Hipot Design Ideas?

I have always been partial to the unijunction (2N6027) oscillator triggering a HV SCR to discharge a cap into the step transformer with secondary to the HV diode and filter capacitor. So you can rectify and double the line for 340Vpk to start. The HV transformer provides isolation. Simple thyristor phase control on the line input and unijunction trigger frequency can adjust the output level. There should be plenty of reference designs around for this: View in a fixed-width font such as Courier.

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Hello Michael,

Ok, here is one: Take an old flyback transformer from a scrapped color TV. Take the transistor from the horizontal final, too, along with the rectifier stack (often a cascade). The only special part you'd need to procure would be a wee high voltage capacitor. In a TV the capacitance between tube metallization and outer shield serves as a capacitor but that would be too large a contraption for your purposes.

Be careful with this stuff. It can generate dangerous and even lethal voltage and energy levels. Even when supplied from a battery.

Regards, Joerg

Watch out for X-rays from those old rectifier valves (tubes).

A TV flyback is probably the easiest way to get 15kV. There are many designs on the net which involve winding your own primary onto a TV flyback, and driving it with a push-pull oscillator built from two 2N3055s. I think that these designs stress the diodes more than would be the case when producing the same output voltage in a TV, since in the TV, the secondary produces a very large voltage in one polarity but a much smaller voltage in the other polarity, so in the TV, the total peak voltage across the HV diodes is only slightly higher than the output voltage. In the push-pull version, the secondary produces high voltage in both polarities, which could almost double the stress on the diodes. Note that in reality, most flybacks have several EHT secondary windings which each has its own rectifier diode, and the whole lot are in series. You probably have some margin to abuse the diodes however, since you are only asking for 15kV whereas the flyback from a colour TV would be designed to produce a higher voltage.

You could drive the flyback much the same way that it is driven in a TV set, and you could even reuse the transistor and HV capacitor that are used on the primary of the flyback. Identifying the windings is possible with an ohmmeter, but it is even better if you have an oscilloscope and a signal generator: wrap a few turns around the core and drive these with a few tens of millivolts of AC from the signal generator, and then measure the voltage an polarity of what is induced in all of the other windings.

There are lots of web pages about driving flyback transformers, some of which are useful.

Chris

In article , Fred Bloggs wrote: [...] Note this:

When I was a kid, I nearly killed myself with a circuit like this. The step up xformer I used was a car ignition transformer.

Modifying drawing to closer to what I did:

In fact, my circuit used a diac as the oscillator not a unijunction, and the positions of the SCR and primary were reversed.

If you want to semi-regulate the output, you can: Add a rectifier on the anode of the SCR and monitor it. When the SCR stops conducting, the transformer is ringing. The ringing couples back into the primary and its amplitude is somewhat controlled by the output voltage.

If I was doing it today, I would look at using a IGBT transistor or two. An average of the supply current would allow you to limit the output current to something less deadly.

Chris - maybe you could clarify this with some specifics (all this makes no sense to me at all so far)?

TV flyback transformers are usually driven by high voltage transistors (eg 1200V), as you only get EHT on the secondary when there is also HV on the primary (usually the flyback pulse when the single-ended transistor switches off). The drive COULD be push-pull, but to what end? To get the same EHT out you'd still need HV on the primary (perhaps as much as 1000V), which would be a lot more difficult to handle than a simple single-ended flyback design running off low voltage DC. The secondary rectifier would then be a bridge, else the push-pull drive would "go nowhere"; the bridge doesn't suffer the voltage problems you mention above.

It all seems hard to justify unless you really need to extract much more power.

And no matter what the circuit, 2N3055s wouldn't even come close.

Tony Tony (remove the "_" to reply by email)

Hello Tony,

They sure don't. If you apply the normal line frequency pulses they'd just go "Huh?".

If you want to make life easier use a HV FET. Much easier to drive.

Regards, Joerg