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Old 11th June 2002, 03:57 PM   #1
jarthel is offline jarthel  Australia
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Default LM337 problems

I built a simple -5V regulator using an LM337 the latest version of kwak-clock.

After soldering all the components, I then connected Vin and ground to the outputs of a DC adaptor. The output of the DC adaptor is +11.xxV and a max of 500mA. Checking Vout using a VOM, the voltage output is around +10.xxV. Well according to my calculations (using page 1 of LM137 PDF from National), the output should be -5V.

I then get the breadboard and connected all the components to it.
R1 is 270ohms and R2 is 820ohms. Output cap is rated at 220uF 25V. This is the same type of cap I'm using on a +5V regulator using an LM317. I then connected the DC adaptor output and the circuit in breadboard is giving me -5V which is what I need.

I then removed the resistors and caps from the breadboard and replaced the corresponding components in the veroboard. Connected the DC adaptor, it's still giving me +10.xxV.

I then got another set of resistors and caps and connected these to the breadboard. When I connected the DC adaptor this time, the output is +10.xxV. I noticed that the cap is getting very hot and I can't even touch it without burning myself.

I have tried 4 LM337.

Any ideas on the solution?

thanks

Jayel
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Old 11th June 2002, 04:27 PM   #2
ALW is offline ALW  United Kingdom
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Default PSU wrong way round?

An LM337 needs to be fed with a negative raw supply, to get +10V out, you must be feeding a positive supply in.

What voltage are you feeding into the reg, I think your problem lies there.

Andy.

P.S. You'll have to replace those caps - reverse biasing them is likely to have damaged them, the 337 might be dead or degraded too.

P.P.S Make sure the caps are the right way round too - negative terminals to the reg o/p/ i/p and adjust.
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Old 11th June 2002, 04:51 PM   #3
UrSv is offline UrSv  Sweden
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Exactly. 11.xx in giving 10.xx out sounds like the inoput minus the protection diode loss of .6 V or similar. Check that you feed the regulator a negative voltage in and as mentioned make sure your caps are correctly mounted with + at the gound terminal and - at the output.

/UrSv
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Old 11th June 2002, 05:02 PM   #4
tiroth is offline tiroth  United States
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Those regs are almost certainly toast. 317/337 don't like being reverse biased.
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Old 11th June 2002, 05:03 PM   #5
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hey guys can you post the schematic for the kwak clock, i will like to have a look at it.
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Old 11th June 2002, 05:54 PM   #6
mlloyd1 is online now mlloyd1  United States
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Just ask Elso. He's very prompt and you get the latest version.
mlloyd1
Quote:
Originally posted by NickC
hey guys can you post the schematic for the kwak clock, i will like to have a look at it.
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Old 11th June 2002, 06:44 PM   #7
paulb is offline paulb  Canada
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Default Re: LM337 problems

Quote:
Originally posted by jarthel
I then connected Vin and ground to the outputs of a DC adaptor.
I think you just need to reverse the leads of the adapter.

I wouldn't re-use any caps that have been too hot to touch (you're lucky they didn't explode). However, ICs can be amazingly resiliant. The LM337s might be okay.
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Old 11th June 2002, 09:06 PM   #8
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To be safer, just insert a (low drop - if your input voltage is not high enough) diode between your DC adapter and the input of your reg, (before the caps !!). I always do that when using an adapter, since it often has a polarity switch, and misconnection is sooo easy

Hope this helps
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