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How to measure voltage output of AC flyback transformer

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MRMILSTAR

Sat Oct 12 2019, 09:39PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

My next project is going to be a Cockroft-Walton voltage multiplier. I plan on using an AC flyback transformer as the power supply. For design purposes, does anyone know how to measure the output voltage of an AC flyback transformer. I would also like to know to measure the output of a DC flyback transformer.

Sulaiman

Sun Oct 13 2019, 01:55AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I use a P6015 high voltage probe that works with h.v. a.c. and/or d.c.
but such probes are not cheap, alternatives are;

1) commercial h.v. d.c. probe, commonly used for measuring the eht of crt, relatively cheap - if available
2) a diy string of resistors to make your own hv dc probe (for multimeter or oscilloscope)
3) for an ac flyback use diodes and capacitors to get + and - eht then measure with a hv dc probe
(use the diodes and capacitors that you would use for your C-W multiplier)

I advise against diy hv ac probes as frequency compensation and testing is a complex affair.

P.S. with high voltage transformers resonance can be your friend or your enemy,
try to make it your friend.
(the large number of turns required for a hv secondary gives significant self-capacitance,
which combined with the intrinsically high inductance (plus the capacitance of external circuitry) causes resonance.
Read up on hv transformers - insulation and self-capacitance are key features.
A few days research is better than a rewind ;)

MRMILSTAR

Sun Oct 13 2019, 03:07AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I already have the AC flyback transformers. They are commercial units. I am not attempting to make one. I have some rather large ones but I don't know what voltage output they have. I have some 30 KV doorknob capacitors that I plan to use for the Cockroft-Walton voltage multiplier. I just want to insure that the voltage rating is high enough. The same goes for the diodes.

Sulaiman

Sun Oct 13 2019, 12:31PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Everything before my P.S. above still applies.

The maximum output voltage will depend mainly upon how you drive them, the load, and the insulation of the secondary.

As they are commercial units, can you point to a reference, or is there a part number ?

MRMILSTAR

Sun Oct 13 2019, 02:38PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

Unfortunately there are no data sheets to be found.

father dest

Sun Oct 13 2019, 06:42PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
Location:
Posts: 28

you can't measure it. my flyback with stock secondary has something like ~100kohm of leakage reactance at the frequency am driving it. my 200mhz o'scope probe with 1:10 divider has 18pf of input capacitance, which is (surprisingly) the same reactance. but there are more serious troubles - unknown capacitances from secondary to infinity, from o'scope to infinity and so on - all that variables vary output voltage like crazy - just swapping secondary outputs may change voltage like 3 times or more.
but you can measure number of turns in your secondary and then calculate voltage.

i think measuring voltage after multiplier may be much more simplier and accurate.

Sulaiman

Mon Oct 14 2019, 01:15AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

By adding a known large capacitance to the secondary, e.g. 1000pF, and measuring the new resonant frequency,
you can calculate the effective secondary inductance and the miscellaneous capacitances.

In flyback mode the output is pulses of fixed ENERGY
the output voltage depends upon the load resistance and capacitance.

You could add a spark gap to limit the peak transformer output voltage.

I agree that the simplest would be to build the C-W multiplier and monitor the first stage voltage
either for manual control, or as a feedback signal to your inverter.

Patrick

Mon Oct 14 2019, 02:02AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

are you trying to measure 60Hz HV ? like a NST ?

or a real square wave intrrupted Ac flyback.

If you squint the purple trace is 52kV i think at 60kHz.

MRMILSTAR

Mon Oct 14 2019, 03:32AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

If it was 60 Hz HV, I could easily measure that. This is an AC flyback transformer with high frequency square wave drive as with any flyback.

I had thought of using my oscilloscope with a suitable HV probe to measure the voltage but it makes me nervous connecting my oscilloscope to a flyback output because I don't really know what the maximum voltage is and I may guess wrong on the probe maximum voltage capability.

father dest

Mon Oct 14 2019, 06:17PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
Location:
Posts: 28

Sulaiman wrote ...

In flyback mode the output is pulses of fixed ENERGY
the output voltage depends upon the load resistance and capacitance.

but we talking about flyback transformer here - not the mode of operation.

By adding a known large capacitance to the secondary, e.g. 1000pF, and measuring the new resonant frequency,
you can calculate the effective secondary inductance and the miscellaneous capacitances.

no you can't : )
i.e. you can't differentiate secondary self capacitance from secondary stray capacitance to ground from example. and from capacitance of the scope/frequency counter to ground/infinity/whatever the heck else that matters.
or if you can - plz explain how one can do it.

and also try to explain, why neon bulb which is held by the glass glows bright when touching one end of low voltage (only ~1kv) secondary, while not glowing at all when touching the other end? neither of the ends are grounded/connected to anything at all.
that's stray/to infinity capacitances at play - how you suppose to measure/take them into consideration?

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