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Old 21st July 2004, 12:16 AM   #1
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Default How do you double the voltage on a PSU?

I happen to have two mains transformers that I got for next to nothing. The secondaries are a nominal 135 volt at over 100 milliamps. There are multi tap primaries. I have heard of voltage doublers and I'd be grateful if some one could explain how I can do that to these secondaries as they would then form the basis of a good twin mono pre-amp supply. I figure I should be able to get 135x2=270 volts plus some extra with the appropriate primary tap. I am aware that the ripple voltage will be the same as the mains supply but after that I'm not sure. Can someone put up an illustration please, preferably with capacitor values/ratings etc?
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Old 21st July 2004, 12:48 AM   #2
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Hi,

A voltage doubler rectifier into a capacitive load would give you roughly twice the primary voltage at half the primary current.

Ripple voltages are much higher due to less adequate filtering so care needs to be taken to further filter it prior to feeding the B+ to the circuit.

GLASSWARE

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Old 21st July 2004, 01:18 AM   #3
PassFan is offline PassFan  United States
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Pay close attention to the first half of Franks answer. The VA rating of your transformer will not change. At twice the voltage you get half the current. Maybe it will still work for you , maybe not.
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Old 21st July 2004, 06:23 AM   #4
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A voltage doubler works by connecting a pair of diodes in opposing polarity to one end of the secondary winding, so as to maintain charge on a pair of capacitors, one negatively and the other positively, with respect to the other end of the secondary winding. The following rough schematic gives the idea.

Xfmr diode
||C--------------|>|------------- +270v
||C | |
||C 135v | = capacitor 220uF
||C | |
||C------------------------
| |
| = capacitor 220uF
| |
-----|<|------------ 0v
diode

It relies on each capacitor retaining its charge long enough so that its voltage doesn't drop significantly between charging pulses, which occur one each mains cycle (i.e. 50 positive and 50 negative pulses per second). The values of 220uF shown in this example should be more than adequate for your purposes.

All this energy has to come from somewhere, consequently what you gain in volts you lose in amps. Your 135v 100mA transformer will be able to provide ~270v but at only 50mA max. However, for most preamps I've seen this should be enough.

The voltage doubler produces 50htz ripple and should be followed by a good filter.
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Old 21st July 2004, 06:32 AM   #5
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Sorry about that bl**dy mess! I spaced it all out properly but the typing collapsed when I submitted it. Basically:

1. Connect one end of the transformer secondary to the junction of a pair of electrolytic capacitors diodes in series.

2. Connect the other end of the secondary to the junction of a pair of diodes in series.

3. Connect the other ends of the diodes to the other ends of the electrolytics, observing correct polarity.

4. Take the output of the voltage doubler from the ends of the electrolytics.
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Old 21st July 2004, 06:41 AM   #6
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Thanks all. Yes I'm up to speed on VA ratings. Frank, I take it that what you have refered to as the "primary" is what I assume is the secondary (of the transformer) IE the primary is the mains side and the secondary is the winding prior to the rectifier. Any way I think I'm clear now. Ray moth your written explanation will make sense of a diagram I have. Thanks.
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Old 21st July 2004, 06:45 AM   #7
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There are several ways to make voltage doublers. On this page http://www.du.edu/~etuttle/electron/elect9.htm some of the most common are shown, full wave, half wave and multiplier type. The fullwave type gives much lower ripple as the ripple frequency is 100Hz but the disadvantage is that both inputs are floating but it doesn't matter so much with transformer coupling.

There are another variant using a bridge rectifier which seems to be less popular but it has an advantage if you want to make a circuit that i easily switched between ordinary rectification and voltage doubling, one example is here http://sound.westhost.com/project65.htm

Regards Hans
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Old 21st July 2004, 06:56 AM   #8
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Quote:
Originally posted by ray_moth
Code:
Xfmr                diode
 ||C--------------|>|-------------  +270v
 ||C            |                 |
 ||C 135v   |                 = capacitor 220uF
 ||C            |                 |
 ||C------------------------
                  |                 | 
                  |                 = capacitor 220uF
                  |                 |
                   -----|<|------------  0v
                        diode
Let's see if the CODE tag helps. If not, hit QUOTE on this or his post to see how it was typed.

The hum output from such a doubler is 120Hz. It's a FULL WAVE doubler. One ''up" pulse to the top cap followed by a "down" pulse to the bottom cap, due to the connection the phases are equal and they both go up.

Does your transformer have a center tap? Have you been using it FWCT? (Typically for tubes; one diode on each 'arm' of the winding, pointing towards the first filter cap, which is grounded to the CT.) If so, just put a FWB (SS or if you want to be annoying, damper diodes) on it and go. Double voltage, half current.

Which reminds me, multipliers are tough on transformers so you'd be wise to derate the current draw a bit. Although, the winding is used in the same way so I suppose it wouldn't be too much worse than the standard RMS 1.5-2x derating used with FWCT or FWB rectifiers.

Tim
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Old 21st July 2004, 06:58 AM   #9
Yvesm is offline Yvesm  France
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Just a little drawing could help
Click the image to open in full size.
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Old 21st July 2004, 07:51 AM   #10
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Appreciate the links and clearer diagram. Thanks. JB
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