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Fly back transformer driver CIRCUIT (UF4008)

2017-07-13 03:52  
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This article describes the Fly back transformer driver CIRCUIT (UF4008). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components:UF4008. 

flyback Fly back transformer driver
General Description and circuit operation:

This is an efficient fly back driver for modern cylindrical
rectified television fly backs. 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 puffs 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 types of
fly back as frequency and duty cycle have a large range.

Frequency range can be increased using multi position switch
for other values of C3 capacitor ,for example 2 of for
80KHz-200000KHz, but didn`t found fly backs 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 electrolytic caps at 300-400V). Unfortunately my
ignition coil died because insulation breakdown (too long drawn

I was able to power a small (20cm) Spark Gap tesla coil Using
these dc rectified fly backs 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 netw1ork 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
betw1een 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)


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)


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.