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Old 11th February 2011, 11:40 AM   #1
markusA is offline markusA  Sweden
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Default Transformer for voltage doubler?

I have a bunch of small toroids lying around.
How can I decide if they are usable with a voltage doubler?
So far I've been looking at a full wave voltage doubler.

The trafo ratings are:
Prim: 230V@50Hz
Sec: 7.5V 1A, 15-0-15V 75mA, 20-0-20V 420mA.

I've been toying with some sims using LTspice and it's causing me some headaches.

The load will draw roughly 26mA.
I need approximately 150VDC from the PSU. (B+ for some 6CG7)
Using these figures everything looks ok if I hook the 15-0-15 and 20-0-20 secondaries in series. I have the trafos already so it's free...

Now, sims will show a peak current draw from the trafo in the 400mA range.

How much peak current can it handle? It's Toroid brand and it's number is
TI-78038-8?

(I'll save the 7.5V secondary for the heater.)
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Old 11th February 2011, 11:52 AM   #2
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You could also just use the 20-0-20 and use a voltage quintupler. With the inevitable losses you might get just about 150Vdc, and you'll remain within the current limits of the xformer. I'd rather do that than use the teeny 2x15V 2VA tap.
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Old 12th February 2011, 12:17 PM   #3
markusA is offline markusA  Sweden
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I took your advice and simmed (LTspice) a quadrupler. So far everything is looking good.
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Old 12th February 2011, 12:24 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
quite unusual to generate B+ from a low voltage transformer, but do-able.
Doublers and more do not like to provide lots of current, but your current load is low.

Doublers and more do have a lot of ripple under load.
I would suggest you aim to go over voltage and bring the voltage back down to operational B+ using RCRC or RCLC or extended to more stages or similar.
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Old 12th February 2011, 12:34 PM   #5
markusA is offline markusA  Sweden
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I'm dropping something like 25V over a symetrical three-stage RC filter.
Thanks for confirming my design ideas.
I know that you know your power supplies and hearing this from you means I'm finally picking up some usable skills in electronics design.
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Old 12th February 2011, 12:49 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by markusA View Post
I'm dropping something like 25V over a symmetrical three-stage RC filter.
if 25Vdrop is too high and leaves B+ a bit low then consider replacing one or more of the R with an aircored L
A 100gm bobbin of 0.5mm diam wire will have quite a bit of inductance and only 5r of resistance. Expect L~4mH to 8mH
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Old 12th February 2011, 12:51 PM   #7
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Why not just use the 240V - 15V to drive the 20V - 240V in reverse. This will give you about 180V. A relatvely small 1K2 1W resistor will drop the extra few volts.

This would have much better power transfer than a voltage doubler.
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Old 12th February 2011, 02:22 PM   #8
markusA is offline markusA  Sweden
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My current sim leaves me with 140VDC at the plate and that's good enough for me.
If the need arises it's always good to have a trick up your sleeve, I think I'll save the inductances for "in case of emergency". Thanks for the tip though.

BTW, does the 100gm stand for 100 grams? or what? (to me gm is something you find with tubes?)

Andy, I'm not sure I'm getting what your saying? It's one Toroid? Either you hook it up to a secondary or a primary? Anywho, I need the 7.5V @ 1A for the heater so I can't run it in reverse.
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Old 12th February 2011, 02:25 PM   #9
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Quote:
Originally Posted by markusA View Post
My current sim leaves me with 140VDC at the plate and that's good enough for me.
If the need arises it's always good to have a trick up your sleeve, I think I'll save the inductances for "in case of emergency". Thanks for the tip though.

BTW, does the 100gm stand for 100 grams? or what? (to me gm is something you find with tubes?)

Andy, I'm not sure I'm getting what your saying? It's one Toroid? Either you hook it up to a secondary or a primary? Anywho, I need the 7.5V @ 1A for the heater so I can't run it in reverse.
Sorry. I thought you had a number of transformers.

In the valve World it is quite usual to use back to back transformers to replicate those bygone age power transformers.

In this case I fed you with a complete load of b******s.

Andy
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Old 13th February 2011, 10:12 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
100gm is 100grams of enameled copper wire.
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