Supply voltage doubler question

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In the attached circuit I've shown a voltage doubler circuit driven by a center tapped transformer ( S1,S2) . I checked the voltage across the capacitors C1 and C2 after switch on. These capacitors seem to have a reverse voltage across ( up to 15 V peak depending on component values) for one cycle followed by a lower voltage on the second cycle and then charges up correctly according to the polarity of the capacitor.

Assuming my simulator is correct , would these two reverse polarity cycles damage the capacitors progressively after multiple switch on cycles ? I added the two diodes D5 and D6 across the capacitor to limit the reverse voltage to a diode drop. Is that OK or even required ? I don't see any diodes like these being used in published circuits . Is that an indicator that they will not create a problem ?

I'll add the voltage plots to show the effect with and without the diode.
 

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In the attached circuit I've shown a voltage doubler circuit driven by a center tapped transformer ( S1,S2) . I checked the voltage across the capacitors C1 and C2 after switch on. These capacitors seem to have a reverse voltage across ( up to 15 V peak depending on component values) for one cycle followed by a lower voltage on the second cycle and then charges up correctly according to the polarity of the capacitor.

Assuming my simulator is correct , would these two reverse polarity cycles damage the capacitors progressively after multiple switch on cycles ? I added the two diodes D5 and D6 across the capacitor to limit the reverse voltage to a diode drop. Is that OK or even required ? I don't see any diodes like these being used in published circuits . Is that an indicator that they will not create a problem ?

I'll add the voltage plots to show the effect with and without the diode.

Hi,

It's an interesting circuit. In my view however I don't think it does what you want it to do. If you use the outer transformer leads without the centter tap and a diode bridge you'll get the voltage you want.
Don't forget that voltage doublers by design have a much larger ripple than 'normal' rectifier circuits and have much higher Zout meaning that the voltage drops significantly with load.

Voltage doublers are generally only used when it is impractical to have a xformer for the full voltage and where small currents are required, e.g. in high-voltage bias circuits.

jd
 
Hi Jd,
This is meant for low operating currents and yes the ripple is ( expectedly!) high.
I don't really plan to use this as I can get a much cleaner supply by just using additional windings. However from the operation point of view I wanted to check if my idea was correct about using those diodes. I forgot to add that the transformer already produces a +/- supply rail for the main circuit and the voltage doubler circuit has to use the center tap as gnd. The HV is being used by a another circuit that needs very low currents. There is enough extra voltage to sacrifice and clean up the ripple .

I'm attaching the voltage build up plot across one of the capacitors , C1.
Without the diode D5 and with the diode installed. Any ideas on that ?
Thanks.
 

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Hi Jd,
This is meant for low operating currents and yes the ripple is ( expectedly!) high.
I don't really plan to use this as I can get a much cleaner supply by just using additional windings. However from the operation point of view I wanted to check if my idea was correct about using those diodes. I forgot to add that the transformer already produces a +/- supply rail for the main circuit and the voltage doubler circuit has to use the center tap as gnd. The HV is being used by a another circuit that needs very low currents. There is enough extra voltage to sacrifice and clean up the ripple .

I'm attaching the voltage build up plot across one of the capacitors , C1.
Without the diode D5 and with the diode installed. Any ideas on that ?
Thanks.

It's not a doubler, it's a halver ;)
You get the same or more voltage from one 30V winding. You'd get the double if you use the outer windings.
I don't think D3 and D4 do anything; they are always opposed by another diode and will never conduct. Take D3 & D4 out and see if it makes a difference.

jd
 
I don't see any diodes like these being used in published circuits . Is that an indicator that they will not create a problem ?

...from the operation point of view I wanted to check if my idea was correct about using those diodes.

No, it's not.

Your circuit is FUBAR.

You don't understand what you're doing. Don't fiddle around with it any more.

Just copy a conventional circuit. Connect a bridge rectifier across one winding and follow it with a cap.

An externally hosted image should be here but it was not working when we last tested it.


w

If you want to be pedantic you can build a full-wave centre-tap rectifier and save 2 diodes. Look it up...
 
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No, it's not.

Your circuit is FUBAR.

You don't understand what you're doing. Don't fiddle around with it any more.

Just copy a conventional circuit. Connect a bridge rectifier across one winding and follow it with a cap.

An externally hosted image should be here but it was not working when we last tested it.


w

If you want to be pedantic you can build a full-wave centre-tap rectifier and save 2 diodes. Look it up...

Don't be such a scientist... ;)

jd
 
Hmmmm....:)

Let me add that apart from those two diodes D5 and D6 the circuit is functional and working for quite a long time. ;)

In fact the circuit is borrowed from one of the threads on this forum. I have shown only one half , the other half produces the -ve doubled voltage.

Wakibiki , you probably haven't understood how this circuit works. If you spend a few minutes you can see how the voltage doubles here. It is quite simple . You also haven't apparently understood my question.

You will find that as the supply switches on , the initial current flow through the capacitor C1 will reverse charge it ( junction with diode D1 and D3 will be -ve) by an amount depending on the ratio of C1 and C3 and the current flow. On subsequent cycles the capacitor C1 will get charged such that the side connected to the junction of diodes D1 and D3 will be positive. After that the voltage across C3 keeps ramping up till it reaches close to double the peak supply voltage. Subsequently C1 will always be positive at the junction of the diodes D1 and D3 .There are some drops in the circuit and other losses and so it will never reach the full doubled voltage. Any load comes in parallel with C3 and hence the greater the load ( more mA) the lower the output voltage. The circuit has it's uses and it's limitations.

Now the "question" was , "are the diodes D5 and D6 of any use ?".

On further reflection I think the appropriate answer is that it does cut out the reverse voltage across C1 or C2 to a diode drop for the first cycle or so. But since this happens briefly only at switch on and not after the circuit is operational plus it happens for just one or two half cycles ( 20 mS), it can be dispensed with. It shouldn't have enough time to cause any problems.

There the problem is solved .;)
AND you've understood how the circuit works !
 
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It's not a doubler, it's a halver ;)
You get the same or more voltage from one 30V winding. You'd get the double if you use the outer windings.
I don't think D3 and D4 do anything; they are always opposed by another diode and will never conduct. Take D3 & D4 out and see if it makes a difference.

jd

I think you never looked at the circuit closely. D3 and D4 are absolutely essential for operation of the circuit . Without them it wouldn't work.

Take for example D1 and D3. On the positive cycle of the incoming signal, D1 conducts and charges up C3 incrementally. On the negative cycle D1 cuts off C3 from the input circuit but current flows through D3 and C1 and charges that up incrementally with +ve side at the junction of D1 and D3. On the next positive cycle D1 opens up again and C3 sees the supply voltage plus voltage across C1 ( they are additive and in series) and hence there is again an incremental increase in voltage on C3. This continues over several cycles till it stabilises at some point, as explained earlier. C1 is also essential as it is the one that helps build up the voltage to double the incoming ac signal.

Amazing. You guys never even looked at the circuit before making those sweeping statements in your posts. :confused:
Never mind !

The attachment shows the voltage build up plot with a 30-0-30 V (peak) 50 Hz supply.
 

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Ashok,

I never said it wouldn't work. It's just that you use a lot of components in a convoluted way to get a double voltage. I just wanted you to think: I said that I believd that D3 etc...

But hey, if it works for you, more power to you. Somebody will pick up YOUR circuit from the forum in a couple of years and there you go ;) Enjoy.

jd
 
I think it was a mistake to start this thread. I should have just thought it over myself.
Jd, as I mentioned earlier. This isn't MY circuit ! It isn't even the DIYaudio member's circuit. It's generic and obviously quite old !

Never mind. This thread is getting to be rather silly.
 
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I think it was a mistake to start this thread. I should have just thought it over myself.
Jd, as I mentioned earlier. This isn't MY circuit ! It isn't even the DIYaudio member's circuit. It's generic and obviously quite old !

Never mind. This thread is getting to be rather silly.

Come on Ashok, you asked for comments! Now that apparently it isn't the most smart circuit, all of a sudden the thread is silly, it isn't your circuit, and it's quite old.

Maybe some comments come across as harsh, and I apologize for that, but in Holland we have a saying "soft doctors make stinking wounds".
Why not ask what the best way is to do a doubler, instead of clinging to a circuit you found somewhere and then act insulted if there is criticism?

A doubler requires 2 diodes and 2 caps. The basic circuit is at least 80 years old. It is best not to limit your searches to diyaudio because 1) it's difficult to decide what makes sense and what doesn't, and 2) this is not about audio anyway.

jd
 
This voltage doubler circuit was to be used with a regular +/- power supply which has a standard center tapped transformer. The circuit was to provide a regulated clean dc supply to the input stage of the amplifier without a transformer change.

This could be achieved easily by using additional coils on the transformer or using a separate transformer. However when this is not possible or desired , this circuit makes use of the existing power transformer to get a doubled voltage . This voltage is much higher than what is required and has significant ripple on it. Since the required voltage is much less than this supply and current load is low , it is possible to easily filter it and regulate it to get a cleaner dc for the input stage.

This one was used with the TDA7294 chip which has some special requirements regarding the power supply.
The doubler supply circuit can be used to improve an existing amp without replacing the transformer or adding another one. Most toroidals also have their center epoxied nowadays and so one can't add on additional turns easily.

It is of course obvious that if one is designing the amp from scratch , the separate supplies can be incorporated in the transformer design itself . This negates the need for a voltage multiplier.

Cheers.
 
The existing supply is about +/- 35 Vdc .
The doubler circuit provides about +/- 60 odd raw volts . This is filtered and regulated to provide about +/- 37 V dc to the input stage.

The original unregulated supply is of course required to power the output stage . The input stage however now gets a cleaned up supply .
 
if additional filtering, regulation is going to be used anyway then just put the 2x V+ cap on top of the main V+ instead of gnd to eliminate the need for the "extra" diodes to prevent pump C reverse V

Hi Jcx,
Can you put up a circuit sketch ? I didn't quite figure out your suggestion.


It's interesting to see that this thread is now discussing something completely different from the intended question which was " is a diode useful across the input caps that help double the voltage. In order to prevent high reverse voltage across it during switch on". I did say earlier that on reflection this was not required .

Now we are discussing the voltage doubler and why it should or should not be used.
Cool. It will lead to another subject and maybe the thread should be split here with another heading.;)
No doubt a lot of learning of several kinds has occured since !:)
 
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