LM337 behaving strange???

deduikertjes · 24th May 2006, 07:10 AM

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

peranders · 24th May 2006, 07:18 AM

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.

deduikertjes · 24th May 2006, 09:28 AM

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

richie00boy · 24th May 2006, 09:45 AM

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.

peranders · 24th May 2006, 09:53 AM

You must also have a cap at the output. Use 1-10 uF or so, see datasheet.

jan.didden · 24th May 2006, 10:02 AM

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

richie00boy · 24th May 2006, 10:35 AM

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

peranders · 24th May 2006, 10:57 AM

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.

deduikertjes · 24th May 2006, 11:47 AM

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

richie00boy · 24th May 2006, 12:53 PM

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

24th May 2006, 07:10 AM	#1
deduikertjes diyAudio Member Join Date: Aug 2005	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

24th May 2006, 07:18 AM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

24th May 2006, 09:45 AM	#4
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

24th May 2006, 09:53 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	You must also have a cap at the output. Use 1-10 uF or so, see datasheet. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

24th May 2006, 10:02 AM	#6
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	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 __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun


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24th May 2006, 10:57 AM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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