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Old 24th May 2006, 07:10 AM   #1
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Default LM337 behaving strange???

Help,

I've build a pretty straightforward negative supply for my pre-amp using a LM337, which behaves well with a ouput voltage of 15.1 V (as designed).

Now comes the strange part: As soon as I ad the Adjustment pin bypass capacitor (10 uF or 22 uF) the voltage drops to about -8V. When I remove Vin the output voltage first increases to about -13 V before decreasing to 0V.

I've tried several cap's. Who knows what's happening and how this can be solved? (Of course I want the adjustment bypass capacitor because of the lower noise and impedance when used.)

thanks and greetings, MArco
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Old 24th May 2006, 07:18 AM   #2
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You are aware of the different pinning compared to LM317?

Can you show us how the circuit look like? Resistor values? It's a bit sensitive not having too large values between OUT and ADJ.
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Old 24th May 2006, 09:28 AM   #3
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Quote:
You are aware of the different pinning compared to LM317?
Yes, I am aware. It works fine without the adjustment bypass capacitor. It would'nt when I've got the pinning wrong. Or would it?

Quote:
Can you show us how the circuit look like? Resistor values? It's a bit sensitive not having too large values between OUT and ADJ.
Well the circuit looks pretty much like the attached. The differences:
R1 = 120
R2 = 1320

C2 tried 10uF and 22 uF. Both with same results.
Added 0.1 uF from Vin to ground.
Tried with and without the protection diodes.

Omitted Cl. I want that one near the load. Now that I think of it, could that be the problem
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Old 24th May 2006, 09:45 AM   #4
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If you read the datasheet again it tells you to keep Cl near the regulator to minimise problems with inductance from the traces reg to cap.
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Old 24th May 2006, 09:53 AM   #5
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You must also have a cap at the output. Use 1-10 uF or so, see datasheet.
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Old 24th May 2006, 10:02 AM   #6
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You may have the cap reversed, that would cause a large leakage current, giving exactly the thing you see. The + terminal of the electrolytic should go to ground.

Jan Didden
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Old 24th May 2006, 10:35 AM   #7
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Quote:
Originally posted by peranders
You must also have a cap at the output. Use 1-10 uF or so, see datasheet.
Cl is the output cap, so he is ok
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Old 24th May 2006, 10:57 AM   #8
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Yes, but if you have too little the LM337 can oscillate and 337 is more sensitive for that than 317.

Jan's idea might also be something.
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Old 24th May 2006, 11:47 AM   #9
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Quote:
You may have the cap reversed, that would cause a large leakage current, giving exactly the thing you see. The + terminal of the electrolytic should go to ground.
The adjust bypass cap (C2) does have the positive side to ground. I actually measured the voltage across the cap to be double sure.

Marco
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Old 24th May 2006, 12:53 PM   #10
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Quote:
Quote:
Originally posted by peranders
You must also have a cap at the output. Use 1-10 uF or so, see datasheet.
Originally posted by richie00boy
Cl is the output cap, so he is ok
Sorry you are right he had omitted Cl.

YOU NEED Cl
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