Flyback secondary burnout

[abstruse1]

I've toasted a few old TV flybacks.  I read recently that they shouldn't be run with an open (infinite resistance) secondary because the voltage gets too high for the insulation.  Is this generally true?

One could put a short spark gap or a high voltage resistor across the secondary output.  The resistor might be preferable, except that it would be a drag on the output voltage all the time, not just when a spark gap was sparking.  What does anyone suggest?  If I used a HV resistor, roughly what resistance?  I have several 240k and up from there.

Another idea would be to use a MOT instead of a flyback. Maybe two MOTs could be run in series to get enough voltage?

Than, guys.

[johnnyzoo]

I've toasted a few old TV flybacks.  I read recently that they shouldn't be run with an open (infinite resistance) secondary because the voltage gets too high for the insulation.  Is this generally true?

One could put a short spark gap or a high voltage resistor across the secondary output.  The resistor might be preferable, except that it would be a drag on the output voltage all the time, not just when a spark gap was sparking.  What does anyone suggest?  If I used a HV resistor, roughly what resistance?  I have several 240k and up from there.

Another idea would be to use a MOT instead of a flyback. Maybe two MOTs could be run in series to get enough voltage?

Than, guys.

I'm not sure about TV/CRT transformers specifically, but in general, it is true that transformers operated in a flyback type of converter (1) or in certain resonant converters may be damaged if they are operated without load.

Which is better, spark gap or resistor, depends on your application. A resistor will dissipate some power all the time. A spark gap will only fire when the voltage gets high enough, but then it will appear almost like a short circuit.


Whether using microwave oven transformers is a good solution also depends on your application. Using them in series is not impossible but it requires great care. Note that the other end of the high voltage winding is usually connected to the transformer chassis. The frequency is low compared to flybacks (50 Hz vs some ... tens of kHz) and you will get around 2 kV from one microwave oven transformer. And perhaps most importantly, a microwave oven transformer can produce a deadly amount of current at a very dangerous frequency if you get a shock from it. TV/CRT flyback transformers are safer, although you should always avoid getting a shock. Maintain distance, use insulated stick, stands etc, and keep your work area clean.



1) Storing energy in the magnetic field and releasing it; TV transformers are operated this way in their normal application. Basic mechanical and transistor-based ignition coil circuits also work this way. But not all "flyback drivers" used by hobbyists operate in flyback mode - some of them may operate like ordinary transformers, others may be resonant.

[Mat]

I've toasted a few also, these days that im doing experiments with one of these chinese ZVS driver. I'm not very good at theory, so maybe there are mistakes in what ill say. Please forgive me and correct me if its the case (and also, any advice on how to care better for flybacks is welcome!). So, according to what ive seen, I'd say its more amp-related than volt-related (with these chinese ZVS and regular TV flyback).
through my lots of tests i have been having often open-circuit voltage (when the gap is to big for making the arc) sometime for a whole minute. Zero amps, zero problem, the flyback kept alive. But as soon as i'd be running the flyback with a jakob ladder doing continuous sparks between 5-10A (measured before the ZVS) i have to cool down the flyback with a fan, use a duty cycle of 20%, never more than 2 minutes, and the flyback gets very hot and burns often.
I guess there is a current limiting inductor in the ZVS design that protects the flyback in case of open-circuit.

[klugesmith]

Pay attention to where Johnny said
>>1) Storing energy in the magnetic field and releasing it; TV transformers are operated this way in their normal application. Basic mechanical and transistor-based ignition coil circuits also work this way.

Whether it's a flyback converter or a spark ignition circuit: Ever think about the unloaded output voltage?
Primary-secondary voltage ratio always applies, during charge and discharge phases. (Both voltages proportional to dFlux/dt.  We might write dPh/dt if we don't want to get the Greek letter with F sound.)

If primary current were interrupted by a perfect switch, and there were no capacitance to receive the stored energy, voltages in the "collapsing field" phase would be determined by breakdown somewhere in primary or secondary circuit.