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runeight 6th April 2004 07:02 PM

Voltage Doubler Problem
 
Say, folks, here is a dumb problem that I'm not yet able to track down.

I have a simple voltage doubler connected a 240VAC winding. The partially loaded xformer is actually producing 270VAC. The doubler is 1n4007s with 220u/450V caps. I am expecting to get about 750volts (no load except cap leakage) from this configuration. I get more like 715?

When I load the doubler with about 23mA, it pulls way down to 638V or so. The xformer loads down to 260VAC.

This loading effect seems excessive to me. I've run PSUD on it and nothing like this happens.

Is this telling me that my doubling caps are leaking heavily? Or something else?

Thanks.

dhaen 6th April 2004 07:29 PM

Runeight,

With 2 ohm ESR caps and 60 ohm transformer resistance, I SIM the results you are measuring.

thoriated 6th April 2004 07:47 PM

Hi -

Are you going half wave? Btw, if you go full wave off of a center tapped secondary, you can use a bridge rectifier for all the doubler diodes, I've found. That's how I'm getting a unipolar doubled voltage in addition to the regular plus and minus fw rectified outputs for my HT amp project. I'm getting good regulation results with my doubler & tripler because I'm using polypropylene caps which have negligible esrs. Of course the values are smaller so there's more ripple, but that'll be cleaned up with a series choke/shunt cap filter at the doubler (& tripler) output in the final version.

Btw, if you are running two caps in series at the output of the doubler, maybe adding a couple of high value voltage equalizing resistors might be worth a try?

dhaen 6th April 2004 07:54 PM

Half or full wave?
 
Good point Thoriated :) My sim was using full wave (as PSUD uses).

runeight 6th April 2004 08:51 PM

Thanks. It's a full wave doubler. The secondary is center tapped, but I'm not using the tap.

The doubler caps are followed by a choke and then two caps in series with equalizing resistors in on the caps.

Thoriated, can you show a drawing?

thoriated 6th April 2004 09:23 PM

2 Attachment(s)
If the choke is before the cap following the rectifiers, that might be killing the regulation.

Sch3mat1c 6th April 2004 11:09 PM

2 Attachment(s)
Um, that's an awful lot of circuit for a doubler. Reminds me of the dual-bridge circuits oft used in SS amps.. :rolleyes:

There's also a lot of circuit surrounding the key area but look below. This is (er, will eventually be) the power supply for a quad 6146 amp, 600V at .5A. :eek: (The big PT would be either tapped or fed with a buck transformer to provide the three output voltages shown.)

The key is low resistance (winding, ESR and diode losses need to be minimized) and high capacitance (which makes high current peaks which require low resistance...). This style doubler can be envisaged as two half-wave rectifiers working opposite half-cycles from the same two-wire source.

Tim

thoriated 6th April 2004 11:54 PM

Hi -

The doubler is in the left half of the schematic, and the choke and second shunt cap are optional, depending on how much ripple is tolerable, so the full wave doubler, aside from the transformer, can be realized with four components. The right half is a conventional dual output supply which is shown only for comparison purposes.

Sch3mat1c 7th April 2004 01:28 AM

Ok.

The nice thing about a FWB is that, if you go overseas a lot, you need only change one (1) connection (which can be done with an SPST switch) to convert from FWB to FWD! That's how switching supplies get dual voltage capability: they run the HF transformer and stuff from +340VDC rails.
Of course this is moot since you'll smoke almost any transformer or otherwise lose regulation (except maybe with a Turner Audio transformer...damn he overbuilds those things) in doing so. ;)

If nothing else... it might be a good way to test double the voltage on a circuit. I suppose. Not really too useful for audio. :p

Tim

runeight 7th April 2004 03:57 AM

2 Attachment(s)
thoriated, I see what you're doing, adding a full wave doubler on top of a full wave bridge.

thanks for the replies and diagrams. i've got more testing to do for sure.

here's a drawing of the doubler in question.


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