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Old 30th June 2006, 11:15 PM   #1
Devius is offline Devius  Portugal
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Default Power supply problems

Hello.

I designed this power supply:
http://devius.oniduo.pt/fonte-esquema.jpg

But it doesn't seem to work properly. Wheh I turn it on only one of the two outputs works, either +15V or -15V. It shifts from one to the other at each power up. In that situation the other output is at the same signal but only 1V. I get +15 and +1V or -15V and -1V. Sometimes both outputs do work at the same time, but I don't know what triggers it. It isn't a bad solder because I can shake the PSU and nothing changes. I tried putting a 100K resistor from +15 to -15 but it didn't help. In fact I think it made it worse because now the two outputs never work.
It seems one of the zeners doesn't start conducting, or maybe one of the series pass transistors, since all voltages are normal except on the regulator side.

Any ideas?
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Old 1st July 2006, 01:04 AM   #2
anatech is offline anatech  Canada
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Hi Devius,
That sounds like "latch up". Try putting one reversed biased diode between each output and ground.

-Chris
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Old 1st July 2006, 08:13 AM   #3
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The resistors biasing the zeners should be connected to the raw DC rather than the output of the supplies.

I think the resistors in the base of the transistors are way too large. Isn't 47R5 equal to 4.7 meg?
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Old 1st July 2006, 09:08 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
it seems like a start up problem, since loading the output pulls the inputs to the opamp to ground and then refuses to allow both to start.

Go and read W Jung and ALW about the start up sequence that is needed to get up to working voltage. I think Walt discusses three operating senarios from first power on to full power out.

It may give an insight for what to look for while debugging.

What is the common mode tolerance for a 5532 when both inputs are at ground when the supplies are at +- raw input?

Or should you be using 5534 with each opamp connected from raw input to ground?
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Old 1st July 2006, 01:46 PM   #5
Devius is offline Devius  Portugal
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Thanks for the replies.

I'm sure that connecting the zener biasing resistors to the raw supply would work, but I opted to connect them to the output because there will be a cleaner supply of DC to the opamp input.

47R5 means 47,5 Ohms It's just to avoid any problems on the opamp due to transistor capacitance.

The NE5532 has a +/-13V common mode input range according to the datasheet, but that's with +/-15V supplies, in my case it should be a little higher.

Hey, the reverse biased diodes worked! Thanks a lot for the help! I thought those diodes were only there for protection in case the leads were reversed by mistake. Everything is fine now
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Old 1st July 2006, 03:04 PM   #6
Devius is offline Devius  Portugal
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False alarm...

Even with the diodes it went back to the previous situation.

So, where can I find that W Jung and ALW stuff? I've searched but didn't find anything related.
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Old 1st July 2006, 03:23 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,http://waltjung.org/PDFs/Low_Noise_P...g_Circuits.pdf
http://waltjung.org/PDFs/IC_Regulated_Power.pdf
http://waltjung.org/PDFs/Regs_for_High_Perf_Audio_1.pdf
http://waltjung.org/PDFs/Regulator_E..._Rejection.pdf
http://www.alw.audio.dsl.pipex.com/M...2.9_iss009.pdf the super regulator manual seems to be down at the moment.

I'm kinda glad you came back so soon, the soldering iron was warming up!
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Old 1st July 2006, 05:57 PM   #8
Devius is offline Devius  Portugal
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Thank you for the site listing! I'm surprised those sites didn't come up when I searched for W Jung...

Anyway, I'm pretty sure the problem was something related to the zeners not being correctly biased upon start up, but I don't know why only one of them worked at a time. I opted for a simple solution: putting a 10K resistor from raw supply to bias the zener at start up. That value should introduce little ripple to the opamp input due to the RC network formed with the 100uF cap.

After giving a look at those articles I now see that my power supply could have been a little better with very simple modifications.
I also see that this thread should have gone to the power supply section... sorry!

Thanks to all that replied
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Old 2nd July 2006, 03:17 AM   #9
anatech is offline anatech  Canada
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Hi Devius,
I'm glad you got it working.

The 78XX and 79XX series regulators used to latch up in some supplies on startup and those diodes seemed to help. I was thinking the discrete version may have had the same problem.

I just include them now when using those regulators.

-Chris
Also, the thread is easy to move so don't worry about it.
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