Startup trouble

I started noticing trouble with my synth power supply units (PSUs) a while ago. Maybe you are having similar trouble too, or maybe you will. Hence this post, because now I know what’s going on, and how to fix it.

I have a couple of Frequency Central FC Power supplies, and a couple based on the Music From Outer Space Wall Wart design. They’re not the same but they’re similar. They take 12 VAC provided by a power adapter — affectionately known as a wall wart — and rectify the voltage. Then a couple of voltage regulators provide regulated +12 V and -12 V outputs.

What I was experiencing was that sometimes when I switched the PSU on, one of the regulators would fail to start up, so I’d get nothing on that output. I found a workaround: If I switched the PSU off and back on again, both regulators would start up and I’d have both outputs. Since I had the workaround, and it was hard to diagnose the PSUs when they were at the back of my synth case behind my modules, I didn’t try very hard to fix the problem. But I did post a query to the LMNC Discourse group, and it turned out a number of other people were seeing a similar problem.

Since then, I’ve added power display modules to my system, which make it easier to see what’s going on. What’s happening more specifically is that roughly a third to a half the time (others report varying frequencies, some seeing it only rarely) the +12 V regulator fails to start. The -12 V side always comes up.

The part of the circuit around the positive regulator looks something like this:

Positive power supply regulator

The LM7812 is the regulator. Unregulated voltage, around +15 V (probably higher, but let’s say 15), goes in on pin 1 and regulated 12 V is produced on pin 3. The resistor marked “LOAD” represents the modules being powered. The two 1N4004 rectifier diodes are there to protect the regulator against capacitor discharge if the input or output is shorted. Actually, according to Texas Instruments Application Report AN-182, the diode from output (pin 3) to pin 2 is to protect against discharge from a capacitance between pin 2 and ground. But there is no capacitor there in this circuit. Granted, there will always be parasitic capacitance, but would that be enough to endanger the regulator? I wouldn’t think so, and so I’d think that diode is superfluous here, but I could be wrong — it’s not doing any harm, anyway.

The negative regulator is connected similarly, so we have this:

Positive and negative regulators

The trouble occurs if the negative regulator gets going first. Then the output of the positive regulator, which in the absence of the load would be floating, gets pulled to a negative voltage. According to AN-182, if the output pin is pulled low enough below ground, the positive regulator will not start up.

That’s just the positive regulator, though. Negative regulators have different circuitry, and will start up even if their output is pulled positive. And that matches what I was seeing — positive regulator sometimes not starting, negative regulator always starting.

AN-182 goes on to say, “Clamping the output to ground with a germanium or Schottky diode usually solves this problem,” and indeed some of the Discourse people reported success with this. The point is that the diode from the positive regulator output to ground will prevent the pin from being pulled negative by more than the diode’s forward voltage, but that’s about 700 mV for a 1N4004 rectifier, and that’s too large. Germanium and Schottky diodes tend to have lower forward voltage, usually around 200 to 300 mV.

So when I installed a third PSU in Kosmodrome for the bottom row, I first removed the 1N4004 on the positive output and replaced it with a 1N5817 Schottky. Because this problem doesn’t exist with negative regulators, I left the 1N4004 on that side alone.

Both regulators, with Schottky diode on positive regulator output

On the FC Power board, the diode in question is this one:

Positive output diode on FC Power board

Since then, I’ve continued to see the positive supply on the top two rows, with the unmodified PSUs, fail to start up roughly a third of the time. But both rails on the bottom row start up every time. Problem solved! Or it will be when I get around to modifying the supplies for the top two rows.

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