# Power supply current?

P

#### Phil Allison

##### Guest
Pimpom wrote:

------------
Ed

The circuit is not intended to produce 500VDC which is not needed
for, in the OP's words, "bog standard AF triodes".
** Fair enough - the OP has been very vague.

500V far
exceeds the maximum plate voltage of common AF triodes.
** But not *power* triodes - like the 300B.

They like 700 to 800V.

230:12 -&gt; 12:230 transformation will theoretically output 230V AC
which,
** Only IF you use toriodals.

The magnetising current with cheap E-cores will spoil that quite a bit.

Look, I admit have done it myself for a low current, tube pre-amp with a pair of 12AX7 / ECCL83s.

FYI:

I admit I have done many things myself I would never recommend on an internet forum while ever I use my REAL name.

There is so much risk of *misinterpretation* of a seemingly good idea.

One reason being, 500VDC is dangerous, concieveably fatal for anyone with little experience.

BTW I think you are one of the better, more serious posters here.

..... Phil

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#### Phil Allison

##### Guest
Pimpom wrote:

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** Using two transformers wired like that is a bad idea and will not do what you asked for. The seconds one will be driven into core saturation.

I used that configuration because small 6- and 12-volt
transformers were easily available at local parts shops and, in
fact, I had them in stock at that time. It worked.
** I do know it can - but no so much at higher currents.

&gt; How will 12V fed to a 12V winding (or 6V to 6V) saturate the core?

** Well, it in fact does.

AC supply transformers are NOT intended to be run backwards.

Take a 15VA, 240V to 6.3V tranny - the off load voltage is gonna be about 7.5V. The gives a turns ratio of 32:1.

On load, 6.3V multiplied by 32 = 201V - so not 240.

Now, the approx 30mA magnetising current has moved from the primary secondary, so multiplied by 32 = 0.96 amps.

In reality you will be lucky to get 180V before rectification, filtering and load.

Been there, done that.

.... Phil

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#### Pimpom

##### Guest
On 4/12/2020 2:53 PM, Phil Allison wrote:
Pimpom wrote:

--------------------

** Using two transformers wired like that is a bad idea and will not do what you asked for. The seconds one will be driven into core saturation.

I used that configuration because small 6- and 12-volt
transformers were easily available at local parts shops and, in
fact, I had them in stock at that time. It worked.

** I do know it can - but no so much at higher currents.

How will 12V fed to a 12V winding (or 6V to 6V) saturate the core?

** Well, it in fact does.

AC supply transformers are NOT intended to be run backwards.

Take a 15VA, 240V to 6.3V tranny - the off load voltage is gonna be about 7.5V. The gives a turns ratio of 32:1.

On load, 6.3V multiplied by 32 = 201V - so not 240.

Now, the approx 30mA magnetising current has moved from the primary secondary, so multiplied by 32 = 0.96 amps.

In reality you will be lucky to get 180V before rectification, filtering and load.

Been there, done that.

... Phil
OK, I'll take your word for it. As I said, I did that "many moons
ago" - decades in fact, and I don't remember the details.

J

#### Jeff Layman

##### Guest
On 12/04/20 10:00, Phil Allison wrote:

230:12 -&gt; 12:230 transformation will theoretically output 230V AC
which,

** Only IF you use toriodals.
About right. Over 30 years ago I used back-to-back toroidals to get a
safer supply to a 230V pond pump 10 metres away in the garden.

The matching toroidals were 230:18-21-24-27-30V and rated at 3A. I used
the 30V output of the first, ran it through 10m of 1.5mm^2 cable, and
connected it the 27V secondary of the second toroidal. The pump was
rated at 50W, and, from what I remember, I got about 220V out from the
primary of the second toroidal when under load. It worked that way
without problem for many years.

--

Jeff

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#### Peter Percival

##### Guest
Peter Percival wrote:
I wish to buy a power supply delivering up to 500V for experiments with
thermionic valves.Â  What would be a suitable current?
So far as making ones own is concerned, I found this
https://www.nutsvolts.com/magazine/article/a-semiconductor-based-high-voltage-utility-power-supply
while looking for something else.