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Old 6th July 2008, 02:45 AM   #1
alexg is offline alexg  Philippines
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Default DC heating, parallel diodes

Newbie question:

I need more than one ampere of 6.3VDC.

I have a 2A 6.3VAC secondary, can I parallel 1n4007 diodes to get higher current?

Am trying to source higher current diodes and I can't find any in the area where I live. If parallel diodes will not work, then I will order from RS.

Thanks.
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Old 6th July 2008, 03:35 AM   #2
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First how much current do you need? Once you rectify and
filter that 6.3v ac you will not have 2A still available. Any way
it's not a good idea to parrell diodes unless you put some resistance like a .2ohm resistor in seriese with each. A 5A
or higher bridge rectifier here would be a better choice.
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Old 6th July 2008, 04:02 AM   #3
alexg is offline alexg  Philippines
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I would need 1.5A (two 6n1p and one 12ax7) of 6.3VDC.

Ok, I will just get the proper rectifier.

Thanks.
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Old 6th July 2008, 04:14 AM   #4
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You can find better conducting low voltage schottkey diodes. Readily
heatsinked tab. Just find a dead PC power supply, a blowtorch or
gas stove, and with pliers from the top side. Start yanking off any
neat looking parts that might come loose in one piece. Its not like
anything you need to be delicate about. Just don't burn yourself.

All salvaged parts are suspect until tested. You have a meter, right?
After all, that power supply was DEAD.

Actually, I'm abusing an empty power supply shell for my latest tube
project... I salvaged a working bridge rectifier, a big high current
toroid choke 0.68uH, a power mosfet (and its twin that measured
very dead), several schottkeys, two oddly shaped heatsinks, a fan,
and a 100uF 450V cap. Several .1uF 450V (film?) caps. Bunches of
power resistors and other parts to be had if I had thought them
worthy of saving...

Someone is always throwing away a PC power supply.
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Old 6th July 2008, 04:43 PM   #5
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Quote:
I would need 1.5A (two 6n1p and one 12ax7) of 6.3VDC.

Alex,

Stop right now! Because of I2R heating effects, you can get only 1/2 the RMS current capability as cap. I/P filtered DC. You need a filament trafo rated for at least 3 A./6.3 VAC.

Construction of regulated 12 VDC supplies is easier than construction of "6" VDC supplies. You can use a 12 VDC heater supply, if you wire the 2X 6Н1П (6n1p) heaters in series. Then, your draw (including the 'X7) is 750 mA. That's within the capability of an inexpensive 7812 3 terminal regulator.

Greinacher ("full" wave) voltage double a 3 A./6.3 VAC winding using Schottky diodes into a stack of 2X 15 WVDC/10000 μF. 'lytics. Follow with a WELL heatsinked 7812.
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Old 6th July 2008, 05:30 PM   #6
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If the load seen at 12V is only 750mA,
why does he need a 3A transformer?

1.5A still seems the right number into
each half of the doubler... And the 2A
he has now, more than plenty...

Where has my math gone wrong?
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Old 6th July 2008, 06:01 PM   #7
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With a doubler the draw from the transformer is about triple the DC load - consult Duncan Amps PSU Designer program for a closer approximation...
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Old 6th July 2008, 06:23 PM   #8
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Tom is correct. TANSTAAFL applies. The amount of energy available doesn't change. You get approx. 1/2 RMS current as DC, using a FW rectification setup. Plan on 1/4 RMS current as DC, using a Greinacher doubler. The observation is especially noteworthy, given the HUGE capacitance in the doubler stack.
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Old 8th July 2008, 01:14 AM   #9
alexg is offline alexg  Philippines
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Thank you very much guys for the very enlightening tips.

Am a newbie, and I really appreciate the lessons!

Thanks.

Another question, on heaters, if I want to build a 12ax7->6dj8 phonostage (saw a nice Thorsten design on this forum), do I need DC on both tubes, or can I get away with 12VDC on the 12ax7 and a 3.15-0-3.15VAC on the 6dj8? Thanks!
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Old 8th July 2008, 01:49 AM   #10
AJT is offline AJT  Philippines
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Default Re: DC heating, parallel diodes

Quote:
Originally posted by alexg
Newbie question:

I need more than one ampere of 6.3VDC.

I have a 2A 6.3VAC secondary, can I parallel 1n4007 diodes to get higher current?

Am trying to source higher current diodes and I can't find any in the area where I live. If parallel diodes will not work, then I will order from RS.

Thanks.

ALEX,
paralleled 1n4007 is not a good idea, there are 3amp rects or even 6amp rectifiers that can be purchased at alexan/deeco in raon at a cheap price, depending on circuit, you can even use 6amp flat type birdges which you can bolt onto your aluminum chasiss for heatsinking...being generous here can save you a lot of grief...


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