Please help me understand\fix this discrete linear power supply

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

What B-E junction? It's gone!
Same for the B-C junction too - in fact the diode test function shows an open circuit on all pins in both directions.
Looks like that TIP32 is dead.

But if there is no base current, where did that 3.53v I got on the +5v rail come from I wonder? Perhaps the TIP32 is also leaky between E-C?
I tried powering up without the TIP32, no voltage on +5v at all so I don't think that diode is leaky, anyway.


It seems weird though that it only died after I put that 15 Ohm resistor on as a load, but then the toasted 33 Ohm resistor already hinted at damage somewhere, so...

[budm]

Re: Please help me understand\fix this discrete linear power supply

Check the RESISTANCE of E-C, E-B, B-C.
You can also put it back in without having the Base connected and see if you get an output.

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Resistance mode shows 0L for everything too. That's on a Fluke 73 series II.

I think diode mode is better, isn't it? Testing with another meter shows the Fluke outputs about 2.4v on diode test mode and 0.6v on resistance mode.

According to the manual, it can do 3.1v on both diode and ohm mode (open circuit), under load the diode output drops to 2v and resistance to 440mV for under 3.2M Ohm and 1.4V for above that.

[stj]

Re: Please help me understand\fix this discrete linear power supply

it has a parallel reverse-connected diode - check the diode

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

it has a parallel reverse-connected diode - check the diode

Check it for what? It does not appear to be leaky or shorted, at least, or surely there would be some voltage on +5v with the TIP32 removed (and there isn't)

Edit: not open either. Decided to replace it anyway just for fun.

[budm]

Re: Please help me understand\fix this discrete linear power supply

That diode is for protection in case the input is shorted to GND.
BTW, the smaller transistor that drives the BASE of TIP32A checked out OK?

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Well, I found some ancient green packaged TIP42As in a box (all I had) and put one of those in and now the supply is working again... to some extent. Regulation is out the window even WITH a load resistor. I get 7.8v now instead of 5v.

I replaced the ZTX214, it seemed OK but I replaced it anyway with a brand new 2N3906, same problem.

I also checked the main filter capacitors, they test at 8200uF for the 10,000uF one and 4200uF for the 4700uF ones. ESR is ~0.03 Ohms for all. The 4700uFs have 0.4v ripple at 100Hz on them. The 10,000uF has 0.3v ripple at 100Hz. This seems OK for the primary filter capacitors of a linear supply?

[budm]

Re: Please help me understand\fix this discrete linear power supply

It sounds to me that the Error Opamp is not doing its job, do you see any change on pin 7 as you adjust the P301?

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

No, while the input to Pin 5 changes as I change P301, the output on Pin 7 stays fixed at around 2 volts. Time for a new Opamp?

I'm going to go to Jaycar now, and as well as a new TL072 I'll get a new TIP32 that was made this century, just in case...

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

This thing is driving me insane!

A new Opamp made no difference. Pulled and checked the little 3.3nF capacitor connected between input and output, seems OK on the meter.

I decided to try something else... disconnected the Opamp and Q303. Connected a 5k Pot in series with the TIP42s base resistor and ground. With my 15 Ohm load I can happily adjust the TIP42 directly on the base to get +5v.

Put Q303 back in and connect 5k Pot from its base to ground instead. Can happily adjust the output voltage into 15 Ohm load and set to +5v just fine.

Put the Opamp back in - everything goes bananas and the +5v rail is at +7v with the load and can't be adjusted with P301. Opamp Pin 7 stays at 2v just like before.

Two dead Opamps somehow, or something else? The circuit doesn't like my TIP42?

[ben7]

Re: Please help me understand\fix this discrete linear power supply

Maybe some resistor(s) have gone out of tolerance?

[budm]

Re: Please help me understand\fix this discrete linear power supply

What DCV do you have on VCC pin 4 and pin 8 of the OPAMP? What do you have on the wiper of P301 when you move the wiper up and down?

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Opamp TL072:
Pin 8: 8 volts (with respect to DGND) Same voltage as on the main filter capacitor, which it is connected to.
Pin 4: -7.04 volts (with respect to AGND), or, 0.77 volts (with respect to DGND).
Pin 5: Adjustable from 3.9v to 4.42v via P301.

P301 Wiper: Adjustable from 3.9v to 4.42v


EDIT:
Since I noticed that the +5v rail's filter capacitor voltage has dropped from 8.78v to 8v, I figured maybe something was a bit funny...

So just now, I tried disconnecting all loads, this time adding a wire to ensure the Vref diode's series resistor was still connected to +7v line, and now everything works fine. I can adjust the +5v rail to +5v and interestingly, the +5v rail's filter capacitor voltage went up to 9 volts.

I wonder if there is a short or overload on the +5v rail? Would this cause the voltage to rise above 5v uncontrollably? Or is the power supply just in such bad shape that with any load it goes nuts?

[budm]

Re: Please help me understand\fix this discrete linear power supply

Per diagram, there are 3 resistors connected in series, 20K on top connected to the +5V output, then the P301 POT (??K Ohms, cannot read the value on the diagram), then 20K on the bottom leg to GND, so you can calculate the range of what Voltage you will get at the wiper pin.
BTW: (-) Input pin 6 is fed with +2.45VDC Vref, the (+) input pin 5 is fed with +3.9VDC, that should make the output pin 7 swing positive, there is also pull-up resistor connected to pin 7 which should make Q303 to be bias less which in turn less bias for the TIP32/42.
But you already proved that the output can be adjusted by changing the bias of Q303 that you experimented with already.

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

P301 is a 5k pot, but since it changes the rail voltage as you change it, the voltage across it will always change, right? How do you calculate that?

Also, I just noticed now:
With the +5v rail working without a load, I saw that adjusting P301 to change the +5v rail shows a much slower response than when adjusting the +/-7v rails with P300.

If I set each pot to one end, then quickly change it to the other end, the rail voltage on my meter changes in about 1 second on the +/-7v rails, but maybe 6 seconds before the numbers stop changing for the +5v rail.

Also, the +5v rail comes up from power off a lot slower than the +/-7v rails.

[budm]

Re: Please help me understand\fix this discrete linear power supply

Are you saying that +5V is putting out 5V now? The output filter will have to change its Voltage so the response will not be very fast, especially if there is not load, when you crank the Voltage up, the cap will be charged to higher level, when when you crank it down, the cap must discharge through the load to bring down the Voltage to the new set level.

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Are you saying that +5V is putting out 5V now?

It was (see below)

The output filter will have to change its Voltage so the response will not be very fast, especially if there is not load, when you crank the Voltage up, the cap will be charged to higher level, when when you crank it down, the cap must discharge through the load to bring down the Voltage to the new set level.

True, but with no load either the 7v rails are still much more responsive to adjustment and come up much faster from power off.


Also, while the +5v rail was OK unloaded, I found that if I put a 10K Ohm resistor across the +5v rail - it dropped to 1.6v!... Apparently even 500 microamps load is too much for it.

Then, I powered off and just went to take a video showing how slowly it ramps up at power on, and now the +5v rail won't go above 3.9v!

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Now the +5v rail is 'working' again. Why? I don't know!

Anyway, I took a video showing how slow it is to come up. See for yourself compared to the 7v rail (apologies for the horrible quality, hope you can see the meter OK):

+7v rail: https://youtu.be/DBl3AwHeQT4
+5v rail: http://youtu.be/OmR1uLveJng (Takes about 16 seconds to finally reach 5v, then I switch off, it starts dropping, but the camera runs to the time limit)

Also added some photos to make this thread a bit more interesting.

[budm]

Re: Please help me understand\fix this discrete linear power supply

If it takes that long, it sounds to me that the PASS transistor TIP32 is being bias on slowly and it is not regulating either since using very small load cause the output to drops that means the transistor is barely turned (has high impedance) on. There must be some bad component in that section.

[Agent24]

Re: Please help me understand\fix this discrete linear power supply

Yeah, I think the opamp is stuffed again, because putting my 5k pot back on the base of Q303 and manually adjusting it can give me +5v just fine into that 15 Ohm resistor.

And it's probably my fault..... I failed to notice that removing LK3 removes -7v supply to the opamp. So with only the +ve voltage supply from the filter capacitor, the opamp probably got toasted.

Guess I'll have to wait until RS can ship me a few more TL072s next week, because I'm not getting any more from Jaycar. Bunch of extortionists!


In addition - All the resistors checked out fine, I swapped the capacitors 'just in case' so I can't think of anything else that might stop it from working this time unless I do something stupid again...