Quick capacitor ?...

[WilliamD]

I have a 40V 40,000 micF capacitor, and a 50VDC transformer. What happens if you run more than the rated voltage through the capacitor? I'm sure if it's too much it would explode, but what is the margin? I have 2 75V caps on the way, but in the meantime I was wondering what would happen if I used the 40V cap.

[NC Cams]

It is OK to run a higher voltage rated capacitor than the applied voltage but it is NOT OK to run a capacitor at a voltage higher than it is rated.

If you do over-voltage a cap, you can let the smoke out of it in a violent fashion. As far as I"m concerned, there is NO safety margin for over voltaging a capacitor.

[2muchstuff]

zaa-pooof

[ViperTX]

...yes some smoke.....and lots of flying debris.....

[2muchstuff]

I remember in high school electronics class taking small low voltage electrolytics and connecting them across the line. In the blink of an eye, there was a pop, a puff of smoke, flying aluminum foil and the can stuck in the ceiling tile.

[NC Cams]

Want to bet somebody will try it anyway???

Should somebody try it, they will quickly learn the difference between knowledge and wisdom.

We've imparted the knowledge - we may never know if wisdom is/was gained.

When one ignores the knowledge imparted by training, wisdom is gained by first hand experiencing the ramifications of someone doing something they were told NOT to do and they did it ANYWAY.

[rboeser]

This is a serious issue with a capacitor of this size. PLEASE don't apply a voltage anywhere near what the cap is rated for. I try to use a minimun of 20% safety margin. This means (using simple numbers for math reasons) if my DC recitfied voltage is 80Vdc, my cap would be rated at 100Vdc min.

It is always good to use a higher voltage rating, NEVER use an equal or lower voltage rating.

Remember, the AC volt reading you see coming from the secondary side of the transformer will read 1.44 times higher when converted to DC through a bridge rectifier.

An example: the secondary AC volts is 55Vac, after rectification, it will read roughly 80Vdc. To do the math, 55Vac x 1.44 = 79.2Vdc.

The 1.44 is not a magic number, it has to do with the RMS (root mean squared). No need to know what RMS is or means, just remember 1.44 x AC volts = DC out volts.

It never hurts to use a large value cap along with a low value cap. The low value cap reacts faster and helps smooth out the DC output, the large value cap does the "heavy" work, the low value cap helps fill in the ripple.

A simple rule of thumb for a value (in microfared) of the cap is 1,000uf per designed amp draw for a simple DC power supply like most of us are making.

Example: My transformer is rated at 10 amps. I need a minimum of 10,000uf cap. This can be 2 seperate 5,000uf caps, 3 seperate 3,300uf caps and so on. More capacity is good, less will give you a ripple on the DC output. Sometimes this ripple is acceptable, most times it is best to use more capacity (larger value caps.)

[WilliamD]

Thanks for the replies guys! I kind of figured, but wanted to be sure. Luckily, I never tried it first! Good to know that there's not even a litte room for error, and will plan accordingly.

[unterhaus]

my understanding is that you can increase the voltage rating by running caps in series if there is a voltage divider limiting the voltage across each one. In some cases, it's worth it.

I wouldn't do it. I hooked up an electrolytic cap backwards once, and it blew up. The lab had an unpleasant smell all day. And the voltage rating of caps is notoriously wrong. Everyone derates caps, except people that don't care if they fail.