Tangent Young-Jung power supply - Page 7

Case work is difficult (for me, anyway) - this was merely tedious. No beer - he's just working off his tuition!

ruZZ.il: What kind of problems did the transformer cause?

Tangent: The sturdier opamp sounds like an excellent idea. Maybe I'll order an OPA604 - if I look for others, are there any other parameters to keep in mind, besides JFET and high voltage?

Not from me, the PS is for a friend. I don't own anything that uses a power supply below 250V anymore. And all the fab for that case was done with a hand held drill, it's too cold in the garage to justify using the press for little stuff like this. Measure twice, use a punch and drill carefully.

Low noise is probably the most important thing. I wouldn't go much higher than what the AD825 puts out, 12 nV/rt.Hz. The 604 is a little lower, at 10 nV/rt.Hz. You can get down to about 4nV or so if you look hard enough, though you might give up too much else.

Don't go experimenting if you don't have an oscilloscope. You need to test this carefully before accepting it. You're pioneering here.

Well, I was using +/-15v AC and dropping it to about 26Vdc which is around a 10v drop for the regulator, which was causing quite some heat on it so I wasnt even fully biasing my M^3. Changing to a +/-12v cut the voltage drop by around 70%.. fully biased and still much less heat.


Nate: Ah. I guess I need a punch :P

This is pretty similar to the one that I use to mark holes before drilling. And a few thousand holes of practice doesn't hurt either.

I have access to an oscilloscope, though I've never used one before. I could get one of the physics teachers here to show me how to use it, I suppose, but I'm not sure what I would be looking for in a power supply - it's not like I could send a square wave through it. Maybe I'll leave the experiments to you. If you try an OPA604 and it works, will you post the results?

I must have done something else wrong if ruZZ.il used the same transformer and the only problem he had was heat/inefficiency. Either that or I got a bad part somewhere. When the new transformer comes I'll start by removing the opamp and one end of R8 and see what happens. In the meantime I'm without my PPA...

In this case, oscillation. It'll be dead obvious. You just hook the scope probe across the output (ground clip to TP3, probe tip to TP6), set it for AC coupling, and set the bandwidth and vertical settings so you can see the scope noise, typically a few mV. That's all you should see; just a nice fuzzy line a few mV high. If it's oscillating, you'll probably see a huge periodic waveform; at the very least, enough high-speed "ripple" that it pokes up over the scope noise.

Do the test with the supply just sitting there idle, then put a dummy load across it, re-test, then put a real load on it and re-test.

So I'm not in danger of frying the oscilloscope by feeding all 30V or so directly into it?! I suppose then that "AC coupling" puts a capacitor in the path or some such thing to block DC? If I'm not liable to commit "oscilloscopicide" maybe I'll try it.

Would a power resistor work as a dummy load?

Depends on the scope and the probe you use with it. A decent scope will have the input voltage limit printed on the front of the scope near the probe connection. The most common probe type is a 1/10x passive probe, which on the 10x setting cuts the signal down by 10x. So, if the scope says 30V but you use a 10x probe, you're still fine. Switch the probe to 1x, and you're right on the edge of damaging the scope.

Yes.

Yes.

I looked at the scope again, and it has a limit of 600V, so there's no problem there. I was puzzled by one thing - appended to the voltage limitation was "(P.P + DC)". Any idea what the "P.P" is? The manual to this thing has long since disappeared (as has the probe!)

AC peak-to-peak, plus DC offset.

OK, I replaced the transformer and retested, with virtually the same result (except of course about 10V less) - still around 1.6 volts down over the LM317. I removed the opamp and lifted R8 - same result, 1.6V (also same result everywhere else). I suppose that pins it down to the LM317 - I'll replace that and post again with the results.

YJPS is up and running

I have mated the YJPS with the USB/DAC. It works.
The DAC is drawing about 290ma at 12vdc. No noise, no voltage sag, Maybe the heat sink are a tad too warm. I had to size them to fit inside the case so they are a bit shy of the correct size.
The PS has really helped the sound on the DAC. I am quite happy...
Thank you Warren

Phil
Santa Fe, NM

Sir, May I have another?

Good to hear it.

There are some more things I want to do before I make the next run. Changes so far:

- Enlarged the mounting holes in the corners of the board

- Added separate test points for CGND and RGND, so you can test the three main sections of the circuit separately. (Changes most of the TP numbers, unfortunately.)

- Lots of tiny little part positioning tweaks and such

Still left to do:

- While assembling my first YJPS v1.1, it seemed to me that the VOUT and jumper pad scheme is probably a touch on the complex side. I want to look back at it and see if I can simplify it, or at least make it more elegant.

- Add another hole to each of the big film caps, C1-C3, allowing use of 12.5mm pitch caps.

- I'm considering moving at least D2 and Q1 to the other side of Q2 so you have the option of laying the transistor down flat for low-profile cases. I'd probably move R5 and R6, too if I did this. You can already do this with U1.

- The heat sink mounting holes are too small. They're going to be enlarged to allow at least #4 screws.

- I'm thinking of adding a zener/SCR type crowbar across the output of the supply to force the fuse to blow if you go over a safe output voltage for the op-amp, to prevent a recurrence of the problem Schubert's having.

Anything else for the wishlist?

Replaced LM317 - no change. Still -1.5V over LM317 with no op amp or R8 in circuit. -1.1V over TP5-TP6. D2 does not light. Any ideas?