Capacitor keeps wanting to blow on power supply

[frosr6]

Hi I built this power supply just recently from the circuit designed by earckens that he had designed about 5 years ago.
https://www.electro-tech-online.com...n-voltage-of-0-00v.150216/page-3#post-1294887

I have it working for the most part but ran into an issue of a capacitor that keeps getting hot and swelling up. The capacitor is is c8 on the schematic. I was testing the current limit and when I go anywhere lower than 9vdc at right around 18 amps the capacitor which is an electrolytic (100v/100uf) starts high pitch whining gets hot and swells up. I’ve replaced it but same thing. Would some metal film capacitors work? Any suggestions?

I’m using a big 1.0kva transformer that has 12/24 secondary taps. Which I have hooked up to a relay and volt sensing circuit that switches the taps automatically to reduce heat on my transistor’s. For those I’m using 2n5686, nine of them on large heat sinks.

 

[danadak]

What do you see on scope on C8 when first powered up ?


Regards, Dana.

 

[crutschow]

Any suggestions?

You might try high ripple-current rated, low ESR capacitors designed for use in switching regulators.
The higher ESR of standard electrolytic capacitors can overheat at the high ripple currents that often occur in switching supplies.

 

[frosr6]

What do you see on scope on C8 when first powered up ?


Regards, Dana.

I never checked it. The first one got hot bulged up and I replaced it thinking it was just a bad one then last night I was messing around with it and the second one bulged up. I shut it down and went to bed. I may change it out later today but seeing if anyone had seen a problem with my setup.

 

[frosr6]

You might try high ripple-current rated, low ESR capacitors designed for use in switching regulators.
The higher ESR of standard electrolytic capacitors can overheat at the high ripple currents that often occur in switching supplies.

From what I’ve been reading it sounds like you may be spot on. Idk what the ratings are for the caps I have but they’re just ordinary electrolytic ones. I’ll have to look for some. Thanks.

 

[danadak]

You might try high ripple-current rated, low ESR capacitors designed for use in switching regulators.
The higher ESR of standard electrolytic capacitors can overheat at the high ripple currents that often occur in switching supplies.

Unless I am mistaken this is a linear regulator application ?

But to your point if the regulator is oscillating at low V OP discusses that cap
may be inadequate to handle the oscillation current/V......


Regards, Dana.

 

[rjenkinsgb]

Hi I built this power supply just recently from the circuit designed by earckens that he had designed about 5 years ago.

If you look at the later schematic in the original article, C5 was changed from 470pF to 100pF as it was causing oscillation.

https://www.electro-tech-online.com/attachments/lm723-labpsu-0v-46v_v3_-complete-final_pagina_1-png.106998/

 

[danadak]

A scope examination should be able to examine phase margin with a
transient load, and at low V the OP says the occurrence of C failure
happens. If its not already oscillating under fixed load conditions.

Other folks experiencing oscillation -

LM723 Small Power Supply - psu as a beginner - Page 1

LM723 Small Power Supply - psu as a beginner - Page 1

www.eevblog.com

LTspice@groups.io | LM723 Model

groups.io

https://www.reddit.com/r/AskElectronics/comments/3oiz9x

Regards, Dana.

 

[crutschow]

Unless I am mistaken this is a linear regulator application ?

Yes, I made a mistaken assumption.

 

[frosr6]

If you look at the later schematic in the original article, C5 was changed from 470pF to 100pF as it was causing oscillation.

https://www.electro-tech-online.com/attachments/lm723-labpsu-0v-46v_v3_-complete-final_pagina_1-png.106998/

Yes I seen that update and used a 100pf for c5. C8 is the cap that’s heating up and about to blow. It’s a electrolytic cap 100uF

 

[frosr6]

Unless I am mistaken this is a linear regulator application ?

But to your point if the regulator is oscillating at low V OP discusses that cap
may be inadequate to handle the oscillation current/V......


Regards, Dana.

Correct linear supply.

 

[Nigel Goodwin]

Yes I seen that update and used a 100pf for c5. C8 is the cap that’s heating up and about to blow. It’s a electrolytic cap 100uF

As has been suggested, if that capacitor is dying, then it's most likely that the PSU is oscillating. Presumably you don't have access to a scope to check it?.

 

[rjenkinsgb]

Yes I seen that update and used a 100pf for c5. C8 is the cap that’s heating up and about to blow. It’s a electrolytic cap 100uF

Can you post a photo of the PSU as you have built it?

 

[frosr6]

As has been suggested, if that capacitor is dying, then it's most likely that the PSU is oscillating. Presumably you don't have access to a scope to check it?.

I do have one though I’m new at using one. I will some time to check it later this week. Thanks.

 

[frosr6]

Can you post a photo of the PSU as you have built it?

I will when I get a chance. It’s still a work in progress, little more fine tunning and will be able to finally close it up.

 

[Nigel Goodwin]

I will when I get a chance. It’s still a work in progress, little more fine tunning and will be able to finally close it up.

Don't you have a mobile phone?

The point of asking for a picture is to see if your construction technique is likely to be causing it to oscillate.

 

[frosr6]

Don't you have a mobile phone?

The point of asking for a picture is to see if your construction technique is likely to be causing it to oscillate.

Here ya go