I just got two old Farnell Industrial power supplies that do 6v each at 30amps.
The voltage and current limit are set by trimmer pots at the moment, that can be accessed via a screwdriver through the front panel. Probably to start people knocking / playing with the settings when they’re being used in labs or industrially. That’s not very easy to adjust however, so I’m looking at replacing them with either a digital resistor or just pots mounted to the panel it’s self.
I can see the values on them. The voltage trimmer is a 2k log pot.
But the current ones are causing me problems. I suspect they’re logs also but, when I try testing the pins, it’s reading a little below 2k (when they have 2k2 written on them) and when I turn them down, they won’t go below 40 ohms.
They squeak like rusty doors and are stiff. REAL squeaky and stiff!
I’ve tried sweeping them back and forth to see if there’s muck on the track, but it’s not doing much. Weirder still, both do exactly the same thing (stop dead on the same points), indicating it may not just be muck.
Is there any possibility these pots have fixed end values on the wiper that aren’t it’s full limits? I don’t want to just drop another 2k2 in and then roast the supplies when it DOES reach the full limits of travel. But I’m also wondering if this may just be that they’re a bit crappy and weren’t sweeping the full range in the first place. They do look surprisingly nasty compared to the rest of the unit(s). Which weigh in at a mere 22kg a piece (ahhhh… linear wastage… :mrgreen:)
I don’t have a can of squirty cleaner, and money is tight at present. Squirty cleaner is mainly DCM and propellant isn’t it? I have a container full of 99%+ DCM, and an airbrush.
The odds that you have some special pot that has an intentional 40 ohms of minimum resistance is minimal. What you would want to do is to measure the voltage across those pots when the supply is in operation. From those you could get an idea of the power those pots are dissipating. Make sure that you don’t overheat your new pots.
For a power supply, replacing a log pot with a linear should be OK.
most power supply pots are linear (log would be a disaster), 2k2 should be 2200 ohms, special pot with a stop at 40 ohms would not be unknown. electronic mail order places and some repair shops sell a spray for shooting into potentiometers that is specially made to lubricate them without injuring them. substitutes not advisable.
Looks like I’m going to have to be careful replacing it.
As I say, these things are fairly old. The regulator inside as a huge array of palm sized heat sinks about a foot across, half a foot deep and half a foot wide, lined with pass transistors. There aren’t any integrated regulators inside, the entire thing looks discrete.
I’m fairly sure the first pot is a log, as the resistance falls very slowly for most of it’s travel and then nose dives. It may be something to do with how they’ve organized the discretes for regulation.
I can also feedback a voltage for regulation. They came from what looks like a Farnell service, with a stamped tag stuck to one saying “set to 5v” and what looks like a big zenner diode stuck across the feedback pins. I have been considering using an LM or maybe even the Jung Super regulators to make that more variable.
I would immediately suspect muck if the pot was flickery or stopping at different points. But the way both stop at 40 ohms (on two different units) suggests that’s a manufacturer defect or it’s been purposefully designed in.
