LM337 behaving strange???

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
deduikertjes

deduikertjes

Member
Joined 2005
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. :confused:

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
 
deduikertjes

deduikertjes

Member
Joined 2005
You are aware of the different pinning compared to LM317?
Click to expand...

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?

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.
Click to expand...

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 :confused:
 

Attachments

  • schema.gif
    schema.gif
    6.6 KB · Views: 437
R-K Rønningstad

R-K Rønningstad

Member
Joined 2006
deduikertjes said:


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?



Well the circuit looks pretty much like the attached. The differences:
R1 = 120
R2 = 1320


---
Why is it that for the LM317 National recommends R1=240ohms while for the LM337 they recommend R1=120ohms?For stability? I have always used 240ohms for both and have had no problems. I have always used close located decouplings, though.
See:
http://www.national.com/pf/LM/LM317.html
and
http://www.national.com/pf/LM/LM337.html

Rolv-Karsten
Click to expand...
 
richie00boy

richie00boy

Member
Joined 2003
readresearch.co.uk
AndrewT said:
Hi,
the CLoad will provide the output to ground capacitance, but the input to ground capacitance is also missing.
The schematic is incomplete.
Click to expand...

No the schematic is fine.

Jan is right. If you look at Iadj for the 317 it is about half that of the 337. So for the 337 to achieve the exact same output as the 317 the resistor must be half the value as in the 337 because of the error voltage drop.
 
R-K Rønningstad

R-K Rønningstad

Member
Joined 2006
A

AndrewT

R.I.P.
Joined 2004
Hi,
sorry to have to repeat myself but the schematic is not complete.
As I said earlier the input to ground cap is missing and the output to ground cap (Cload) is NOT optional.

Missing one and/or both of these could be causing the instability when the cap is added to the adjustment pin.
 
deduikertjes

deduikertjes

Member
Joined 2005
Ouch !

Yes, Yes, Yes,

I know I've been stupid.:cannotbe: It really is the output cap. To be hounest, I've been searching for the error for a few hours.

Inserted it and everything works fine. To be sure I'v inserted one in the positive side of the PS as well (altough that one was working well without it).

thank you all.

greetz, MArco
 
W

wkbdgeorge

Member
Joined 2010
Hello All,
This is an old thread, but I'd like to say that I was having a similar problem. I did some searching and found this thread and it solved my problem. I had a .01uf on the output of a lm337 and it fed into another lm337 so I didnt think I needed the 10uf in there as well. But I did!
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom