Problem with capacitance multiplier power supply

[mjf]

hello.
the voltage drop across q4 should be 2,3v (=28,8-26,5)........;
looks like the problem is (around) q3...............
greetings.........

[jscherer]

Originally posted by kaos
The 20.8 V across Q4 doesn’t make sense relative to your other measurements, I’m guessing a typo? 27.3 V on the emitter of Q1 and 8 V on the emitter of Q3 sort of narrows it down to a faulty connection or trace at Q3, or a bad Q3. Any chance the base and emitter are interchanged on Q3 (easy to do on a TO-3 package that’s been hand wired)? I'm assuming nothing's getting hot.

You were absolutly right kaos, on both accounts!

1st, The voltage across Q4 was a typo. It should have read 1.8V.

2nd, I also thought that all roads pointed towards a bad Q4. I had swapped Q4's between the +Ve and -Ve several times and the problem never followed transistor. Also, the voltage output from the -Ve side also seemed about right. So I ASSUMED that the pass transistors were ok. The transistors were in fact ok, but I had the emitter and base swapped in my circuit. Wow. So now I feel a little foolish that I didn't catch this myself sooner. Well it was my 47th birthday yesterday so I will blame everything on that!

My goal now that I have this running is to clean up everything, put it in a chassis shorten the wires up and do some measurements. Are there any good websites that would get me going on doing the power supply measurements?

If I do some very basic measurements using the formula:

Vac/Vdc * 100 =% ripple

I get the following

Vac/Vdc * 100 =% ripple
%ripple=.002/26.9 * 100
%ripple = .007%

Thats .002 VAC measured from +Ve to ground and 26.9V DC from the same point. This seems very low indeed! Below is a picture of the PSU Board

Thanks to everyone who helped me on this. I really love this form everyone is a big help. Its just great to have a place to turn to for questions on this stuff and they actually make you think!

[AndrewT]

2mVac of output ripple is low and that would be consistent with a low output current. What was the test current during those measurements?

The nice thing about a capacitance multiplier is that as the output current rises the output ripple increases much more slowly than on a conventional PSU. This suits amplifiers that draw a high quiescent current and have a low PSRR. low ripple = low hum.

The normal way to quote ripple is Vpp. The DMM and any analogue voltmeter when set to AC tries to measure the average voltage and then scale it to read an effective RMS voltage.
Vpp~ 3* Vac.
Your ripple if measured on a scope would be ~ 6mVpp.

BTW,
some DMM do not work as your's did measuring AC superimposed on a DC voltage. They try to add some portion (or all of) the average DC voltage to the average AC voltage and give a very high total voltage reading that can overload or even damage an unsuitabel meter.
Check any other meters you have starting with the highest Vac scale to ensure you don't overload them.

[jscherer]

Hi Andrew,

I didn't measure the actual current at that time however I did have a 50W 10 ohm resistor at the +Ve output and gnd. If my calculations are correct that should be about 2.7 amps. I'm planning on doing more testing later today. The board is setup to have two power supplies on the same board and I still have the second one to finish wiring up so I will do that too.

The meter I was using was a Radio Shack 22-168. I also have an old Simpson model 467 "True RMS" meter and a 200 MHz Tektronics Oscilloscope that I will try as well.

Cheers,

[Lumba Ogir]

jscherer,
I would speed up the driver like this and would also use this configuration for Q1/ Q3.

[SY]

Meter tip: if your meter has a reasonably high input impedance (most do, 1M or more), just AC couple to the rail to measure ripple. A 1u capacitor in series with the voltmeter probe will do it.