frying PSU capacitor, any idea why?

[pierrer]

Hi,
I built the filter board project presented here:
http://archive.siliconchip.com.au/cms/A_30278/article.html

I did almost everything as they advise: same component refs/values save for filter cutoff tuning resistors/condensers, but I replaced the 1N4004s by UF4007s. Here is the schematic of the PSU:

On powering up, the 100uF cap following the 7915 fries. I have checked its polarity and it is installed the right way round. It doesn't happen to the capacitor placed after the 7815. The recommended 0.5A fuse also blows. I tried to measure the output voltage of the 7915 beforehand and it seems to be fine.

Any ideas what might be going wrong here?

 

[JohnRoberts]

check for a shorted diode.

JR

 

[radardoug]

The circuit will work without that cap fitted, but check your diodes, and make sure you have clean dc across the 1000uF, and then that the 7915 is giving you 15 volts.

 

[Rob Flinn]

The rectified 15v will be 1.414 x 15.  This is getting close to the 25v rating on the 1000uF caps.  Personally I'd use a higher voltage rating in that position.

 

[pierrer]

I made some checks: no shorted diode, transformer secondaries both deliver close to 15Vac as expected (14,8 Vac to be precise),  and yes Rob i measured about 20Vdc across 1000uF caps pins.

I am aware that the 100uF cap is not necessarily required but i believe it is still recommended to reduce oscillation on rectified +15/-15. But the fuse blows, too, and I don't have time to measure what the rectifiers actually deliver.
I put a 1A fuse and it didn't fry, so I measured the voltage across ground and 7915 output and read -0,8Vdc or so - i believe it fried too... but I think I read something like 18Vdc across the 7815's out and ground pins, how is that even possible??

Any more expertise welcome!

 

[JohnRoberts]

One thing to be careful about the heatsink tab on the 7915 is not ground but - unregulated,

JR

 

[SSLtech]

78xx series are pinned out:  in/COMMON/out.

79xx series are pinned out:  common/IN/out.

...where the middle (bold typeface) terminal is connected to the heatsink and mounting hole.

I can't think of any reason why a correctly-terminated capacitor would ever blow, other than extremely-random one-off failure.

 

[pierrer]

extremely-random one-off failure

do you mean failure of the regulator?
If so, it should be sorted by simply replacing the 7915 by a new one?

 

[JohnRoberts]

The image you posted above shows the different pin out for 79xx vs 78xx regulators.

Capacitor failures can be caused by several things.

1- an electrolytic cap installed backwards can draw excessive current and fail.
2-an electrolytic cap with higher than rated DC voltage can fail.
3-an electrolytic cap with too much ripple current (like from a shorted diode so the cap is driven with AC) could fail.

Check the wiring of the regulator while that should not affect the cap.

JR

 

[pierrer]

i did check and recheck the wiring of the regulator and of the cap and they are correct. No shorted diode either. Im really at a loss here. Will try putting a new regulator on sometimes this weekend and measure the output voltage - if it is fine i'll put a new cap in and see from there

 

[envelope]

Hello

If changing the regulator does not solve your issue, Is it possible for you to post detailed photos of both the solder and component side of your built circuit?

 

[pierrer]

i just realised that i never posted here to say: problem solved.

I simply replaced the 7915 and it fine with the new one in. I have no idea what happened but it's been running smoothly since.
thanks to everyone who cared to help though!

 

[gswan]

You really need the 0.1uF caps between input and ref pins and output and ref pins on these regulators to prevent oscillation, depening on layout and brand of regulator used.