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

[stingray_65]

OK,

This does not address your actual question (which I cannot give you help with), If your powering a fan, why all the heavy filtering?(4700uf) .

Better stated, what is this CR>=1 formula you mention.

[tubeswell]

OK,

This does not address your actual question (which I cannot give you help with), If your powering a fan, why all the heavy filtering?(4700uf) .

Better stated, what is this CR>=1 formula you mention.

It is a constant which is a benchmark for how well the type of filter performs in regulating the power supply where R represents the load and C represents how much capacitance is available. If this case, 4700uF x 150R (the DC resistance of the fan for the load) gives (0.0047 x 150) = .705. And according to the graph in Fig 3.2 in Merlin's 2nd book (on power supplies), a half wave voltage doubler with a score of .7 delivers just over 90% of the theoretical maximum DC voltage that is achievable.

[sluckey]

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?

I bet you could get away with it. However, I wouldn't bother. I'd just use a bridge and a single 470µF or 1000µF to put about 8 volts on the fan. The fan will still be happy.

[PRR]

> 2 x the peak AC voltage. I take it that's the peak DC voltage rating.

DC doesn't have "peak"??

> caps should be rated at 6.3 x 1.4142 x 2 (i.e.: 17.9V).

Yes; a bit less due to diode drops.

But a bit more because transformer sag is uncertain, utility company has spikes, you should not run a cap AT rating, caps in this size are not very expensive to over-design.

> So would I get away with 4700uF caps rated at 16V? Any bets?

Placed my bet 30 years ago. 37V on large "35V" caps. Bet it would work a while. Was still working fine 2 years ago.

> why all the heavy filtering?(4700uf)

For "12V 0.08A" load, values near 470u give two volt ripple, values near 4,700u give a quarter volt ripple. If fan wires run near preamp, 2V ripple is a lot.

Why do you want a fan?

[tubeswell]

> 2 x the peak AC voltage. I take it that's the peak DC voltage rating.

DC doesn't have "peak"??

Duh! I meant the PP AC voltage (which should equal the maximum achieveable DCV in a perfect world - but of course there are losses etc)

Why do you want a fan?

T'was an amp I built a couple of years ago where I squeezed the output tubes quite close together and it really heats up, and I tried standing a mini fan behind it last time I used it at a gig and it kept it nice'n'cool. So rather than go to the trouble of relocating the output tubes at this stage, I thought I try a little permanently mounted fan.

I have got the rectifier circuit and the fan mounted in the chassis and working now, but when I stick the chassis back in the (combo) cab, the fan is right next to the speaker magnet and it stops working. So I have a little more mucking about to do with it yet.