I've been thinking about making a variable constant voltage, variable constant current PSU for quite a while now, when suddenly I stumble upon these little things. They look like something that would work nicely for my low budget, low expectation variable PSU.
The problem is that once mounted in some sort of a box, the trimpots are not accessible. Those would have to go to the front panel.
How does one go about attaching a finger-friendly knob on such a small device? Or should I just abandon idea and replace them altogether with something larger?
Without actually having the device and measuring, my guess is that those would be 10 turn 10K linear trimpots? Am I in the ballpark?
Here I hit another roadblock. I'm unable to find such trimpots on ebay - most are single turn, and I like the idea of fine control. On Farnell they start at about 10 pounds, which is more than the PSU (and I'd need four since I fancy the idea of having two channels).
Any suggestions regarding either the knob attachment issue, or the procurement of pots that are somewhat more affordable?
That is true.
My thinking was to put a shaft between the trimpot and the knob. The knob would be fixed to the sturdy shaft which would be restricted from much movement (except rotation, obviously) by the enclosure of the PSU. The shaft extends to the trimpot and ends with some sort of a socketed screwdriver that goes over the little screw on the trimpot. Trimpot is fixed to the enclosure as well.
Hi, yes, replace the trimpots with panel mounted pots, their value will be marked on them or you can measure them with a DMM.
The trimpot concept is a set and forget. low usage component. Also physically it would not take the punishment.
Now there is a difference between a 10t trimpot and a 10T multiple pot.
The trimpot is really just a 270Deg pot with some wormdrive gear.
The 10T Multiple are a long spiral track pot, no gears, usually very expensive, any of these pots under $20.00 I would be very suspect as to quality (noise) and its lifespan.
I ordered a few, so when they arrive I suppose I could take one apart to see what's inside.
As for longevity, I can always replace them if they start showing wear, and if the PSU shows promise, I might even buy the good expensive stuff (or likely scavenge it from some old equipment somewhere).
Digital potentiometers might be a way to go also.
Have a rotary encoder to send it pulses to step it up/down in resistance.
I don't know what resistance range you need. You can get 1024 steps of resistance. For example:
And do the same - 2 in series for coarse & fine adjustment
100K for 100 ohm increments, and 10K for 10 ohm increments.
Can even get 2 pots (of same value) in one package, use 2 with different values for controlling 2 output levels.
10 ohm increments does seem a little finite compared to multiturn trim pot.
CrossRoads:
Digital potentiometers might be a way to go also.
Have a rotary encoder to send it pulses to step it up/down in resistance.
I don't know what resistance range you need. You can get 1024 steps of resistance. For example: MAX5484EUD+ Analog Devices Inc./Maxim Integrated | Integrated Circuits (ICs) | DigiKey
And do the same - 2 in series for coarse & fine adjustment
100K for 100 ohm increments, and 10K for 10 ohm increments.
Can even get 2 pots (of same value) in one package, use 2 with different values for controlling 2 output levels.
10 ohm increments does seem a little finite compared to multiturn trim pot.
The problem with this is that digipots that go much above 5v are few and far between, but they have to be able to take the output voltage (IIRC, it formed the high side of a voltage divider - i forget the details). MCP makes some that go up to 36v; that's the best I was able to find when I was doing much the same thing - unfortunately, the minimum resistance of these is too high, and I found that I needed a second pot between the voltage set and ground (with computer control managing these two and working out the right settings for a given voltage) to get the full range of output voltages - and the damned things only come in TSSOP-14, one pot per package. I think there are some ones that go up to 10-15v that might work (that wasn't high enough for me, so I didn't investigate those).
CrossRoads:
You could use 2 in series; a high value as a coarse adjustment, and a smaller value as a fine adjustment.
This is what I would do, even over a 10 turn panel mount pot.
To set a voltage, you turn the fine knob to mid scale, turn the coarse knob to get you close to the desired value, then adjust the fine knob to dial it in to the final value. It quickly becomes second nature.
Besides added cost, the disadvantage of a 10 turn pot is a lot of cranking when you want to make large changes.
CrossRoads:
Digital potentiometers might be a way to go also.
Have a rotary encoder to send it pulses to step it up/down in resistance.
That could have some merit as well, but it introduces extra cost and complexity. (But when has that ever stopped a hobbiest from building something cool. 8) )
From a usability point of view, it adds the ability to have the turning speed of the knob control the adjustment step size: Turn the knob quickly to make coarse adjustments with a large step size, then turn the knob slowly to make fine adjustments with a small step size. It's all a matter of software (and finding a digital pot with a wide enough control range and small enough step size.)
But then, there are the concerns brought up by DrAzzy... Hmmm...
Yep, two panel mount pots: one coarse, one fine. That's the way I'd go.
