The Voltage Doubler

[pwdrive]

A picture or two says a thousand words, the full wave voltage doubler explained easy style:

http://www.youtube.com/watch?v=byCtTlUlHXo

Lawrence.

[Radio Fixer]

Its well explained but I think being in his class for days on end would be a bit of a strain.

Not mentioned was transformer derating: http://www.hammondmfg.com/pdf/5c007.pdf

Voltage douber isnt given but to me the derating would be the same as half wave, into capacitive load, as effectively thats what a doubler is and stacking the cap voltages. So for HW and Doubler I DC can only 0.28 of the transformer rated current. Lets say that is 1A then I DC can only be a max of 280 mA.

F. Langford Smith in Radio Designers Handbook says transformers get very much hotter when rectification takes place on the secondary compared to it supplying just AC into the load. “This severe heating is due to the poor form factor of the rectified current”.

Gary

[Skywave]

It covers the basics, but I was confused (at first) by his use of electron flow as opposed to conventional current flow. He also stated the voltages across the two capacitors as "200-v.", but failed to specify, in numerical terms, the secondary voltage of the transformer. Finally, whenever I have seen (and used) that voltage doubler cct., I have always seen (and installed) a reservoir cap. across the load (in addition to the two caps. that are integral to the voltage-double operation). That cap. did not feature in that lecture.

Al.

[AlanBeckett]

Yes Gary, he was pretty droll, but imagine the same thing now:-

A Celebrity Presenter would dash onto stage to loud music and, while panting, explain that there was only 5 minutes available to explain everything.
The explanation would be accompanied by much zooming in and out, strange camera angles and more loud music.
There would be animations and simulations, probably incorporating fast cars, farm gates and tanks of coloured water in order to make it 'Accessible'.
Finally the Presenter would vanish in a puff of smoke with a bit bang,

I think I'll stick with the old version.

Alan

[Skywave]

Yes Gary, he was pretty droll, but imagine the same thing now:-
A Celebrity Presenter would dash onto stage to loud music and, while panting, explain that there was only 5 minutes available to explain everything.
Alan

The reason for such a hurry is because he will be doing it "on the double!"

Finally the Presenter would vanish in a puff of smoke with a bit bang,
Alan

And that would probably be because he didn't choose his diodes with an adequate PIV rating - just like the presenter in that video didn't mention it either.

Al.

[pwdrive]

It covers the basics, but I was confused (at first) by his use of electron flow as opposed to conventional current flow. He also stated the voltages across the two capacitors as "200-v.", but failed to specify, in numerical terms, the secondary voltage of the transformer. Finally, whenever I have seen (and used) that voltage doubler cct., I have always seen (and installed) a reservoir cap. across the load (in addition to the two caps. that are integral to the voltage-double operation). That cap. did not feature in that lecture.

Al.

He said that in the first instance that the capacitor charged up to the peak voltage of the transformer and the voltage across that capacitor when fully charged was 200 volts.
I don't think the lecture was supposed to be an exercise in voltage doubler design theory but rather an explanation of how the voltage doubler worked, to that end I think he did a good job, reminds me of our tutor when I did my City & Guilds, concise and to the point with no distractions.

Lawrence.

[Skywave]

O.K., Lawrence; "transformer voltage": I guessed I missed that bit.

Al.

[Mark Hennessy]

That is the arrangement for every switched-mode PSU that has a voltage selection switch. Instead of one smoothing capacitor, there are two. For 230Vin operation, they are in series, but when the 115V switch is operated, neutral is connected to the junction of the two capacitors. Of the diode bridge, two diodes become non-operational, and the other two form the voltage doubler in conjunction with the capacitors. Very neat. Additional capacitors aren't needed.

Of course, modern power supplies with active power factor correction use a boost converter ahead of the main smoothing capacitor, and this performs the function of automatic voltage selection while making the current draw look somewhat sinusoidal. Also very neat.

In short, power supply design is fascinating

[Radio Fixer]

Hi Lawrence

I wasnt knocking the technical presentation of the topic and only mentioned the required transformer de-rating as its easy to over look and not so commonly publicised.

Gary

[pwdrive]

Hi Gary, no worries, you highlighted a valid point with regards to transformer ratings and waveform shape, I once read a post on another forum regarding the pros and cons of replacing a valve rectifier in a vintage receiver with one of a lower heater current in order to reduce the heatr dissipated in the old transformer, the upshot was that it made very little difference, most of the heat generated was because of the rectifier/filter current draw etc. I have to admit though that transformer theory was never a strong point of mine.
This raises the question of say a typical HT transformer eg: if a transformer has 250v 0 250v (center tapped) @ 100ma stamped on it what current can it safely supply in a typical rectifier/filter situation?

Lawrence