Flyback Transformer Drivers

The 555 Driver Basic

Flyback transformers are found in monitors, TVs or anything with a CRT, and
are sometimes known as Line OutPut Transformers, or just LOPT. They are
used for generating high voltage for the CRT, which is needed to create an
electric field, which in turn accelerates electrons towards the screen, which
finally excite phosphors and create the image you see. Flybacks are designed
to work best anywhere between 15 to 150 kHz, so some experimentation is
required to find the intended operating frequency. TV flybacks are generally
designed for upper audio frequencies, which is the cause of the high pitched
noise heard from a muted TV. (If you're over 40 you will need to confirm this
with your kids.) The optimal operating frequency can have many harmonics,
which will work as well as the actual optimal frequency to some extent. Since
flyback transformers use a ferrite core they need vastly different operating
conditions than an iron cored mains transformer. In fact flyback transformers
aren't really conventional transformers at all, but coupled inductors which
means they should be driven differently. Fyback transformers are generally
either driven in "flyback mode", or some push-pull topology. The first two
drivers on this page drive the flyback in flyback mode, while the last two use
push-pull topologies. To obtain a high frequency variable duty cycle drive
signal we can use the 555 timer. This simple driver circuit is quite efficient if
tuned correctly, and in some cases quite powerful. It is currently set to run
between 17 50 kHz, which should be a large enough range to sweep
through any harmonics a flyback may have.

This is a pretty standard 555 astable design. All parts except the timer and
mosfet are non-critical. Input power should be 12-16 volts, the current draw
can reach a few amps. For the mosfet I used an IRFP450, though any mosfet
with a breakdown voltage above 200V and "avalanche rated" will work. Make
sure you use a mosfet and not a bipolar junction transistor, the symbol in the

datasheet should resemble the one in the schematic. For a different

frequency range you can use a 555 timer calculator or just experiment to
find new capacitor and resistor values. As mentioned above this driver drives
the flyback in flyback mode. What that means is that the mosfet is turned on
by the timer, and current starts to flow through the primary winding. After
some time the timer will turn off the mosfet again and the current will be
forced to stop. However, this is not possible since the primary has significant
inductance. The current then causes the voltage at the mosfet drain to
increase in an attempt at allowing current to flow. The voltage will rise up to
the breakdown voltage of the mosfet, where it stops (since the mosfet is
avalanche rated this does no harm, and only produces heat in the mosfet).
The voltage at the mosfet drain will potentially be equal to the breakdown
voltage of the mosfet, meaning the primary voltage will be hundred of volts
now. Due to the large turns ratio of the flyback the few hundred volts at the
primary become several thousand volts on the secondary. Since some energy
is avalanched in the mosfet, adequate heatsinking of the IRFP450 is required.
Winding your own Primary
I recommend winding your own primary for several reasons. For one thing
you dont have to worry about finding the built-in primary, and you can
adjust the primary turns according to the drive voltage or desired output
voltage. Also you don't need to worry about destroying the internal primary
during the experimentation phase. The primary must be wound directly onto
the exposed ferrite core. The number of turns varies, and is determined by
operating voltage, on-time and core cross-sectional area. For general use, 3
to 10 turns should be right for this driver. Fewer turns mean higher voltage,
but increased mosfet power dissipation. Start with 10 turns and remove them
until the MOSFET gets too warm or the spark too big.

Standard Monitor Flyback Transformer with new primary.

Pinout

For those of you who have never seen a flyback transformer before, it may
be a bit tricky to know where the primary, ground and other pins are. The
ground pin can be found by finding the pin the HV arcs to the most. Simply
take the HV lead and bring it near the pins on the bottom. The internal
primary can be found by measuring resistance. It should be around 1 ohm.
Some flybacks may have several winding which will appear to be primaries,
in this case the real one can only be found by measuring inductance. A
typical primary inductance is often 300H.
Pictures

Sparks at 5v. This was before I knew anything about electricity, so 5V was all
I considered safe for the driver.
Troubleshooting
Of course something can go wrong, so if youre unlucky check these points.

If you hear a high pitched whine but you dont have any HV, youre
close. This is caused by a false primary or wrong phasing. Try switching
the primary leads, since the flyback is rectified by a single diode, which
makes polarity is important. If the output is weak, the primary polarity
may need reversing. Check to see if you have found the correct ground
pin.

Nothing. Silence. This is what I hate the most. Basically just check that
everything is wired correctly, if it is, check that all the parts are
functioning. You can check for a frequency by replacing the flyback
with a speaker. If you get a high pitched whine its alive. If youre
getting a drive signal out of the circuit, you are probably not connected
to the correct primary. Wind your own if you fail to find the internal
primary.

Always check that the 555 is still functioning.

555 Driver MKII

About 18 months later I decided to try this again, only this time I knew what I
was doing. I whipped up this new 555 driver, which works quite well. The
max voltage is 50V with an IRFP450, due to the primary energy becoming to
much for the mosfet to avalanche without dying. If you want to power it from
an even higher voltage, use a snubber or stronger mosfet instead. The
additional circuitry simply isolates the 555 from the "power" supply, so it can
be increased beyond the 16V rating of the NE555. Keep in mind standard
7812 regulators should only run from 30V maximum, so for 50v you need to
cut out the 7812 and run the logic section from a dedicated supply.

Make your own PCB!

I've designed a PCB for this driver which can be edited with ExpressPCB. A
pdf of the copper traces and components layout is also included for those
without ExpressPCB. flyback_driver_MK_PCB.zip. I recommend using a
5.45mm block connector for the IRFP450, in case it needs changing. As with
the basic driver, heatsinking of the IRFP450 is required. I've used an old
processor heatsink with a small 12V fan. If powering the fan the 7812
regulator will need a heatsink as well.

Pictures

24v input, 3 cm+ sparks

Dual sparks
modified)

24v in juicy spark

70kV from 50 volt supply (driver was

MORE POWER!!!
Tired of measly 2 cm sparks? Want more power? Try the Mazzilli ZVS flyback
driver! This driver is capable of pumping upto a kW of power, so impressive
arcs can be made. Normal operation from 12V only results in about 100W,
but that's still several times what the 555 drivers will process. The circuit is
has very few parts, is simple, and very elegant. If it weren't more deadly than
the 555 drivers I would have made it my recommended newb driver. The arcs
produced by this driver are very hot, the copper ground wire goes white
quickly, and anything brought close to the arc in incinerated. The current in
these arcs can kill, so be carefull. The primary winding is center-tapped, so
two windings must be wound in the same direction. I usually intertwine two
wires, and then simply wind one winding with the pair. The start of one wire
and end of the other are connected and serve as the center-tap. The primary
should be between 3 + 3 and 10 + 10 turns, depending on voltage. The
amount of turns depends on supply voltage, and resonant frequency of the
circuit. The highest voltage Ive heard of people running the Mazzilli driver at
is 100V. As you can imagine, the arcs were insane! The resonant frequency is
determined by the total primary inductance and parallel capacitor. It is a

simple parallel resonant circuit, so the resonant frequency is easy to find with
the parrallel resonant LC formula.
Circuit designed by Vladimiro Mazzilli.
The circuit works by one mosfet turning on due to differences in the gate
resistors or internal structure of the mosfet. Once on, the opposite mosfet
will be held off by the fast diodes. The voltage across one primary half will
rise up an fall again in a half-sine wave. Once at zero the mosfet that was on
will be forced off, and the mosfet which was held off will be allowed on. The
cycle repeats in opposite this time, before returning to where it started. The
large inductor serves as a "current capacitor", providing constant current to
the driver. Thanks to the resonant action of the circuit, it benefits from ZVS,
or Zero Voltage Switching. This means that the mosfets switch on with no
voltage across them, so while they transistion from off to on they won't
dissiapte power. (P = I * V)

Pictures

Arc at 25V supply voltage provided by my

MOT PSU. Susposed to be 50V but it dropped to 25V when pulling an arc, I
should have done a better job of rewinding the MOT!

Off-Line Flyback Driver (The walls the limit)

Running power through multiple transformers just to power another
transformer seems a bit absurd, so I though it was time for a direct mains
powered flyback driver. This driver rectifies mains and produces a 320V DC
source. The circuitry from the TL494 to the GDT creates alternating pulses
for the IRFP450s at any desired frequency and duty cycle. The IRFP450 halfbridge feeds a square wave at +/- 160V to the flyback primary. 160V at
considerable current, stepped up to some kV and mA produces some
impressive arcs. The driver itself is my "multipurpose inverter". Contrary to
the 555 drivers this driver provides actual AC to the primary, and due to DC
flybacks being half-wave rectified you'll likely have problems with saturation
regardless. I've run DC flybacks offline several times, but they have all failed
eventually, and they cause the driver to heat up. Unrectified AC flybacks are
much more suitable for this driver, and cause very little heating of the
mosfets. See my article on making HV transformers for details.
The frequency and number of turns are a matter of tweaking and design, but
for safe testing 30 turns and 100kHz have worked fine for a wide selection of
flybacks. The main issue here is saturation of the transformer, which
prevents the core from becoming further magnetized. Practically speaking, it
means the transformer inductance will drop suddenly when saturation

occurs, causing incredible current draw from the inverter and most often
destroying it. (Or with my multi-inverter, simply tripping the OC protection.)
I've put a calculator in the multipurpose inverter spreadsheet for determining
the minimum number of turns required to achieve a specific flux density. Use
0.25T as a starting point if you don't know the saturation flux value of the
core.
Since you wont be running the flyback transformer in flyback mode anymore
you can remove the air gap to decrease the idle current drawn. Remove the
metal bracket, and pull the core halves out. In-between the core halves is a
thin plastic spacer which creates the air gap.

Entire driver assembled in case. This was before it was upgraded into the
Multipurpose Inverter.

Awesome arcing action.

Nice.

Some interesting things to do with HV arcs are coloring them with salts, or
making magnet vortexes. Common table salt will give red/orange arcs, and
boric acid will give green arcs. Check the Internet for flame tests to see
which salts will give which colors. The effect is hard to capture since the
camera saturates and can't see the green color. Arcs drawn with boric acid
are much brighter than usual, and are uncomfortable to look at even with
sun glasses on. Arcs drawn with salt (or iron, I'm not sure which is causing
the arcs to change appearance) on the other hand, are actually pleasing to

look at as they are dimmer than usual. Of course, coloring plasma with salts
in more impressive with Tesla coils, since they aren't as bright.

This blurry picture is all I have to suggests that the

arcs are green.
Another thing to do with DC arcs is to make a plasma vortex. Arcs consist of
plasma which is easily affected by magnetic fields, hence the awesome
vortex. See the 4HV thread for more info and videos.

A magnet plasma vortex can be made with the round

ferrite magnets found in microwave ovens.