I think my wall wart may have been pushing the 25V caps. It’s rated at 18V which I calculate is just over 25V… But the brick is putting out 22vac
That looks like it smells like an old organ on fire.
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Cheese’n’rice-You might want to check the rest of those caps for Xenomorphs!
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Fixed for now. 16V adapter is the lowest I had arround. it’s running arround 19V so just over margin on the caps. I will find something lower
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or higher rated caps ?
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possibly, not sure there is space.
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The regulators turn the difference between input and 12 V into heat, and 22×1.41−12 is almost 4× more than 12×1.41−12. So yeah, you can beef up the capacitors, but you’ll only get 1/4 the power out of the supply with the same heatsinks, before overheating.
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Ok thanks for that, I will go and find a 13-14VDC supply and take the rectifier out… Then I should hopefully be in the right ballpark…
Cheers
Rob
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No, the microbus cannot make −12 V out of +13-14 V (and it cannot make +12 V out of it either, the 7812 needs a bit more to work with). There’s a reason they tell you to use a 12 V AC supply 
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Ah yeah my heads a bit fuzzy been looking at P-to-P figures but only taking half the wave LOL…
Spent half an hour at my workshop again rumaging through boxes, all sorts in there.
Found a 18V DC supply, but it was more than just a Transformer and rectifier…
Found an old lighting desk power supply… That was actualy a board for +/- 12V DC… But the -12 components were unpopulated…
Found a 100VA 12V AC supply… IDEAL LOTS OF POWER… Plugged it in and it was dead, replaced the fuse, there was some sort of control board inside, the battery ( i assume ) on that went bang… removed the control board and although the fuse was still ok, the transformer made a loud hum… Gave up on that one…
One of the supplies I brought home yesterday, looked at it again. 12V AC 1.5A… But the Plastic earth pin is broken… Fortunatly in this case it’s not solvent welded…
So Hopefully that will work ok…
Cheers for the pointers. I must have coverd this 30 years ago in my School electronics course, but Too many years and glasses of wine have taken place inbertween.
Rob
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If your AC value is 22 Volts and you add a bridge rectifier to it and some capacitors, the DC top voltage will be about 31 volts (22 / 0.5 * sqrt(2)). So much higher than the capacitors in the picture can handle. Therefore it is not surprising that they blow up.
18 volts AC would be border line as it leads to 25.5 Volts DC.
So you need to lower the input voltage or exchange the capacitors with say 40 Volt or 64 Volt types.
What was the recommended / required voltage of the caps in the BOM?
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@twinturbo With 12VAC input voltage those capacitor values are fine. That will result in 17.9 V DC, so well below what the caps can have. So the problems were caused by not using the recommended input voltage / the right transformer / wallwart in this case.
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yeah… I have hopefully a 12VAC psu in working order.
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And here I was just wondering if I could put 16v caps instead of 25 in my new PSU when I found this thread…
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Yeah, the given AC voltage is a root-mean-square (RMS) average of the actual voltage on the wire. This is convenient if you talk about power transfer, but you have to be careful if you’re looking at peak (or peak-to-peak) voltages – the peak is sqrt(2) = 1.41× times higher, peak-to-peak (the full wave) twice that, or almost 3×.
So with 12 V AC in, the capacitor will see voltages up to 12×1.41 = 16.9 V 
However, there’s a voltage drop over the rectifier diode (VF), which the datasheet says is 1.1 V, so you get 12×1.41−1.1 = 15.8 V 
However, the 1.1 V is a max value, and can drop below 0.7 V if you’re not pulling a lot of current through the diode. That would put you above 12×1.41−0.7 = 16.2 V
Also, a lightly loaded transformer will output a higher voltage than what’s on the label, often 10% or more. 1.1×12×1.41−0.7 = 17.9 V
So ok, you’re only 12% above the rated voltage (OP was +22% when things blew up, I think) but going for the next higher voltage rating is probably worth the effort.
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Stay safe with the 25’s the cost is insignificant.
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I just happened to have some extras. They shall remain forever alone.