Step-up transformer built from two standard types?

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.

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Chris Jones wrote in news: snipped-for-privacy@corp.supernews.com:

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.

snipped-for-privacy@ip-64-139-1-69.sjc.megapath.net (Hal Murray) wrote in news:b42dnYwOxeX3Yk_bnZ2dnUVZ snipped-for-privacy@megapath.net:

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.

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

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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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?

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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