Voltage doubler putting out 3 to 4 times input voltage

[Joeriz]

I recently purchased a small 12vdc fan to mount in my Deluxe Reverb to keep it cooler. My thought was that I could tap into the 6.3v filament supply and, using a voltage doubler, run the fan from there. So I built the circuit outlined on this page:

http://www.learningaboutelectronics.com/Articles/Voltage-doubler-circuit.php

However, when I use the 6.3vac as the input voltage, my output voltage measures about 26vdc. I also tried using a 9vac wall wart I have on hand as the input voltage and got about 33vdc out.

I’m confused for two reasons. First, why is the output voltage not the same multiple of the input voltage in both cases? And second, how can I get more than 2x the input voltage using only 2 capacitors? My (limited) understanding of voltage multipliers is that one would need 3 caps for triple, 4 caps for a quadruple, etc.

I have checked my wiring multiple times and I don’t see anything obviously wrong.

Any thoughts?

Thanks!

[sluckey]

It's either miswired or your meter is lying. But why fool with a doubler. I suggest using a simple bridge rectifier and one cap. That will produce about 8.9VDC and will operate that fan just fine, although a bit slower but also a bit quieter.

***CAUTION*** Do not connect any part of this circuit (or your doubler) to chassis!!!

[glass54]

+1 with Sluckey 
Used it on many Marshalls, Boogies esp .22/.22+ (stops that R132 resistor falling out of PCB!!) and Voxs esp AC30.
The slower fan speed reduces noise to almost inaudible from front of amplifier.
see attached example.
Kind Regards
Mirek

[Joeriz]

Ok, thanks. Still a head scratcher as to how I am getting so much more voltage than expected. My meter seems fine...I checked it with a few new batteries and my wall voltage. And I have no idea how I could wire two diodes and two caps and get that much excess voltage.

I have 4 diodes to go the bridge route as you recommend. Any particular size cap I should use?

[sluckey]

Still a head scratcher as to how I am getting so much more voltage than expected.

Exactly how are you measuring the voltage? Seems impossible to get 26Vdc from 6.3VAC using that simple doubler.

I would use a bridge module, only about 1" square and uses a single screw to mount to chassis. 470µF/25Vdc cap fits directly onto the bridge. Two AC wires input, two dc wires output to fan. Small, neat, simple.

[Joeriz]

Seems impossible to get 26Vdc from 6.3VAC using that simple doubler.

My thought exactly. I am just measuring where the schematic says “voltage output”...black lead to - and red lead to + with meter set to DC volts.

I already have the doubler built on a terminal strip using on-hand parts and was going to mount that on the chassis. If I can’t figure out the issue I will go the module route as you suggest.

Thanks!

[sluckey]

 Did you have a load resistor across the pos and neg output? What kind of meter? brand and model

[Joeriz]

No load resistor across the output.

[sluckey]

Put a 100Ω resistor across your meter leads and measure again.

[Joeriz]

OK, better. I tried using the 9vac wall wart and now I got about 16vdc out.

Thank you.

[sluckey]

Measure the resistance of your fan.

[Joeriz]

I can’t seem to get a reading. I just get “OL” when I measure across the fan’s power leads.

EDIT: Regardless of the above, I went ahead and ordered the bridge module as you suggested...just seems like a much more elegant solution. Thanks!

[sluckey]

I can’t seem to get a reading. I just get “OL” when I measure across the fan’s power leads.

What kind of meter? brand and model

[Joeriz]

YFE YF-3220

[sluckey]

Meter looks fine. It would be interesting to see how another meter reads that voltage. Even more interesting to see it on an oscope.

[shooter]

I just get “OL”

fwiw;
I wired ups some old 24vdc fans, was gonna "switch" polarity so they would suck or blow, NOT!  they have an internal diode(?)  was a first for me

so measure resistance both ways across   

[PRR]

Most DC PC fans are "dead" until they get several Volts to wake-up the internal chip. So a 0.2V Ohmmeter won't read right.

The "Doubler" doubles the PEAK voltage.

If you have perfect Sine 9.0VAC the peak is 12.7V, so 25.45V DC ideally.

A simple transformer that says "9VAC" and a load current, with NO load is going to read 10%-30% higher (everything sags).

If your power line has spikes on it, an un-loaded cap-input filter will charge-up to those peaks. It may not take much load to integrate narrow peaks so the output is more like what you expect.

[Ed_Chambley]

I use fans on the AC side. 120vac. Look at a traynor schematic. Easy and no noise with a cap.  I have them in all my amps and it sure keeps them looking better. Tubes are not hot either.

[dude]

Try this, maybe only 6 or 7 volts to fan but quite and works fine, have fan suck air from tubes, use a 220uf/10 to 20v cap.  1N007 diodes

[sluckey]

Try this, maybe only 6 or 7 volts to fan but quite and works fine, have fan suck air from tubes, use a 220uf/10 to 20v cap.  1N007 diodes

You would need a 6.3-0-6.3 (12V w/CT) PT for that circuit.

[glass54]

  This one is really simple, works on 6.3V AC Heater with 12V DC fan (tried and tested by many folk)
Works regardless whether 6.3V is centre-tapped or pseudo tapped ie 100R resistors to chassis.
(don't wish to offend anyone)
Kind Regards
Mirek

[dude]

Try this, maybe only 6 or 7 volts to fan but quite and works fine, have fan suck air from tubes, use a 220uf/10 to 20v cap.  1N007 diodes

You would need a 6.3-0-6.3 (12V w/CT) PT for that circuit.

The voltage wouldn't be 12 vdc to the fan, but normal common heater circuit with CT would supply around 7 maybe 8 vdc, enough to run that 12vdc fan.  The voltage might not be high enough to kick the fan on but my experience is that it is, and it runs slower, quite but enough air flow to keep tubes cooler, true...?                                                                                                                                           
Does he has a 5V rectifier tube tap:

[sluckey]

but normal common heater circuit with CT would supply around 7 maybe 8 vdc,

Not with that 2 diode conventional rectifier circuit you posted. That circuit will only give about 4.5VDC.

[Joeriz]

Thanks to all for the help!