MFOS wall wart style PSUs crapping out - why?

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harpooneer
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MFOS wall wart style PSUs crapping out - why?

Post by harpooneer » Wed Oct 13, 2021 4:35 pm

I've got a case with 7 modules with a fairly modest combined current draw of approx. 230mA from the +12V rail and 150mA from the -12V rail.

I've also got two identical PCBs of an MFOS wall-wart style half-wave rectified PSU design with integrated 8-spot bus board. The circuit has the usual extra diodes around the regulators, which are fresh Mouser stock and have heatsinks. The 12VAC wall wart is rated for 1.5 amps.

Both PSUs are perfectly happy powering all 7 modules on their own for extended periods of time and the heatsinks on the regulators don't get very hot. No sag, no hum, no issues.

But - I'd really like to put more modules in this case, and run both PSUs in parallel, which I've done successfully in the past with different instances of the same circuit on stripboard. However, as soon as I power them in parallel from the AC supply and move some selection of 3 or 4 modules over to the other PSU, one PSU or the other will lose negative rail voltage after 10 to 30 minutes. It can be either unit that craps out, and it doesn't follow which modules are on which PSU. I'm confident that I have the AC inlets in parallel and I believe if I didn't, it wouldn't work at all. It's always the negative rail of one or the other that quits. Let it sit for a bit, turn it back on, good for another 10-30 minutes...

One difference between this and the past setups where it's worked ok, is this case is banana modules and the ones that worked were Eurorack.

Any ideas, things to check? (besides "buy a real PSU" - I'm working on it)

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EATyourGUITAR
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Re: MFOS wall wart style PSUs crapping out - why?

Post by EATyourGUITAR » Wed Oct 13, 2021 4:45 pm

Not enough load on the -12v rail. You are creating extra heat in the -12v regulator because there is not enough load and not enough heat sink on the -12v regulator. You could also fix it by getting a transformer with a slightly lower output in AC volts. If you are using a 12VAC you should measure it to see if it is actually 14VAC. Replace it with a proper 12VAC transformer or replace it with 11VAC transformer. Or just don't have your power supply unloaded. If your modules add up to 150mA on paper the actual current draw could be more or less. You can also simulate more.load by decreasing the size of the cap on the -12v rail.
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harpooneer
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Re: MFOS wall wart style PSUs crapping out - why?

Post by harpooneer » Wed Oct 13, 2021 4:53 pm

EATyourGUITAR wrote:
Wed Oct 13, 2021 4:45 pm
Not enough load on the -12v rail.
Totally counterintuitive, to me anyway, but plausible & easy enough to test - all my modules use Eurorack power connectors so I can through a few into this banana rack to increase the load.

The wall wart is a decent one that doesn't run too high unloaded, but I've also got a 900mA version of the same that would be less over-rated, so I'll play with that too.

The heatsinks are decent sized and the current draw is measured, not on paper.

Thanks for the quick response!

harpooneer
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Re: MFOS wall wart style PSUs crapping out - why?

Post by harpooneer » Wed Oct 13, 2021 9:44 pm

EATyourGUITAR wrote:
Wed Oct 13, 2021 4:45 pm
Not enough load on the -12v rail.
I connected the PSUs and loaded them with seven modules each, just stuff that was handy but probably minimum 150mA on each negative rail, and let them play a sample & hold sequence for an hour and a half. No crapping out and none of the heatsinks got so hot that you couldn't lay a finger on them indefinitely; no noise, hum, or droop. I'll do a longer test tomorrow, but we may be good!

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Re: MFOS wall wart style PSUs crapping out - why?

Post by EATyourGUITAR » Thu Oct 14, 2021 7:34 am

Measure the voltage of your transformer. Check to see how close it is to 12VAC. The more information you have, the better it is to decide a solution. You may still have opportunity to decrease heat in the regulators by decreasing the voltage of the transformer but only if that is out of spec.
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harpooneer
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Re: MFOS wall wart style PSUs crapping out - why?

Post by harpooneer » Thu Oct 14, 2021 11:10 am

EATyourGUITAR wrote:
Thu Oct 14, 2021 7:34 am
Measure the voltage of your transformer. Check to see how close it is to 12VAC. The more information you have, the better it is to decide a solution. You may still have opportunity to decrease heat in the regulators by decreasing the voltage of the transformer but only if that is out of spec.
So, last night's test took place with a lot of stuff out of the case and spread out on a table. Buttoned it all back up this morning, wiggled a bit, started getting problems again with -12V crapping out. Pretty much by luck I found a way to reproduce it at will and - found a chain of three intermittent mechanical contacts that, when working together just right, were shorting a stripboard module's -12V rail to ground! I fixed it at the stripboard and can no longer reproduce the problem.

With the seven current modules all running off a single PSU, my smallest 12VAC transformer reads 13VAC under load, which doesn't seem too bad. But I'll watch it as the remaining space is filled, and further optimize it if I can.

Thanks again for your help!

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Re: MFOS wall wart style PSUs crapping out - why?

Post by EATyourGUITAR » Thu Oct 14, 2021 1:35 pm

Oh this is interesting. Happy you found the problem. Intermittent problems are the hardest to troubleshoot. Fixing one of these is a badge of honor for any technician.
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harpooneer
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Re: MFOS wall wart style PSUs crapping out - why?

Post by harpooneer » Thu Oct 14, 2021 3:24 pm

EATyourGUITAR wrote:
Thu Oct 14, 2021 1:35 pm
Oh this is interesting. Happy you found the problem. Intermittent problems are the hardest to troubleshoot. Fixing one of these is a badge of honor for any technician.
This particular stripboard, at the time I first built the module in question, was a big pain in the butt and had made me swear off using ones that didn't have any provision for mounting. Naturally the last one would come back to bite me.

Meanwhile I'm psyching myself out over this PSU arrangement which was only ever a thing because for once, I was trying to have a power surplus in advance instead of maxing out and playing catch-up... there's always something.

This episode has made me realize that with bananas (and with weirdnesses fixed), my panels are not grounded or referenced to the circuit voltages in any way - would it be worth grounding them on purpose?

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