would solve all my problems but I can't find one anywhere.
 

crutschow

Joined Mar 14, 2008
24,115
....................

I'm still doing a lot of research and am amazed at how many different ways and combinations of connecting pots / resistors there are. Ideally a 25K Lin 3- or 5-turn pot would solve all my problems but I can't find one anywhere.
Yes, the ones I found were all 20k. You could use one of this in series with a 5k resistor (or a 5k pot if you want the full adjustment range).
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
Yes, I can't find a 25K anywhere. Nearest is a 22K.

So I'm thinking of keeping the original 25K pot and adding a 1K in series, the 25K for getting the approximate voltage and the 1K for nailing it. From both what I've read and actually operating the existing pot I think adding a normal 1K 270deg will suffice.

I could also add a switched resistor network so common voltages may be easily selected.
 

crutschow

Joined Mar 14, 2008
24,115
Yes, I can't find a 25K anywhere. Nearest is a 22K.

So I'm thinking of keeping the original 25K pot and adding a 1K in series, the 25K for getting the approximate voltage and the 1K for nailing it. From both what I've read and actually operating the existing pot I think adding a normal 1K 270deg will suffice.

I could also add a switched resistor network so common voltages may be easily selected.
That should work fine.

If you also use a switched resistor network you may need to add some trimpots to tweak the voltage at each tap to the desired value.
 

RamaD

Joined Dec 4, 2009
328
Too easy! ;)

One of the first things I thought of but I read that it wasn't a good compromise, although I can't remember exactly why.
The description in the image talks of a variable resistor. I am at a loss to understand why a resistor and a potentiometer (one end connected to the tap) in parallel is not good. I am eager to learn if someone could highlight this, please.
 
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