Constructing a high-voltage transformer for safety and ruggedness

I have some ferrite cores from old flyback transformers which I'd like to use to wind my own coils around. I have an idea of how I want to put things together to make sure it's safe from arcing to anything (including me), and I'd be interested in any feedback on my design plans. No pics unfortunately so try and visualise if you can:

The coils themselves will be wrapped around PVC conduit. It seems to be a good diameter for the job and I have plenty of offcuts knocking around. Between each layer of the secondary I will use PTFE tape.

The windings will be resting on some plastic stand-offs inside an ABS project case. Each end of the windings will be brought out to a suitable terminal (which I have yet to choose) that I can screw into from outside the box. Once I'm happy with the electrical characteristics of the thing, I intend to fill the WHOLE thing with epoxy potting compound, so each and every part of the transformer is covered and surrounded.

I'm hoping to achieve an output of around 1kV, for various cap-charging activities and possibly other experiments. Other safety considerations are PPE (I have gloves good to 1kV, so they say on them) and I'm pretty sure somewhere I have an air-pressure switch which would make a suitable remote control.

What have I overlooked? Does this seem like a good way of going about things?

Why not simply use the line transformer as it's already manufactured (by professionals who knew what they were doing) and control the excitation voltage to give the output you want. Transformer ratio times the input voltage determines the output so a suitable inout voltage will give you whatever output you desire. I take it you appreciate that ferrite cores require high frequency excitation (in the order of 10s of kilohertz)

Because I only have the cores, that’s why :wink: True, I COULD scrounge up complete transformers… but I was more thinking about the best way to get what I want with the scraps, bits and bobs that I already have. Aside from suitable terminals I pretty much have all the stuff I think I need. Besides, on the whole I thought it might make for a much more interesting and rewarding build, as long as there’s no glaringly obvious safety issues I’ve overlooked.

I’ve got a sort of miniature version of the driver circuit built up, it’s working with a camera flash transformer at the moment - somewhat under-spec for charging up large cap banks, and I doubt it’s very efficient :stuck_out_tongue: But it DOES work… I can get about 760V post-diode out of it at the moment without magic smoke/bad accidents.

When putting together your prototype driver circuit, include the ability to vary the pulse frequency. These transformer cores have a natural frequency that will allow the most efficient/effective operation. You can even vary output voltage by keeping voltage constant, but simply changing the pulse frequency.

wanderson:
When putting together your prototype driver circuit, include the ability to vary the pulse frequency. These transformer cores have a natural frequency that will allow the most efficient/effective operation. You can even vary output voltage by keeping voltage constant, but simply changing the pulse frequency.

Ahh yes, good advice :slight_smile: Should I build the oscillator stage so that both freq. and duty cycle can be tweaked, or is there no real benefit in altering the duty cycle? It seems like building the capability into the first prototype might be an idea, just to mess around and see what it does.

Do some more research on winding HV coils. The get wound from one end to the other with out layers or with minimal over lap. This keeps the potential voltage from one winding to the next fairly low.

BulletMagnet83:

wanderson:
When putting together your prototype driver circuit, include the ability to vary the pulse frequency. These transformer cores have a natural frequency that will allow the most efficient/effective operation. You can even vary output voltage by keeping voltage constant, but simply changing the pulse frequency.

Ahh yes, good advice :slight_smile: Should I build the oscillator stage so that both freq. and duty cycle can be tweaked, or is there no real benefit in altering the duty cycle? It seems like building the capability into the first prototype might be an idea, just to mess around and see what it does.

It has been at least a decade since I didn't any experiments with this, but I think I remember that duty cycle wasn't as important (within a given range) as frequency. Personally I would use an ATtiny85 (or a full Arduino for testing) as the pulse generator for the circuit so that you could simply program different combinations and see what voltages get produced.

tjbaudio:
Do some more research on winding HV coils. The get wound from one end to the other with out layers or with minimal over lap. This keeps the potential voltage from one winding to the next fairly low.

The flyback transformers, that the OP is using for the core routinely have multiple layered coils. The layers require some extra protection, which it sounds like the OP is aware of. These transformers easily produce voltages in the 35kV range.

Before you go for HV output it might be an idea to wind an LV output coil so that you can establish the core operating conditions without too much effort. From memory of my transformer days you also need to ensure the faces of the cores mate completely since any increase in reluctance ( a beautifully illustrative word) due to face separation has a major effect on performance.