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Old 26th April 2015, 02:35 AM   #1
2bamp is offline 2bamp  Canada
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Default Diode rating for filament voltage doubler

I would like to power some 12v sweep tubes from the 6.3v winding of my PT using a voltage doubler. Total draw will be 1.35A at 12.6v based on the datasheets. I know that the current will be significantly higher when the filaments are cold. How hefty do the diodes need to be? Will a pair of 3A diodes give enough margin, or do I need to go bigger?
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Old 26th April 2015, 03:35 AM   #2
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Check the surge ratings of the diodes you a planning to use. 3 A. rated parts might be OK. Personally, I'd use 5 A. or even 10 A. rated Schottkys. You are going to need large valued 'lytics in the doubler stack.

Now for the bad news, you need a 6 A. rated 6.3 VAC filament winding. A 5 A. rated winding could survive, but ...
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Old 26th April 2015, 12:45 PM   #3
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I have 4A available, so I guess I won't be using the doubler...

That seems like a really high draw. Roughly, how do you come up with such a high figure? I read somewhere that you need a factor of 2x for the doubler, which put me at less than 3A.

This is unfortunate, as I have boxes of 12v sweeps.
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Old 26th April 2015, 01:39 PM   #4
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You need 2X the DC draw, when a "routine" full wave rectification setup is employed. Double the DC voltage and 4X AC RMS is needed. TANSTAAFL.

Try to find a surviving "Rat Shack" and look for 273-1511, which is a 3 A./12.6 VCT filament trafo.
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Old 26th April 2015, 06:56 PM   #5
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by 2bamp View Post
how do you come up with such a high figure? I read somewhere that you need a factor of 2x for the doubler, which put me at less than 3A.
That's correct. The total RMS and average currents in a voltage doubler will be roughly double what they were for a simple bridge.

If the load current is 1.35A then each diode needs to have an average current rating in excess of 1.35A.

The total RMS ripple current in the transformer might 'nomally' be about twice this figure, but since this is a voltage doubler it will now be about four times as much, or perhaps 5.4A. However, each diode only sees about 1/^/2 times as much, or 3.8A.

The peak and inrush currents are not easy to predict, but SS diodes take into account tyical usage so you don't usually have to consider this.

Your 4A diodes are presumably 4A average, and will have similarly excessive RMS rating, which is more than enough.
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Old Yesterday, 03:35 AM   #6
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Check your local supplier but my Hobby Electronics Shop here in Oz sells STPS1545F 45V 15 Amp rated Schottkys in a TO220 pack.
http://www.st.com/web/en/resource/te...CD00000819.pdf
That is what I use for voltage doubling heater supplies when the 1N5822 40V 3 Amp devices won't do.

Why use Schottkys? - to limit any diode switching noise coupling back through the power tranny into B+ or (worse) bias supplies from the same transformer.
If using a separate tranny then this doesn't apply.

Cheers,
Ian

Last edited by gingertube; Yesterday at 03:58 AM.
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Old Yesterday, 08:01 AM   #7
12E1 is offline 12E1  United Kingdom
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And Schottky diodes will generally have a lower forward voltage drop - usefully reducing heating effects at the currents involved and lessening the losses of the multiple diode voltage drops found in multiplier configurations.
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Old Yesterday, 02:07 PM   #8
2bamp is offline 2bamp  Canada
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Awesome info, guys. You have probably saved me from ruining a new PT! I will use the 6v version of the sweep tubes on this build. I built a killer SET with 6AV5GA, which sounds great with 6FW5, so I am going to use 6GE5 for this build. They clearly have the same guts as 6FW5. I have many pairs of the 12v version, but I will save them for use with a heftier PT.
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Old Yesterday, 02:26 PM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by 2bamp View Post
You have probably saved me from ruining a new PT!
Why, what's the PT rated for?
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Old Today, 12:26 PM   #10
2bamp is offline 2bamp  Canada
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6.3v at 4A. It's an Edcor, so I suppose that may be conservative.
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