Calibrating a Thurlby PL320

[WaveyDipole]

Now that I have recapped both PL320 PSU units I decided to check the calibration. I have the older units in the brown case and they have likely not been checked for a long time although they did not seem very far out - just a few millivolts on the DC voltage.

I first used a voltage reference to calibrate the voltage reading on my DMM and then used this to check the PSU following the Thurlby instructions. I believe I have identified the correct service manual version, but I am unsure about the last step. This reads as follows:

Set VR2 and VR3 (course and fine voltage controls) to maximum. Set VR1 (current setting control) to minimum. Adjust PR2 such that output voltage collapses to about 10 volts.

I have added the clarification brackets. Now this is very difficult to set as the voltage never actually settles, hence it says 'about 10 volts'. The issue though is that on all the previous steps which I was able to follow precisely, the settings were fairly close the value required and just needed a bit of a tweak. On this one, however, the voltage on both PSU's was originally around -3 to -4v, a long way from 10v. I've set it to around 10v as per instructions, but am wondering whether this is actually correct? I can undestand one unit being far off for whatever reason, but not both and by a similar amount?

[mhennessy]

Don't worry about

This is just a fine-tweak of the offset voltage of the current regulator section. In theory, if it's wildly wrong, then the preset current (DC output switch off) wouldn't match the actual current limit (when DC output switch is on).

Whenever I've made PSUs of this topology, I've not bothered to include the adjustment.

I did one for a course we run at work, and over the last decade or so, we've probably built between 50 and 100 of them. Very, very occasionally we find one that differs by a couple of counts, and to fix it, I just swap out the op-amp! But the spec is quite loose anyway (+/-2 counts). If you're interested, a (very) old version of the handout is here: https://dl.dropboxusercontent.com/u/...-%20Mk%202.pdf and the schematic is here: https://dl.dropboxusercontent.com/u/9900549/PSU.gif

Lining up those PL320s is a nightmare - the presets have too much adjustment range and are far too "touchy". Classic design error. Good luck

[WaveyDipole]

Yes, although one or two of those presets were quite straightforward to set, a number of them were rather 'touchy' and so tricky to set accurately. Thanks for your insight and I will not worry about it too much. I have also downloaded your handout and schematic and will have a look at them later.