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Old 4th Oct 2022, 4:30 pm   #1
Radio Scotland
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Default Homebrew PSU Problems

Put together a PSU from an old 4 amp battery charger, Used a 4700 uf cap for smoothing and a LM317 For regulation which gives me an adjustable voltage from 1.6 - 18.5 volts, Problem is the regulator keeps blowing after a few minutes even with no load on it, Before the regulator blows it works as it should.


Do i have a batch of faulty regulators ? They did come from china, Can't think of another reason why they keep blowing.
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Old 4th Oct 2022, 5:51 pm   #2
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Default Re: Homebrew PSU Problems

Check the LM317 datasheet for the correct suppression capacitor values.
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Old 4th Oct 2022, 5:57 pm   #3
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by kellys_eye View Post
Check the LM317 datasheet for the correct suppression capacitor values.
Output 1uf
Input 0.1uf
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Old 4th Oct 2022, 5:58 pm   #4
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Default Re: Homebrew PSU Problems

The LM317 has a max input voltage of 40V so it is unlikely that the battery charger is giving too high an output for the regulator.
Does the regulator get hot before it blows?
Do you have the recomended protection diode across the regulator (1N4002 Cathode to input, Anode to output)?
Are you using the original metal rectifiers in the charger (you did say it was old), if so change them to silicon types (if the 4700uf is getting hot it indicates that one of the rectifiers is breaking down and ac is getting onto the capacitor.

Have you got a 0.1uf capacitor close to the input terminal (input to 0V), this is recomended
"Ci is recommended, particularly if the regulator is not in close proximity to the power-supply filter capacitors. A 0.1-F or 1-F ceramic or tantalum capacitor provides sufficient bypassing for most applications, especially when adjustment and output capacitors are used."
And don't forget the capacitor from the adjustment terminal to 0V.

Just a few thoughts

Peter

Edit, looks like we are all thinking along the same lines.
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Old 4th Oct 2022, 6:09 pm   #5
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by Electronpusher0 View Post
The LM317 has a max input voltage of 40V so it is unlikely that the battery charger is giving too high an output for the regulator.
Does the regulator get hot before it blows?
Do you have the recomended protection diode across the regulator (1N4002 Cathode to input, Anode to output)?
Are you using the original metal rectifiers in the charger (you did say it was old), if so change them to silicon types (if the 4700uf is getting hot it indicates that one of the rectifiers is breaking down and ac is getting onto the capacitor.

Have you got a 0.1uf capacitor close to the input terminal (input to 0V), this is recomended
"Ci is recommended, particularly if the regulator is not in close proximity to the power-supply filter capacitors. A 0.1-F or 1-F ceramic or tantalum capacitor provides sufficient bypassing for most applications, especially when adjustment and output capacitors are used."
And don't forget the capacitor from the adjustment terminal to 0V.


Just a few thoughts

Peter

Edit, looks like we are all thinking along the same lines.

No protection diode across the regulator, The rectifier diodes are silicon, And the only capacitors in the circuit besides the smoothing capacitor are a 1uf 50v on the output and a 0.1uf on the input,

Edit, No it's not getting hot,
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Last edited by Radio Scotland; 4th Oct 2022 at 6:38 pm. Reason: Extra info
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Old 5th Oct 2022, 5:54 am   #6
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Default Re: Homebrew PSU Problems

Take a look at the design requirements from the data sheet and make sure you have both diodes and the Cadj fitted. I attach the relevant section from the data sheet.
Peter
Attached Files
File Type: pdf lm317-1.pdf (58.4 KB, 51 views)
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Old 5th Oct 2022, 7:10 am   #7
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Default Re: Homebrew PSU Problems

How big is the heatsink you have the LM317 bolted to.


Joe
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Old 5th Oct 2022, 8:10 am   #8
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by Electronpusher0 View Post
Take a look at the design requirements from the data sheet and make sure you have both diodes and the Cadj fitted. I attach the relevant section from the data sheet.
Peter
Cheers peter

Could be that the 1uf on the output is discharging into the regulator and destroying it,



Jay
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Old 5th Oct 2022, 8:11 am   #9
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by joebog1 View Post
How big is the heatsink you have the LM317 bolted to.


Joe
About 3 x 3 inches joe



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Old 5th Oct 2022, 9:46 am   #10
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Default Re: Homebrew PSU Problems

As a quick and dirty test i tacked a diode across the output and the result was the same, The writing seems to rub off the LM317 as soon as you touch them. I'm thinking they're fake,

Actually planning on using the LM338 Regulator, But still waiting on them, The LM317 Are only being used for testing purposes
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Old 5th Oct 2022, 9:54 am   #11
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Default Re: Homebrew PSU Problems

Recently used one of these regulators to power some LED's in a radio, Just tested it and its working fine, Kinda stumped now
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Old 5th Oct 2022, 10:56 am   #12
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Default Re: Homebrew PSU Problems

The writing rubbing off does sound deeply suspect....

I have used the LM317 and its negative voltage version in loads of equipment, both the plastic TO220 version and the steel case TO3 variety. They are both rather good if genuine. Alas, these days fakes abound!!
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Old 5th Oct 2022, 11:34 am   #13
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Default Re: Homebrew PSU Problems

It's worth mentioning that Ci and Co should be as close to the actual connection pins as possible. No long wires just in case there is a chance of instability. I've used lots of these without a problem but I do buy them from RS or Farnell. It is important to use the protection diode and fit Ci and Co.

It does sound like you might have some dodgy regulators.....! If you have access to a scope, you could check the output to make sure it's not unstable.

Two of these in regular use....one in the PSU for my Bush BA11 providing 2V for the valve filaments, the other is in the PSU for my stereo generator. That's been operational since 2006. Generally they are reliable regulators.
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Old 5th Oct 2022, 11:59 am   #14
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by Sideband View Post
It's worth mentioning that Ci and Co should be as close to the actual connection pins as possible. No long wires just in case there is a chance of instability. I've used lots of these without a problem but I do buy them from RS or Farnell. It is important to use the protection diode and fit Ci and Co.

It does sound like you might have some dodgy regulators.....! If you have access to a scope, you could check the output to make sure it's not unstable.

Two of these in regular use....one in the PSU for my Bush BA11 providing 2V for the valve filaments, the other is in the PSU for my stereo generator. That's been operational since 2006. Generally they are reliable regulators.
The caps are connected via wires but not overly long ones,


Now here is a strange one, I tested the last ''blown'' regulator and it didn't show any shorts between it's pins so it got me thinking that it must be the potentiometer thats acting up, Swapped it out and so far so good It was a brand new pot and tested fine, Funny how swapping the regulator out temporarily fixed the problem, Very weird
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Old 5th Oct 2022, 12:07 pm   #15
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Default Re: Homebrew PSU Problems

The old pot still tests fine, I had it wired as a rheostat and the one i replaced it with which is working as intended isn't wired as a rheostat, I wonder if that' was the source of the problem, Still doesn't make any sense
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Old 5th Oct 2022, 12:08 pm   #16
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Default Re: Homebrew PSU Problems

Maybe the heat from unsoldering the first regulator and soldering in a 'new' one was enough to make the potentiometer play properly for a while??
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Old 5th Oct 2022, 12:31 pm   #17
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Default Re: Homebrew PSU Problems

Quote:
Originally Posted by G6Tanuki View Post
Maybe the heat from unsoldering the first regulator and soldering in a 'new' one was enough to make the potentiometer play properly for a while??
But the potentiometer is nowhere near the regulator, Very weird
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Old 5th Oct 2022, 1:26 pm   #18
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Default Re: Homebrew PSU Problems

Forgot to add that the replacement pot is 10k and the original was 5k
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Old 5th Oct 2022, 1:41 pm   #19
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Default Re: Homebrew PSU Problems

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But the potentiometer is nowhere near the regulator, Very weird
Maybe that's contributing to the problem, specially if the leads between the pot and the regulator are inadequately decoupled to ground??

The setups I designed had about half an inch of PCB track between the IC and the pot; that's about the same as the ubiquitous Vellemann kits thta use the LM317.
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Old 5th Oct 2022, 1:47 pm   #20
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Default Re: Homebrew PSU Problems

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Quote:
Originally Posted by Radio Scotland View Post
But the potentiometer is nowhere near the regulator, Very weird
Maybe that's contributing to the problem, specially if the leads between the pot and the regulator are inadequately decoupled to ground??

The setups I designed had about half an inch of PCB track between the IC and the pot; that's about the same as the ubiquitous Vellemann kits thta use the LM317.

Not a clue pal, All i can tell you is it's working fine now after swapping the 5k pot for a 10k
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