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Old 24th May 2020, 6:05 pm   #1
Valvepower
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Default Halfwave voltage doubler psu.

Hi,

Just thinking about the grafting of a mains transformer from an Armstrong 222 amplifier onto a Decca SG188 as discussed in an earlier thread.

The power supply in the Armstrong 222 and the Rogers Cadet Mk3 use a half wave voltage doubler power supply. Looking at these had me thinking why did they use such a power supply when they could have just as easily used a full wave rectifier and higher transformer voltage. Below are a few reasons why I felt they might have done it this way, excluding they just wanted to do it this way

Less turns of a higher gauge wire on the transformer possibly making it easier and quicker to make and thus marginally cheaper?

Only two diodes as opposed to four when they were quite expensive at the time?

Use of two 100uF 275V capacitors, which were in wide use in TVís at the time and thus being cheaper?

A down side to the half wave voltage doubler power supply is a higher transformer current to that of the full wave rectifier. Maybe the trade off of a higher transformer current was justified elsewhere?

Just mulling things over in the Ďole napper

Regards
Terry.
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Old 24th May 2020, 7:20 pm   #2
Herald1360
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Default Re: Halfwave voltage doubler psu.

Quote:
Originally Posted by Valvepower View Post

The power supply in the Armstrong 222 and the Rogers Cadet Mk3 use a half wave voltage doubler power supply.


Terry.

Looks like a full wave voltage doubler circuit to me. At any rate ripple is 100Hz.
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Old 24th May 2020, 7:33 pm   #3
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Default Re: Halfwave voltage doubler psu.

I always thought the voltage doubler on the Armstrong was full wave.
See the attached circuit.
One diode conducts on the positive half cycle and charges the upper 100uF capacitor, the other diode conducts on the negative half cycle and charges the lower 100uF capacitor. Thus current flows on both half cycles, that's whole wave surely?

Peter

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Old 24th May 2020, 8:13 pm   #4
Valvepower
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Default Re: Halfwave voltage doubler psu.

Hi,

Doh!... I can see how I made the blooper and referred to half wave as I was looking at the voltage on each capacitor relative to the common point between the two capacitors and transformer winding. I hadn’t considered the ripple voltage across the positive and negative of the two capacitors.

In the past I’ve used this power supply to derive a ‘split’ positive and negative supply from just one Transformer winding without a centre tap. The positive and negative are relative to the common point between the two capacitors and transformer winding which is the earth or 0V.

Regards
Terry

Last edited by Valvepower; 24th May 2020 at 8:15 pm. Reason: Triping errot
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Old 24th May 2020, 11:57 pm   #5
TrevorG3VLF
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Default Re: Halfwave voltage doubler psu.

The voltage doubler uses two capacitors in seies so the voltage on these is less. No resistive circuit is necessary to equalise the voltages. They do however use single capacitors further down the smoothing path so suitable capacitors were available. Perhaps lower voltage diodes were adequate. What diodes were available. BY100?
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Old 25th May 2020, 7:02 am   #6
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Default Re: Halfwave voltage doubler psu.

The Armstrong used a pair of RS240AF diodes.

Peter
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Old 25th May 2020, 11:19 am   #7
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Default Re: Halfwave voltage doubler psu.

Much earlier, metal rectifier stacks were used in some sets using this arrangement.
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Old 25th May 2020, 1:04 pm   #8
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Default Re: Halfwave voltage doubler psu.

One advantage of this circuit is that it has intrinsic current limiting, set by the size of the capacitors.
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