Snubberized LM338 regulated PSU problem

[Nuuk]

You're like me Rob - did all this so long ago, I can't remember what I put in and what I didn't!

I'll try the zener mod, and when I get them I'll add some 0R47 10 watt resistors before the regulator inputs (if I can find the room )

 

[Nuuk]

You could try something a bit simpler - like a zener diode across the regulator. Say a 1.3W type, 33V. I have no experience of such solutions, but it sounds like it might be worth a try to limit the transient overvoltage.

Am I correct in assuming that the zener goes across the input/output the opposite way round to the protection diode, ie cathode to output?

 

[abraxalito]

No, same way around as the protection diode - zener diodes work in reverse breakdown so the cathode is more positive. You won't need the protection diode once you've installed the zener, it''ll do both duties.

 

[Nuuk]

Thanks again. I didn't realise that I can remove the protection diode as well. I was going to solder the zener to the bottom of the reg legs but it would be neater to replace the IN1002.

 

[sasmit]

When you say "across the regulator" do you mean across the input and output pins?

Do you have the protection diodes that are required if you're having large capacitance on either side of regulator.
I have built LM338 based regulators and still use them running from a 0-30,0-30 - 330 VA trafo with no soft start.
If you do not have the protection diodes the capacitors will discharge through low current paths of LM338 destroying the regulator, this is more likely to happen at switch off.

 

[Nuuk]

Do you have the protection diodes that are required if you're having large capacitance on either side of regulator.
I have built LM338 based regulators and still use them running from a 0-30,0-30 - 330 VA trafo with no soft start.
If you do not have the protection diodes the capacitors will discharge through low current paths of LM338 destroying the regulator, this is more likely to happen at switch off.

Yes, I had both protection diodes fitted from the start. I've now removed the one from input to output, and replaced it with a 27 volt zener as abraxalito suggested. When it's cooler, I'll cobble in the 0R22's and re-test.

BTW what VA is your traffo?