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Flyback Transformer Driver
- Jonathan Filippi
- jonathan.filippi@virgilio.it
- 40.304 Views
- moderate
- Non tested
- 0 Likes
General Description and circuit operation
This is an efficient flyback driver for modern cylindrical rectified television flybacks. Many sites doesn’t provide circuits driving these transformers, they simply say that they are bad.
I don’t agree. In fact I built this circuit. I spent a lot of time for finding resonant frequency (around 15Khz) and duty cycle. These transformers best work at around 90% duty cycle. You may notice corona breakdown at terminals and pfffff sound (as well as the ozone smell) when adjusting the off time trimmer to near 500-300 ohms. Of course it will work for other tipes of flyback as frequency and duty cycle have a large range.
Frequency range can be increased using multiposition switch for other values of C3 capacitor ,for example 2 nF for 80KHz-200000KHz, but didn’t found flybacks with so high resonant frequencies, in addition with higher values of c3 , eg 200nF, 2uF the
frequency will drop making possible the use of ignition coils, and rectified power transformers @50Hz to charge high voltage electrolitic caps at 300-400V). Unfortunately my ignition coil died because insulation breakdown (too long drawn arcs)…
I was able to power a small (20cm) Spark Gap tesla coil Using these dc rectified flybacks to charge primary tank capacitor.
The operation is simple
The 555 is wired as an astable and the capacitor is charged only through the 4,7Kohm trimmer (notice the diode) and discharged only through the 2.2 Kohm trimmer, making the duty cycle full adjustable. The square wave is then feed in a totem pole made up of a 2N3904 and a 2N3906, which are cheap, and easy to find. The totem pole ensures the gate being charged and discharged very fast (approx 50nS i think). The IRF840 is a cheap (i found it for 4euros) reliable and powerful power mosfet, it has current capability of 8 A continuous and 32A pulse, 800V drain source voltage, protecting internal zener diode. There is a snubbing network to ensure that voltage spikes are kept low (unless the insulation of the transformer start to leak) protecting both transistors and 555 IC. 100 ohm is a compromise between decay time and voltage spike.
Comments and specifications
The 100 ohm snubber must me a 5W resistor, or it will burn at long operations
The led is only for safety purposes
Use a dead man switch (pushbutton) for safety
The power supply must supply at least 2-3 A if you want decent arcs (20000 KV)
Dangers
The flyback driven in this way can supply a significant current, aldough the heart fibrillation starts at 30mA I recommend caution to avoid painful arc-burns. The arc is a hot plasma, never operate the circuit in presence of flammable substances. Charging high voltage capacitors is a serious life threat, so if you arent unexperienced just draw arcs and no more This device when rectified generates static voltage that can be a little annoying…. (or fun, i sprayed with corona a plastic pen from positive terminal and then i was able to attract little pieces of paper)
Disclaimer
I don’t assume any responsibility of the damages or discruptions dove by this device, to persons or things. Any irresponsable action would be a serios danger. This is high voltage threat it with respect.
Dear Jonathan,
Thank you for this circuit. This is the only one among so many in the web that seems to me suitable for long time uses.
I have 3 questions:
1. Is this circuit suitable for a DC flyback (I mean one wit internal rectification), as suggested by the “virtual_diode” in the schematic?
2. Is it possible to use this circuit to obtain more than 20000V, say 25-30kV ? if it is, how to do that? (increasing the input voltage?)
3. and finally, is it possible to tune the output voltage of this circuit from DC to max_voltage, say, by varying one of the pots?
thank you in advance.
My name is Flavio.
My question is about the resistance of snubber.
Why 100 ohms. ?
How many turns, and what gauge wire on the flyback primary?
Flyback transformer can be sourced from an old CRT TV, we don’t have the specifications to build your own.
Thanks for the reply. I saw another schematic where the builder wrapped 10 turns of 14-18 gauge wire on the ferrite frame. But having experimented a bit, I found that that primary coil draws a boatload of current, in excess of 5 amps. So many different flybacks out there, some even having built-in focus and screen adjustment circuitry. And almost no spec sheets on line. So a little experimentation is the only way to find a setup that works for any given flyback.
Can i use the 154-177B flyback transformer for this project??
Almost any fly-back transformer will work with this project. I would suggest to give it a try.
I have been experimenting with transformers from old TV. It was easy to find circuit diagrams of these TV’s, wich were helpful for finding the correct pins. However, i found also that in the original circuits, the supply voltage for the output stage was usually 100-150 V. That might be the reason for making a new primary coil with just a few turns.
What coul you suggest for finding the correct number of turns?
Doubt the 555 will last long. The inductive spike is only partly contained. I would favor an optocoupler instead of q1 and q2.