Step-up transformer built from two standard types?

[Lostgallifreyan]

Can anyone advise if this is safe?

I have an idea for a laser pulse drive using an 850V capacitor.
If I feed 15VAC into the secondary of a 240V mains 6V output transformer,
I get a calculated 600VAC out, which after a bridge rectifier (*1.414)
gives me a 848.4V peak (no-load) voltage.

That looks ideal, and cheap, so I want to try it, but what other
calculations should I do to set a safe VA or current rating for each
selected transformer, assuming a maximum continuous current of 20 mA on the
600V output current. (There will be a brief surge as the capacitor charges,
but I will limit that with a series resistor).

I'll come back to this after hunting for a small 600V output transformer,
but I don't hold out much hope of finding any, let alone as cheap as a
combination of two small standard transformers.

 

[Hal Murray]

I'll come back to this after hunting for a small 600V output transformer,
but I don't hold out much hope of finding any, let alone as cheap as a
combination of two small standard transformers.

600V center tap used to be common back in the days of tubes.
They may not have been as small as you want. Fist size or larger
is the right ballpark.

You might be able to find one is a surplus/junk place. When I was
a kid, my junk box had lots of them. Maybe check out some junked
modern tube audio gear.

--
These are my opinions, not necessarily my employer's. I hate spam.

 

[Chris Jones]

Lostgallifreyan wrote:

Can anyone advise if this is safe?

I have an idea for a laser pulse drive using an 850V capacitor.
If I feed 15VAC into the secondary of a 240V mains 6V output transformer,
I get a calculated 600VAC out, which after a bridge rectifier (*1.414)
gives me a 848.4V peak (no-load) voltage.

That looks ideal, and cheap, so I want to try it, but what other
calculations should I do to set a safe VA or current rating for each
selected transformer, assuming a maximum continuous current of 20 mA on
the 600V output current. (There will be a brief surge as the capacitor
charges, but I will limit that with a series resistor).

I'll come back to this after hunting for a small 600V output transformer,
but I don't hold out much hope of finding any, let alone as cheap as a
combination of two small standard transformers.

Unless you increase the frequency, you cannot increase the voltage of the
transformer much above its nominal value without making the core saturate.
(saturation makes the inductance suddenly become very low and so a large
current flows and blows your fuse, assuming you have one, or burns out
something else if you don't.) This is because the rate of change of
magnetic flux is proportional to the voltage, so the total magnetic flux is
proportional to the integral of the voltage over one quarter cycle. It's
quarter because in one half cycle of the sine wave, the flux goes from
negative maximum flux, to zero, and on to positive maximum flux. If the
cycle takes longer (lower frequency) then the integral of the voltage over
a quarter cycle will be a greater number so the flux will be greater, and
may saturate the core.

If you want 600VAC from a 240V transformer then you will have to more than
double the frequency (which you could do with an inverter). (You should
also make provision for the possibility that the insulation is not up to
the job in the long term.)

I would suggest investigating the Cockcroft-Walton voltage multiplier, so
that you can use the transformer at its nominal voltage, or consider
connecting several transformers in series to achieve the voltage that you
want.

A small flyback converter might also do what you want.

Chris

 

[Lostgallifreyan]

Chris Jones <lugnut808@nospam.yahoo.com> wrote in
news:13d65safru9ap7f@corp.supernews.com:

Unless you increase the frequency, you cannot increase the voltage of
the transformer much above its nominal value without making the core
saturate. (saturation makes the inductance suddenly become very low
and so a large current flows and blows your fuse, assuming you have
one, or burns out something else if you don't.) This is because the
rate of change of magnetic flux is proportional to the voltage, so the
total magnetic flux is proportional to the integral of the voltage
over one quarter cycle. It's quarter because in one half cycle of the
sine wave, the flux goes from negative maximum flux, to zero, and on
to positive maximum flux. If the cycle takes longer (lower frequency)
then the integral of the voltage over a quarter cycle will be a
greater number so the flux will be greater, and may saturate the core.

If you want 600VAC from a 240V transformer then you will have to more
than double the frequency (which you could do with an inverter). (You
should also make provision for the possibility that the insulation is
not up to the job in the long term.)

I would suggest investigating the Cockcroft-Walton voltage multiplier,
so that you can use the transformer at its nominal voltage, or
consider connecting several transformers in series to achieve the
voltage that you want.

A small flyback converter might also do what you want.

Chris

Nice post. I hadn't considered the saturation issue but I had considered a
higher frequency, and that led me to find a few DC-DC converters that will
do ok, saving me a lot of trouble.

I might also consider the voltage multiplier, but it looks like the DC-DC
converter is a nice easy cop-out. The cost is low enough in the cases I
found to justify it too.

 

[Lostgallifreyan]

hal-usenet@ip-64-139-1-69.sjc.megapath.net (Hal Murray) wrote in
news:b42dnYwOxeX3Yk_bnZ2dnUVZ_ofinZ2d@megapath.net:

I'll come back to this after hunting for a small 600V output
transformer, but I don't hold out much hope of finding any, let alone
as cheap as a combination of two small standard transformers.

600V center tap used to be common back in the days of tubes.
They may not have been as small as you want. Fist size or larger
is the right ballpark.

You might be able to find one is a surplus/junk place. When I was
a kid, my junk box had lots of them. Maybe check out some junked
modern tube audio gear.

Not got access to it, or I might (it all seems to be on the web now
anyway)... Also, if I found something good I'd want to be able to repeat
it. I did find that Maplin (who I'd long since thought had fallen into the
pit and subsided) still sell a valve power supply transformer, but it's
240-0-240, not enough. Might be useful enough to get a couple anyway
though.

I decided to go with DC-DC converters fed by standard low-volt transformer
and rectification and smoothing. The total cost is still low, and it's
repeatable and safe.

 

[LVMarc]

Lostgallifreyan wrote:

Can anyone advise if this is safe?

I have an idea for a laser pulse drive using an 850V capacitor.
If I feed 15VAC into the secondary of a 240V mains 6V output transformer,
I get a calculated 600VAC out, which after a bridge rectifier (*1.414)
gives me a 848.4V peak (no-load) voltage.

That looks ideal, and cheap, so I want to try it, but what other
calculations should I do to set a safe VA or current rating for each
selected transformer, assuming a maximum continuous current of 20 mA on the
600V output current. (There will be a brief surge as the capacitor charges,
but I will limit that with a series resistor).

I'll come back to this after hunting for a small 600V output transformer,
but I don't hold out much hope of finding any, let alone as cheap as a
combination of two small standard transformers.
For real step up voltage fun, I like a 36 foot neon sign transformer.

output > 30 kV. Alternatives include, transformer from microwAve oven,
and oil burner starters. These are the classic three for making
relatively high 1 to 42 KV...Happy sparking

Marc

marc


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[Hal Murray]

Unless you increase the frequency, you cannot increase the voltage of the
transformer much above its nominal value without making the core saturate.

Is that correct?

Doesn't the input current depend on the load as well? Wouldn't it
still work (aka not saturate) if the load was reduced?

--
These are my opinions, not necessarily my employer's. I hate spam.

 

[Chris Jones]

Hal Murray wrote:

Unless you increase the frequency, you cannot increase the voltage of the
transformer much above its nominal value without making the core saturate.

Is that correct?

Doesn't the input current depend on the load as well? Wouldn't it
still work (aka not saturate) if the load was reduced?

Yes, the input current depends on the load current, but the magnetic flux
density in the core is almost unaffected by the load current. From the
core's point of view, any change in the load (secondary) current is
practically cancelled by the change in the current in the primary winding.
This means that even with no load, there is a limit to how much voltage you
can apply to the primary winding before the core saturates. You can try
this if you get a transformer and a Variac and an ammeter. It is an
interesting experiment.

Chris