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Re-calibrating bench PSU

I need another PSU on my bench and I remembered I had this thing in my junk closet, a SMPS-based supply from

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Something was wrong with it I couldn't remember but all I recalled was this is what the power PCB looked like, from the "factory" mind you:

sweet baby Jesus I don't know what they did to it. Looks like a return someone hacked back together in a shed in China. I plugged it in again and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and the switch (2SK1358) with a STW9NK90Z: , and the NTC with an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the display is off-calibration, and I doubt I'll ever find a service manual for the thing. On the controller PCB there's a button which seems to reset the current display to zero, it reads about 150 mA on all voltage settings with nothing connected otherwise. There are also two smt pots labeled "RP1" and "RP2". RP1 seems to adjust the voltage display value, and RP2 does...not very much as far as I can tell. They seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit settings, load, and pot tweaks I should try to calibrate this thing?

Reply to
bitrex
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Here's a schematic of the display/current and voltage sense logic board:

Reply to
bitrex

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.

Reply to
sea moss

I can wrangle current sense trimpot "RV1" for days and the voltage across C10 never changes so shorted pot, looks like. :(

Reply to
bitrex

Yep that is right. Schem:

Display reads the correct voltage but wrong current. The voltage across current sense output cap C10 never budges much no matter how long I spin that trimpot so looks like I've also got a shorted pot, there.

There are two more pots on the control board to adjust the current and output voltage limits of the knobs so I guess you could "mod" this into a higher voltage supply straightforwardly, so long as you stayed within the limits of the associated components and tweaked the current limit down to keep max output power limit the same.

Reply to
bitrex

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Does the trimpot have a mechanical start and stop point, beyond which you c an't turn it? Some of them will allow you to keep turning forever, and onc e it hits the end of the resistance range the value doesn't change anymore as you continue to turn. So you might have to back up by like 20 revolutio ns to find the active range.

Reply to
sea moss

They're not using low-offset op amps or precision resistors here as far as I can tell so getting the display readout as good as it can be is I think one of those things like intonating a guitar, tweak it at the low end, turn the voltage up to max and it'll read high there, tweak it down a bit, turn it back down to the low end and adjust it again there, and just kinda do the best you can

Reply to
bitrex

No mechanical limits, but yeah I should check if I haven't just spun it out of the active range by accident

Reply to
bitrex

Meh, can't find anywhere on it "active." Retouched the solder joints that didn't help. Have to pull it and see what's what.

Reply to
bitrex

Intermittent pot, looks like. Resistance drops to 1.5 ohms as you turn it at various points. I'll try bodging in a better 1k trimmer this and weekend see how it do.

Reply to
bitrex

I can't read your schematic, but normally, there should be two pots for calibration. One for gain and one for offset.

You adjust the offset at a low voltage, and then turn the voltage to high. If it reads high, you reduce the gain and then go back to low and readjust the offset. Keep going back and forth until you are happy.

--
RoRo
Reply to
Robert Roland

'Fraid there are only two adjustment pots on the sense amp board total, for both current and voltage signal input to the uP ADC.

Looks like the output voltage and voltage across the current sense shunt are both fed to a LM358 gain/buffers and then the trimpots are the series component of a voltage divider hung off the output of the '358 buffers, prior to going into the ADC.

The trimpot in the current sense line was bad. Replaced that and it's looking better, I can tweak it to adjust the current display value. Problem is now it won't go high enough, 10 volts across 50 ohms reads

100 mA and I can use the trimpot to push that up to 120 mA and down to 0, but not up to 200mA where it needs to be.

There's a button on the PCB that zeros out the current display so now I guess I have to figure out where the trimpot and other controls need to be set so I can hopefully zero it appropriately and push that up to the correct value

Reply to
bitrex

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