Voltage doubler questions

db!

Member
2009-11-21 4:53 am
Ontario
Do the capacitors on a voltage doubler need to be non-polar? Is there a type of capacitor recommended for use with a voltage doubler circuit?

I have also noticed that there are different "topologies" of doublers (delon/bridge, CW, villard); do they all perform the same?

I'm trying to pull [email protected] from a 120V 0.5A transformer; is this doable?

thanks
 
Doubler capacitors can be standard electrolytics, for your application 470 uF 200V electrolytics in a full wave doubler configuration will give you a little over 300V with more than enough current.

Most older computer power supplies use this method when operating on 120V changing to a bridge rectifier with series capacitors when switched for 240V operation. So you can strip a computer power supply for parts if need be.
 
You will not get quite double the voltage. Do you mean 270VDC from 120VAC? 120mA from 500mA sounds reasonable. Capacitors do not need to be non-polar, but you need to be a little more careful about polarity.

I use a Delon half-wave voltage doubler for a microphone pre-amp, to get +/- 10VDC from a 9VAC transformer.
 

db!

Member
2009-11-21 4:53 am
Ontario
Thanks for the replies!

I plan to use the doubler to raise the voltage and an RC filter circuit for the final output. I have simulated it on PSUD2 and it theoretically should work. Also, how should an electrolytic capacitor be connected in terms of polarity? Most VD circuits don't indicate polarities...

metalsculptor - Thanks for the tip, I just garbage picked a few PSUs last week.
 

Elvee

Member
2006-09-08 2:04 pm
Do the capacitors on a voltage doubler need to be non-polar? Is there a type of capacitor recommended for use with a voltage doubler circuit?

I have also noticed that there are different "topologies" of doublers (delon/bridge, CW, villard); do they all perform the same?

I'm trying to pull [email protected] from a 120V 0.5A transformer; is this doable?

thanks
For the Greinacher/Schenkel topology, the input capacitor should be non-polar, as it can see a reverse in polarity when powered on.
In addition, the ripple frequency of these doublers is the line frequency, which is why the Delon/Latour topology is preferable where possible: but the input and and output have no common connection, which can be a problem for some application.
With properly sized capacitors, you should be able to get your 270V out.
 
..................I'm trying to pull [email protected] from a 120V 0.5A transformer; is this doable?
I don't think so.
The doubler still effectively loads the transformer with a capacitor input filter.
The manufacturer of the transformer will have a de-rating factor for this type of load.
It is commonly around 0.7, but does vary from type to type and from manufacturer.

120V @ 0.5A is 60VA. Applying the de-rating factor results in a maximum continuous wattage of 42W.
At 270Vdc, 42W allows a maximum continuous current of 155mA.
At this output level the transformer is running at 100% capacity.

But the voltage of the output has been pulled down from ~340Vdc to 270Vdc to attenuate the output ripple. This, I believe, means that 340Vdc should be used to determine the transformers maximum rated DC current from the doubler.

340Vdc @ 120mA is equivalent to 40.8W or about 97% of rated capacity.

I think the transformer will run hot.
 

Elvee

Member
2006-09-08 2:04 pm
Using realistic parameters for the transformer, this looks possible (to be confirmed by measurements).
A series bleeding resistor R2 has to be added; its actual value depends on the copper resistance of the windings.
 

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I don't think so.


But the voltage of the output has been pulled down from ~340Vdc to 270Vdc to attenuate the output ripple. This, I believe, means that 340Vdc should be used to determine the transformers maximum rated DC current from the doubler.

You are assuming that a transformer is a perfect AC source with the the winding resistance in series. It is not, there is a a reactive component as well which is quite high in small transformers especially split bobbin types, this makes the transformer behave like it has a small choke in series with the filter capacitor. Effectively this reduces the output voltage without generating additional heat. A toroidal transformer will be closer to your worst case assumption.

Microwave ovens also use a voltage doubler and deliberately introduce additional leakage inductance to better utilise the transformer (split bobbin with low permeability spacer between windings)
 

db!

Member
2009-11-21 4:53 am
Ontario
Thanks for the replies!

Sorry for not mentioning earlier, 120mA is my "conservative estimate" for the maximum current draw. The quiescent current will be about 32mA; if it get anywhere near 120mA, the transformer will be the least of my worries. The max plate current and dissipation is 31mA(+/-9mA) and 8W respectively, I doubt it will draw any more current. The transformer I was planning to use is a hammond 167G120.
 
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megajocke

Member
2003-01-11 8:01 pm
It should work fine.

Iac(rms) = 1.8 * Idc is often a good estimate for a middle-sized transformer, bridge rectifier and sufficiently large capacitors (ripple less than about 40% or so).

For a doubler the AC current doubles with the waveform being the same. This also assumes relatively large smoothing capacitors.

Iac(rms) = 3.6 * Idc fits well with Elvee's simulation, giving 468mA for a 130mA load. (469mA simulated) :D