Cap Voltage rating for 1/2-wave voltage doubler - question

[tubeswell]

In Chapter 3 Merlin's power supply book, there is a statement about cap voltage rating for a 1/2-wave voltage doubler I wish to have clarified please. Please refer to the attached schematic. According to the book, C2 must be rated for 2 x the peak AC voltage. I take it that's the peak DC voltage rating.

I have some 4700uF caps rated at 16V, and I would like to get the doubler to work in order to supply a 12VDC fan (draws 80mA) to blow little breeze on the output tubes. I don't have any 4700uF caps with a higher voltage rating handy - the next biggest voltage rating/higher capacitance I have is 220uF 25V, but they won't cut the mustard for the CR>/=1 equation (for the performance of the supply).

My understanding of all of the above is that the caps should be rated at 6.3 x 1.4142 x 2 (i.e.: 17.9V). So would I get away with 4700uF caps rated at 16V? Any bets?

TIA

[defaced]

Don't forget to include the diode voltage drops (1.4v for this design). For the sake of proofing it out, you might want to build it with the 220u cap and measure the final voltage.

Here's a page I found while digging up some info on this: http://blazelabs.com/e-exp15.asp

Also, you can run the fan at less than 12v, it will just spin slower which may or may not be an issue for your design goals.

[R.G.]

What happens if the AC power line goes 10% high for a while, and/or if your "6.3Vac" is really 7Vac unloaded (i.e. when all the tubes are pulled), or both?

So the answer to "can I get away with it?" is maybe; depends on how long you insist on getting away with it and when you run into some bad luck. I'd say it's perfectly OK to do for testing things out when you're right there and if a cap vents you can catch it and clean up. But I would not do it for any other reason and certainly would not count on it working long term reliably. But I'm big on designing for reliability. I don't ever want to fix something I build when it breaks.

Capacitors do have some "surge voltage" rating that the manufacturer specifies. It's often 10% or 15% higher than the DC rating. They have special caveats about how much and how long they will warrantee their caps will work under an overvoltage surge. But betting that a surge will never get bigger or longer than that is like drawing to an inside straight.

[pdf64]

I've successfully run 12V pc type fans with about 9V dc, derived from the 6.3Vac heater supply via a FWB rectifier and reservoir cap (~330uF).
No perceptible induced noise, EMI etc and plenty of airflow from 60 and 80mm fans.
Fans mounted to get some airflow around the PT and power tubes.
Based on that, I wouldn't bother with the voltage doubler method - you'll need to cut it back to 12V somehow anyway.
I got the idea from
http://www.blueglow.de/FenFan.html
Pete.