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

[martin manning]

The first cap sees 1x Vp-p, so no problem there. The second one will see just about 16VDC if the secondary voltage is 6.3VAC. Right on the rating, but there is very little ripple. Are you going to add a 12V regulator to the output?

[Firestorm]

Are you going to add a 12V regulator to the output?

If all you plan to run is a DC fan, voltage regulation isn't especially critical. DC fans don't care (AC ones do). You could even supply a DC fan simply by putting a FWB on the 6.3V winding and filtering the output. The fan will run just a little slower than it would at a full 12VDC. Variable speed DC fans usually have just a voltage divider in the supply line.

[martin manning]

I was thinking a regulator would take the 16V down to 12 and not run the fan quite so hard.

[ToneMerc]

IMHO, I would config it as a doubler, use a LM7812(LM217 if I wanted variable speed) with 220uf on the input and output. There's very little ripple on the output of a linear regulator, so the 220uf would be fine.

TM

[FYL]

Any bets?

Using 16V caps on a 6.3V AC source should be OK. But voltage ratings are only part of the specs : half-wave voltage doublers are very hard on the caps. C1 sees 120 Hz pulsating DC; C2 gets a lot of ripple. They should be fully qualified for high ripple and preferably be very low ESR models.

[tubeswell]

Any bets?

Using 16V caps on a 6.3V AC source should be OK. But voltage ratings are only part of the specs : half-wave voltage doublers are very hard on the caps. C2 gets a lot of ripple.

Yes that's why I was pondering about it. The inrush-charging peak is high, but is it 2 x 1.4142 x VAC high? Or is it some lesser figure for Half-wave rectification? And does the forward voltage drop in the diodes alleviate things somewhat?

[martin manning]

I believe the current pulses are the issue. How much current does this fan draw?

[tubeswell]

I believe the current pulses are the issue. How much current does this fan draw?

One model I'm thinking if using is a tiny 4cm diam model that draws 80mA because it will fit more neatly in the available space. The other model is about 8cm wide and draws 200mA.

Both are 12VDC brushless fan motors

[martin manning]

How about wire it up with the 4200u caps and the little fan and see if the caps get hot?

[tubeswell]

Well the fan works with a FW Bridge and 2 x 4700uF (16V) in parallel for the reservoir, delivering about 7.6VDC. The trouble is where its mounted, its sits right next to one of the speaker magnets, and that stops it from working. (Worked fine with the chassis out of the cab). Not sure if getting any more Volts to the machine would overcome that. Will have to relocate the fan.

[martin manning]

Try the doubler. You'll get about 14.5-15V according to a simulation I threw together.

[Tone Lover]

Not to hijack the thread but does using the fan add any noise into your heater circuit that could be picked up any where.
I have thought about using a fan a couple times but have always been worried about adding noise.
sorry for the newbie question.
Thanks Bill

[tubeswell]

Not to hijack the thread but does using the fan add any noise into your heater circuit that could be picked up any where.
I have thought about using a fan a couple times but have always been worried about adding noise.
sorry for the newbie question.
Thanks Bill

Not with a DC fan. An A/C fan can put quite a lot of noise into the signal path though, depending on where its situated