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Projects => Power supplies => Topic started by: raq on February 15, 2010, 07:09:31 PM

Title: JSR03 going a bit nuts on me
Post by: raq on February 15, 2010, 07:09:31 PM
P-A,
I hope you're doing great.
I haven't been using my JSR03 for some months now, but as I'm going back to my DDDAC1543 project, I put it back in action but get some weird behavior.

I initially used it with C13,14 and R15.16 in place, and I am pretty sure I wired my load to X6. Important to note - the solderside jumpers were not jumped/soldered.

Funny/bad thing - I do not remember the exact reasons I did this, but it worked just great. With my values - which you might remember - I was getting a rock solid 7.1 or 7.2V out.

Yesterday, in an impulse to clean up (how many times the impulse to "clean up" totally messed something up for yah, huh?...) I took out both caps and resistors, and soldered the jumpers together.

Now I'm getting a different voltage out every time I fire it. I tried to tune it to work by varying the input voltage. But it would definitely not land on the expected voltage out.

If you're wondering, both H1 and DZ1 (where I have a blue LED) are lit in operation.

Shall I just put the Rs and Cs back and take the jumpers out? I would still need to understand what I am doing.

In a whim, I think I'll swap the opamp for a known good tonight.

Thank you very much for your help, as always, P-A.
Best,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on February 15, 2010, 08:21:10 PM
Just wondering, but why remove R15, R15 and C13, C14? The will be disconnected if you'll short them??

It sounds that something is floating.

Voltage at the inputs of the opamp?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 16, 2010, 12:29:49 AM
P-A,

I see your point, but don't forget I've been using this with jumpers not in place. Therefore, whichever way I was using the circuit, the Rs and Cs in discussion needed to be there.

Can you expand on the current sense outputs? How am I supposed to use them? For some reason I decided to use them when I put this together, but I cannot figure out how.

Thank you for your help. I will also be able to measure voltages on the opamp later on tonight, my time (US CET).

Best,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on February 16, 2010, 03:11:42 PM
The Sense inputs are for removing wire resistance, especially important if you have long wires and/or want to have low output impedance at the load. The 220 ohms resistors are just a protection so those inputs not will be floating (not good) if you forget to connect them.

If you'll check Walt Jung's articles you can see how you wire up in the best way.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 16, 2010, 04:17:43 PM
P-A,

Here are the voltages I read early this morning (so early that it was more like 'your time' than 'my time').

My Vref is 2.5V, which I see at both ends of R7 (I used 649ohm for that, as my R9 = 1k, R10 = 1.87k).  For Vref I use an LT1034/2.5

Opamp IN- is at 3.6V, while IN+ is at 1.78V. I got 6.72V at OUT. I am feeding it from the output (using R11) with 10.72V.

Needless to say, that is also what I'm getting out of the regulator.

My DZ1 is a blue LED with 3.76V Vdrop.

For reference, R3 = 3.48k, I am using R10 and C9, and my opamp is a MAX410.

I am going back to my homework of refreshing my memory with Walt's articles. It's been a while. In the meantime, please give me your input based on these numbers.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on February 16, 2010, 04:28:47 PM
It seems that your opamp is broken. You should have 2.5 V at your +IN. The transistor connected at the opamp output could also be broken. Do you have 0.65V across base and emitter?

What did you do exactly?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 16, 2010, 04:47:23 PM
Well, P-A, good question.

Firing this up was part of a major assembly of my DAC, so if messing this up sounds crappy, consider it in the context of much more work, the majority of which went absolutely flawless (this is therapeutic for me, so I'd feel better, just go with it. ok? :))).

In any case, in a nutsheaven, having some CRCRC filters in my unregulated side and registering some larger than expected voltage drop over the series Rs (fairly high current draw...), my entire DAC didn't fire up at all, initially (I had something like 2.6V out of the superreg., for 5 something in).

Therefore, I had to take all of the series Rs out, so I would have enough Vunreg for the superregulator to operate. Vunreg became a little under 12V. Vout what I said before (little over 10V).

If at this point  the opamp died... I don't know. It also is possible it has been damaged by weeks of carrying the little kit around with me (ES discharge or something).

I will replace the opamp tonight and let you know of my findings. You assumption that the opamp is bad coincides with my sense out of all this.

Thank you for your help.

Best,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 16, 2010, 04:56:01 PM
P-A,
What should IN- voltage be for 7.2V out? Per my calcs, exactly the same as IN+ voltage... is that correct?

(I have a 1.87k/1k voltage divider of 7.2V => 2.508V @ In-, which is exactly what I see over Vreg, my 2.5V reference).

Thanks!
aR
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on February 16, 2010, 10:32:42 PM
If the opamp works and you have feedback you should have the same voltage at both inputs of the opamp, only some mV difference.

If you have 2.5 V reference you should have 7.2/2.5 gain => 2.88 gives you 1.88 in quota in your feedback network which you seem to have. Remember to have at least 11 volts in.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 17, 2010, 04:39:12 PM
P-A,

The situation is back under control. The opamp was busted, I guess. It's true that after replacing it, the SR went nuts for a good 10-15 seconds of regaining stability (auto-oscillation?) , during which my blue LED (DZ1) went on and off, off and on, flickered, blinked, etc.

And then - perfect, solid operation. I ran it for hours and hours yesterday on and on - no problem whatsoever. rock solid. I am not sure what happened exactly. Maybe a defective opamp drove this into a non-stability zone, possibly oscillating. The new good opamp canceled those effects, but it took some time to stabilize?... I'm not sure.

It's all good now. Running like a charm.

If you have any good insights into what happened, please let me know. I'd be very interested.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on February 17, 2010, 05:29:22 PM
Do you have a controlled start-up behavior now?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on February 17, 2010, 09:10:01 PM
It exhibits the most stable start-up behavior now.
aR
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 02, 2010, 04:25:12 AM
P-A,

I have to take that back...

This thing is flipping on me every time I turn around. I thought the CCS current might be too small for my output demand (390mA), so I jumped it up to around 25mA (R14 = 47.5ohm).

It didn't make a change. It would usually be solid when not attached to load, but when I put it at work,regulation ceases and it puts out around 12V (I'm putting around 15V in).

I do not know what else to try.

Please let me know what you think.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 02, 2010, 05:09:39 AM
...thinking that the 15V feed might have damaged the op amp when regulation was not happening (although I never measured over 12V @ OUT, where the op amp feeds), I swapped my initial choice of MAX410 for LM49710, which can withstand up to +-17V VCC... (MAX410 maxes out at 12V VCC).

Now the regulator seems to... regulate, but I am getting 6.3V out, which does not whack with the mathematics of this, as far as I understand it (i.e.: I did not change any passives with the change of op amp; the passives require everything to concur on getting 7.2V out).

Can you make sense of this?

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 02, 2010, 06:14:48 AM
The voltage at the opamp pins? Can you measure those?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 02, 2010, 01:38:11 PM
2.5, as the reference, both non-inverting and inverting inputs.
aR
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 02, 2010, 10:12:18 PM
The other pins?

If you have the same voltage (within mV's?) I'll suspect the opamp is working. The "sense" connections are tied to the output and ground?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 02, 2010, 10:23:39 PM
Yup, same voltages present.

While the op amp is indeed working now, except for the strange Vout, I suspect it is going to flip on me again. Let me give you an example. yesterday, after changing the op amp again (this time to a different type, from MAX410 to LME49710), I let it run with no load for 7-8 hours, just to make sure. It did great. When I sat it inside the enclosure, the blue LED would lit for a second, then turn off and I'd get 12V out (by the way, everything in the load should fry at 12V, I'm very lucky everything's still working).

Can you figure what's the reason for instability? Did I make some strange choice of parts? (I'm thinking things such as: maybe the op amp is too wide in bandwidth, and therefore it oscillates?... I don't know).

Can we maybe think of alternative scenarios?

Thank you,
Radu.
Title: PS
Post by: raq on March 02, 2010, 10:24:32 PM
yes, indeed, the 'current sense' outputs are jumped through solder underneath.
aR
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 02, 2010, 10:32:52 PM
Is H1 a red or blue LED? A blue LED is not recommended if you have set the LM317 to 2.5V. The blue LED has a higher voltage drop. I think you must tune up the LM317 to 3.5-4V. This means 6-8 V more in than than out.

Do you have 12 V out? What is the output voltage of the opamp?

(You could also use parts according to my recommendations.)
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 02, 2010, 11:09:39 PM
Is H1 a red or blue LED?
>red

A blue LED is not recommended if you have set the LM317 to 2.5V. The blue LED has a higher voltage drop. I think you must tune up the LM317 to 3.5-4V. This means 6-8 V more in than than out.
>I have a blue LED as DZ1 - what you are saying above still applies?
If yes, how would I go about tuning the LM317 for higher voltage? Can I actually get rid of it altogether?

Do you have 12 V out?
>when things go bust, that is what I would usually measure at the output of the SuperReg.

What is the output voltage of the opamp?
>you mean, V at the output pin of the op amp? I will need to measure that tonight.

(You could also use parts according to my recommendations.)
>there is no BOM for 7.2V out, and besides, I am following the recommendations concerning each part. Most parts are taken from existing BOMs, and for the others I calculated everything by the given formulas, or tuned them by the given recommendations.

Do you want me to send you a scan of the schematic with all parts marked exactly as I have them?

Thanks,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 03, 2010, 06:21:40 AM
If you have a red LED in H1 position, that's fine.

Is the opamp in a socket? Are you sure you don't have a bad contact?

Can you check the opamp inputs when you are having 12 V out? If you have bad contact the inverting input (pin 2) should be more than 2.5 V and the output should be high. If you could take a photo so I can see the parts it would help.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 03, 2010, 06:54:12 AM
Hi P-A,

I just measured Vdc @ OUT (pin 6 of opamp), it is 1.787V, on average, it goes up and down ever slightly.

This is, of course, during stable operation. I will have to jolt it somehow, (maybe connect the DAC) to get it to flip and output 12V. I am currently having it run with no load, and during such operation, it is usually stable.

The op amp is indeed in a socket.  I do not know if a bad contact is at fault. The board is really small and a bad contact could go unnoticed unless I'd have a microscope to be 100% sure.

I will try to take some pictures and send over.

Thanks
R.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 03, 2010, 12:53:42 PM
My guess is a broken transistor. Could you check the base-emitter voltage? Should be 0.65 V or around that. The output of the opamp should follow the output voltage with a DC shift only, LED+ Vbe.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 03, 2010, 05:47:31 PM
P-A,
The transistor you are talking about is the one at the output of the opamp, correct?
Thx,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 03, 2010, 09:57:45 PM
Yes.

Checked your pictures.

R10 should be 0 ohms and C9 not mounted. Those are "just in case".

T4, which type? Can't see.

Are the heatsinks isolated, not connected anywhere?

If the T6 is OK you could test to feed the opamp from R12 (Remove R11). MAX410 should work from 4.8 volts but you'll never know.

Nothing is connected to the output but to the sense inputs. You have tin blobs on the solderside in order to tie these inputs to the output and the ground?
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 04, 2010, 12:01:46 AM
P-A,
Replies below.


Yes.

Checked your pictures.

R10 should be 0 ohms and C9 not mounted. Those are "just in case".

I think I will quickly build another one (have three more PCBs available). Will get this in there as indicated. Also, shall I keep R14 = 47.5ohm? Is sets 25mA CCS current.

T4, which type? Can't see.

D44H11.


Are the heatsinks isolated, not connected anywhere?

They are attached to the T's body, so I guess that's "suspended." No contact with anything.

If the T6 is OK

I hope it's OK, I just changed it when we thought the opamp was bad.

you could test to feed the opamp from R12 (Remove R11).

I am a bit more confident to do this with LME49710, as it can take more Vcc.

MAX410 should work from 4.8 volts but you'll never know.

Nothing is connected to the output but to the sense inputs. You have tin blobs on the solderside in order to tie these inputs to the output and the ground?

Affirmative.

Will keep you posted.
Thank you.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 04, 2010, 03:10:06 AM
testing... I made all of the changes, now it's working under load and I have my backlit fluke sitting there so I'd see the B+ at all times...

Will test it tonight until the end of the day (and a bit beyond, probably).

Will post here again, my morning.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 04, 2010, 01:28:37 PM
Still good, P-A... I think we nailed it down.

Something tells me that feeding the opamp from the input instead of output made its operation more stable and fixed some delayed effects (I think there was something similar to a 'hysteresis' effect going on here, bouncing waves aggravated and amplified by the opamp loop). Rough startup and a positive feedback effect (making the voltage creep up slowly from the intended 7.2 to 12V out), due to the extra parts around the opamp, maybe.... This is sort of my understanding of what was happening.

It seems to be stable. I will keep you posted on this. A 5V and a 3.3V might follow (whoa!).

Thanks for your assistance, appreciated,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 05, 2010, 04:49:25 PM
P-A,

Whenever you get a chance - further comment on all this...

As I am running about 25mA on the CCS, I thought I should monitor the temperature of T1/T6. T1 is in a very safe region, of a little over 30C. T6, though, heats up to just over 60C.

Would you regard that as safe? I am not sure what temp as should regard as bulletproof in that position T.

Please let me know.

(By the way - I've been monitoring the voltage - no problems this far).

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 06, 2010, 08:18:34 AM
You should be worried if the temperature is over 100 deg C. 60 degrees is not much for the transistor itself but electrolytics may be "tired" shorter than expected.

The current should be not more than needed for good operation.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 11, 2010, 04:25:10 PM
P-A,
Operation continues to be stable.

Let me ask you - what do you think about using AD797 or LT1028 with my voltages and everything else? For reference, I am right now using LME49710.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 11, 2010, 09:19:47 PM
LT1028
SUPPLY VOLTAGE
> 9 volts supply voltage
INPUT COMMON-MODE VOLTAGE RANGE
> 2 volts in at the inputs

AD797
SUPPLY VOLTAGE
> 9 volts supply voltage
INPUT COMMON-MODE VOLTAGE RANGE
> 2 volts in at the inputs

If you can make sure you'll have 10-12 V for the opamps they might work but I haven't tested.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 11, 2010, 10:36:49 PM
All checked... With R12 used, the Vcc should be between 12V and 15V (15V most likely).
Vref is 2.5V, so this is accomplished too (Vin+, Vin-).

If I build the 3.3V PS, I might use 1.25Vrefs, things will not check anymore.

Thank you,
Radu.
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 11, 2010, 11:13:05 PM
You must be aware of the common mode limits. It means how near the inputs can be the supply voltage and ground. If the inputs are out of the common mode limit the opamp may not work properly but no harm in testing.
Title: Re: JSR03 going a bit nuts on me
Post by: raq on March 11, 2010, 11:39:30 PM
Alright then - assuming I use any of the mentioned op/amps, I believe likely have a common mode limit above 2.0V. If Vref stays 2.5V, with an R9 of 1k, having R8 = 320ohm should hit my target of 3.3V - is that correct?

DZ1 could be a green or red LED provided it hits 1.65V over itself.

Please let me know - thank you,
Best,
Radu
Title: Re: JSR03 going a bit nuts on me
Post by: peranders on March 11, 2010, 11:49:36 PM
Using a 2.5 V ref is wise and some sort of red, yellow or green LED may work. Make sure that the opamp output is within it's limits. You can measure this but if you have a stable output voltage it will probably work. the potential at R10 must be exactly the same as the + input. If this is the case the opamp is regulating properly.

Correct with the resistors.