Exploding Capacitor

I was testing my auto reverse model train controller and let the smoke out of a cap. Well, more like yellow snot, all over my breadboard.

Here is the relevant piece of my diagram.

TinyDRV8838 - Copy.png

I wanted to test that I was getting 5v out of the regulator. I was. 4.98v by my DMM from point A to point B.
Hmm… Something smells hot. Yup cap gettin hot. Tried to unplug the transformer but not it time.

I triple checked polarity before and after the Genie escaped and am reasonable sure I got it right.
Then I saw the voltage rating on the cap. 330uF 10V. The others in my sample kit are 50v and various numbers of farads.

If I feed 20VAC into the diode bridge and put a 330uf 10v cap across, is the expectation smokey?
I ordered some others in various voltage ranges.
Will 330uf 35V do the trick?

Hi, 20Vac will give you 20 * 1.414 = 28.2Vpeak on the DC side of the rectifier.

This is what your 10V rated capacitor would have had momentarily across it.

Use the 50V rated cap.

Check that the step-down module is rated to 28Vdc on its input.

Tom.. :)

Transformer output voltage ratings are for full load - under no/low load the voltage will be about 10% higher. If your mains voltage is higher than average that's another 5%, and if you get power surges (lightning, strip-lights switching, fridge motors switching, even the model trains themselves!), derate some more.

So that 28V out from the bridge could easily get to 35V occasionally, the 50V is a wiser choice than 35V for the cap (although the capacitor itself will help swallow any votage spikes). Adding a MOV on the incoming supply also helps cut out spikes which is a good idea.