Flyback Transformer Driver for Beginners




Introduction: Flyback Transformer Driver for Beginners

The schematic has been updated to include basic transistor protection in the form of a capacitor and diode. The step 9 page "going further" now includes a way to measure these illustrious voltage spikes with a regular volt meter.

This instructable will show you how to make a simple driver circuit for obtaining high voltage arcs out of a component called a flyback transformer.

A flyback transformer, sometimes called a line output transformer, are used in older CRT TV's and computer monitors to produce the high voltage required to drive the CRT and electron gun. They also have auxiliary low voltage windings built into them which the TV designers use to power other parts of the TV, they are usually customized for a particular model of TV.

For the high voltage experimenter they are used to make high voltage arcs, which is what this instructable will show you how to do with just a few simple electronic components.

You can get flyback transformers out of older CRT monitors and TV's. They are the ones that have a big heavy chassis. There are also other instructables on this website showing how to remove them from the chassis and circuit board.

I am in no way responsible if you mess up with this circuit. If you mess up you have no-one to blame but yourself.


1x Flyback transformer.

1x Transistor, I've personally tested MJE13007, 2SC2373 and MJ15003. MJ15003 was by far the best performer with this circuit, it just seems to have the right parameters to produce the best output without getting too hot. 2n3055 just sucks for reasons I'll go into further on.

1x Heatsink (bigger is better)

1x 180 ohm 1 watt resistor for 6v or 220 ohm 2 watt resistor for 12v or 470 ohm 2 watt resistor for 18v.

1x 22 ohm 5 watt resistor.

2x Fast recovery diodes, one rated for 200v 2 amps minimum and the other at least 500mA and 50v.

1x Bipolar film or foil capacitor rated for 150vac minimum, 47nF for 6v or 100nF-220nF for 12v or 220nF-680nF for 18v (assuming a 100v+ transistor).

Wire for winding two coils and hooking up other parts of the circuit, 2.5 meters/8ft 20 AWG is more than enough.

A power supply capable of supplying 6,12 or 18v at a minimum of 2 amps (3 would give more headroom).

A non conductive stick for drawing arcs without getting your hands too close to the business end.

Soldering iron, other ways of connecting the circuit up include alligator/crocodile clip jumpers, terminal blocks or even just twisting the bare wire ends together really tightly. You could even use a breadboard if you keep an eye on the transistor heating.

Step 1: What You'll Need

What you will need:

1x Flyback transformer

From an old CRT TV/monitor or purchased online (don't get ripped off, these things are worth about $15 max when new). TV flybacks seem to perform best with this circuit, monitor flybacks don't put out as much.

1x Transistor such as MJ15003.

2n3055 is the classic transistor often paired with this driver on the internet, but the 60v rating really limits its usefulness and more often than not results in it being destroyed. The peak collector to emitter voltage easily soars above this 60v rating and clips when the transistor breaks down causing extensive heating and eventual failure of the device. So please don't use it, if you do you'll need a large capacitor like 470-1uF across it to limit the peak voltage. This will make the arcs very small too.

MJE13007 is ok but doesn't perform as good as MJ15003 without further modifications.

NTE284 and 2N3773 are reported to give similar performance to MJ15003 whilst KD606 and KD503 are said to be the best with this driver. The KD's are hard to get hold of cheaply these days and were more common in Eastern Europe.

A good transistor has decent current gain (Hfe), for example MJ15003 measures about 30 with my Chinese tester and low turn-off delay (storage time) and fall times.

It also needs to be rated for several amps to handle the peak currents and at-least 100v, but below 250v is preferred as the higher voltage parts such often fail to oscillate in this circuit.

It would appear many audio and linear application transistors possess these parameters.

1x Heatsink with mounting screws and locking nuts

(Bigger heatsink is better). The MJ15003 uses TO-3 case style whilst the MJE13007 uses TO−220, TO-3 hardware is generally more expensive than TO−220. Those who are handy with metalwork could probably fabricate their own heatsink out of scrap by drilling the required mounting holes, just google TO-3 or TO−220 transistor technical drawing for more info.

A thermal pad or paste/grease is recommended for better thermal transfer between the transistor and heatsink. The cheapest and nastiest stuff you can find on ebay is adequate for this, I had some leftover from a PC build but you could salvage enough from old LED light bulbs or the TV you took the flyback from! A pea sized amount is plenty and the transistor will squash it down and spread it out.

1x 2 watt resistor

Your power supply voltage determines the value of this resistor. 150 ohm for 6v, 220 ohm for 12v, 470 ohm for 18v. I will be making a 12v driver so will reference a 220 ohm resistor from now on.

1x 22 ohm 5 watt resistor

The resistor values do not have to be exact, the next standard value up or down would work also, they need to bias the transistor on by feeding about 0.7-1v to the base, the upper one needs to allow a few tens of milliamps through whilst the bottom one sets the turn off current when the transistor comes out of saturation, making this a lower value will increase output but cause more stress on the transistor and heat.

2x Fast recovery diodes one rated for a minimum of 200v 2 amps with a reverse recovery time below 300ns, the other rated for 500mA and 50v minimum (UF4001-UF4007 works well here).

They protect the transistor from negative going voltage spikes, I just used ones found on the TV board.

For the 200v 2 amp diode I used (BY229-200) but anything which meets those minimum requirements will do. MUR420 and MUR460 are the cheapest available at my local electronic store, EGP30D to EGP30K could also work along with UF5402 to UF5408.

For the other diode on the base I used UF4004, this one protects from the negative going pulse there and prevents transistor gain degradation.

With random diodes you may have laying around simply google the part number on the side of the diode and check the datasheet.

1x Capacitor

This should be a film or foil type rated for a minimum of 150vac and between 47-680nF. This capacitor forms a quasi-resonant snubber and helps to protect the transistor from the positive going voltage turnoff spike, a larger capacitor will limit the output voltage but give extra protection, I used a 200nF (code 204) with mine. With a higher voltage transistor you can reduce the capacitor value and allow the voltage to ring up to a higher level thus producing more voltage on the output.

I'll include a technique to measure the peak collector to emitter voltage with a multimeter on the "going further" page.

Wire (any old scrap will do).
For the primary and feedback coils, any wire between 18 AWG (0.75mm2) to 26 AWG (0.14mm2) will do but no larger as it won't fit! Unwanted low current mains appliance power cords are a good source, just cut away at the outer insulation and strip back to reveal the color coded inner cores. I used 1 meter for the primary and 70cm for the feedback, with 12v this gives plenty of extra length for experimenting with more turns.

Enameled copper magnet wire is just too expensive per spool these days for me to recommend it, plus it has a nasty habit of scratching and shorting against the core.

Some way of connecting the components such as solder or alligator clip jumpers.

A breadboard could be used but mind the transistor and resistors don't cause it to melt!

6,12 or 18v power source at a minimum of 2 amps (more on this further on).

Step 2: Wind the Two Coils

Wind two separate coils on the core, one in one direction and one in the other. Don't worry about getting it mixed up as you can simply flip the ends later on.

There is no one size fits all when it comes to number of turns, but 8 turns primary and 4 turns feedback is a good starting point for 12v, a bit less of both for 6v and a few more primary turns for 18v. Experimentation is recommended.

If you've used enamel coated wire like I did then use something sharp to scrape away the enamel insulation at the ends. I don't recommend enameled copper wire for homemade windings as the insulation layer has a habit of being scratched off by the edges of the core and shorting to it, plus its expensive these days! The core is actually conductive measuring about 10kohm end to end, so any damaged parts of the wire insulation is like connecting a parasitic resistor between them.

You do not need to worry about finding the HV return pin yet.

Question: Why can't I use the built in coils?

Answer: I've done this in the past with some success, but the arcs aren't as big and the frequency drops down into the audible range and sounds loud and screechy like a car doing 100mph and suddenly slamming the breaks on.

Plus it can be a nuisance finding which coils to use, best bet is to google search for your flybacks model number and see if places like HR diemen have schematics.

Step 3: Mount the Transistor to the Heatsink

Apply a dab of thermal compound or insert the thermal pad now, then mount the transistor onto the heat sink.

The heatsink is important as the transistor gets hot. I just bought the cheapest heatsink I could find, but bigger is better. The transistor I used is of the TO-3 case style

Don't let the legs of the transistor touch the metal heatsink when tightening the screws or else you'll be shorting the base and emitter to the collector. I just used some random screws and nuts I found in the garage, but they're pretty cheap on places like ebay or local hardware stores.

Q: Can I use a PNP transistor?
A: Yes, but you will have to essentially build the circuit backwards for a positive ground.

Q: Is the heatsink really needed?
A: Yes, if you are wanting to use this circuit for more than 10 seconds the heatsink is vital as the transistor gets hot.

Q: Can I use a MOSFET?
A: No, a MOSFET will not work for this circuit (other self oscillating circuits designed for single MOSFETs are out there).

Step 4: Putting the Circuit Together

In the graphical diagram, the red coil is the primary coil with one end connecting to the positive "+" of the power supply/battery, the other end connects to the transistors collector which is actually the metal casing of the transistor itself if a T0-3 style transistor is used.

The green coil is the feedback coil with one end connecting to the middle point of the two resistors, and the other to the base of the transistor (looking at the T0-3 underside this is the pin on the left).

Step 5: Powering the Circuit

To power the circuit I recommend a power source which can supply a minimum of 2 amps, lower will most likely work but will limit the output.

Add more turns on both windings to increase power, (contrary to what I've read online), this lowers the operating frequency and allows more primary current to ramp up. The number of turns appears to give a rudimentary form of adjustable current limiting.

Bench power supply Self explanatory really.

Wall Wart/charger You can use these, but be mindful of their voltage and current ratings. The switched mode variety will most likely go into self limiting/shut down if the maximum current rating is exceeded.

Salvaged transformer Done this myself for my 12v driver, a 48VA transformer which puts out 9v AC will give roughly 12v DC 3 amps when rectified and smoothed. A 4700uF 25v capacitor will give plenty of smoothing, I'd go with 50v 4 amp bridge rectifier diodes minimum.

Lithium cells in series are great as they can supply lots of current.

Drill batteries are fine, most are 18v so use the 18v circuit.

AA batteries in series are fine, the arcs will just gradually become smaller and smaller as they become depleted. An AA cell is considered spent when it drops below 0.9v at rest, but many can still power other loads even when they're no longer able to supply the juice for this circuit.

A 12v lead acid battery is a very good way of powering this circuit and is what I personally used.

12v car battery see above.

6v lantern batteries will power this circuit for a long time before the arcs start getting small. These are not too common nowadays and pretty expensive in many places, don't waste your money if cheaper options are available!

AAA batteries will work for a while but won't last as long as the larger AA cells, they also have a higher internal resistance so will waste more power as heat.

9v/PP3 batteries will give a few minutes play when new before the arcs become smaller and the circuit stops working. The upper resistor will probably need to be around 180 ohms for 9v, but I didn't make a 9v driver schematic as it would probably lead people to using 9v PP3 batteries and disappointment.

Step 6: Safety First!

When drawing arcs...
I strongly urge you to make a "chicken stick" which is an insulating stick of some sort where you can tape the high voltage wire to the end of instead of touching it directly. PVC pipe is very good for this, wood is fine too as long as its dry.

Sharp pointed nails work well and give slightly larger arcs than rounded electrodes, I used a piece of scrap metal for mine. Attach one end of a wire to the chicken stick electrode and the other end will be attached to the HV return pin on the flyback transformer (either solder in place or use an alligator clip or terminal block).

Scary warnings.
Including the obvious risk of electric shock another thing to take note of is the arc is VERY hot and can easily burn or set to fire to anything it touches. Even the cable insulation will burn if you draw the arc onto it.

If you insist on burning pieces of paper or other objects take that into account and have some way of putting the fire out.

  • Never touch the high voltage wire or any of the flyback base pins when the circuit is turned on.
  • Make sure you can easily cut power to the circuit.
  • Do not use this circuit on an unsuitable surface such as a bare metal or easily flammable surface.
  • The transistor heat-sink can get hot, watch out not to burn yourself.
  • The primary coil and transistor collector can ring up to a few hundred volts, don't touch these either.
  • Keep any high voltage cables away from other parts of the circuit.
  • Keep pets away. As well as the risk of shocking your pet from the sparks many household pets like to chew things such as wires, the high frequency noise can upset many animals too.

I am in no way responsible if you mess up or hurt yourself or others with this circuit.

Step 7: Finding the High Voltage Return

To find the high voltage return first attach your chicken stick to the high voltage out (the big thick red wire) and turn the circuit on. You should hear a high pitch noise, if you don't hear this noise then go to the troubleshooting page at the end.

Bring the chicken stick close to the pins on the flyback and go past each one individually. Some of them may give a slight spark but one of them should give a solid constant HV arc, this will be your HV return pin. You should now disconnect your chicken stick from the HV out and connect it to the HV return pin instead.

In the images above are some arcs produced by my driver and some different flyback transformers, most flybacks will give around a 1-2cm arc with 12V input and upping the input voltage only increases heating on the transistor in my experience.

Have fun and remember the heatsink may become too hot to touch.

Step 8: Troubleshooting


  • If nothing happens then try reversing the connections to one of the coils.
  • If it works but the arc is small try reversing both the primary and feedback coil connections. Around 2cm is the maximum arc length you are going to get out of this driver, 1cm may also be the maximum on some flybacks. Also try adding more turns to both coils, this lowers the operating frequency and allows more current to ramp up in the core.
  • Make sure all connections are secure and nothing is shorting out. Enamelled copper wire is notorious for bad connections, soldering doesn't always break through the enamel. I use a knife to scrape some off and prepare for a good connection.
  • It works but the transistor gets very hot. This is normal for this circuit, the price to pay for a simple design! The way the switching cycle ends is by the transistor coming out of saturation as the collector current increases beyond a certain point, a transistor passing a few amps out of saturation will cause heat. To combat this add more thermal mass to the heatsink, either by swapping it for a larger one or attaching more metal to the current one.
  • The 22 ohm resistor gets hot, this is normal. It my case it dissipates 1.96w, but that's enough to get most little resistors too hot to touch. If you're not comfortable with components running too hot to touch then increase the thermal mass, in the resistors case increase the wattage rating.
  • Broke the core? Glue it back together, dampening the mating surfaces with water first will help certain types of glues stick.

Step 9: Going Further

You can measure the peak voltage spike across the transistor with the method shown in the picture, it is important to keep the peak collector to emitter voltage below the maximum rating of the transistor along within the safe operating area (about 80v at 3 amps for the MJ15003).

A transistor may appear to clamp the peak drain voltage for a while but this quickly leads to failure of the part.

PNP transistors can be used by flipping a few things around.

Try making a jacob's ladder by placing two rigid conductors like thick copper wire in a vertical V shape, the arc forms at the closest point near the bottom and rises at it heats the air.

HV capacitors are also interesting, you can make one by taping two pieces of foil on each side of an insulator such as a plastic container lid and running two wires to each sheet. Now connect one plate to the HV out and other to the HV return, the arcs will turn into a series of loud bright snaps! Just don't touch it as it really hurts.

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237 Discussions


4 months ago

Hi, I love your article!
I currently only have access to 2N3055 transistors. If I use those in the circuit, is there anything I can do to extend their lifetime until i get access to the MJ15003? I just want to test a bit with the 3055.


Reply 4 months ago

Hi! Use a larger resonant capacitor value (C1) to limit the peak collector voltage along with both protection diodes, also using less primary and feedback turns increases the operating frequency and reduces the peak voltage stress. The diode on the base prevents emitter-base reverse breakdown which slowly degrades the gain.

As a starting point I'd suggest somewhere around 470-680nF, junked power supplies and the CRT circuit the flyback came from often have a few of these inside them, you can put smaller ones in parallel and their capacitance will add up. To see what they look like just google image search for "film capacitor types". All of mine came from old circuit boards and they work fine.

You can use this method in the attached image to measure the peak voltage spike without an oscilloscope, for 2n3055 the peak needs to stay below 60v (100v for the HV version). Just keep putting suitable capacitors in parallel to C1 or reducing coil turns until the peak voltage stays below 60v, it'll change depending on arc loading too so watch out for that.

When it goes above 60v it doesn't blow right away, instead it acts like a crappy self destructing zener diode and slowly destroys itself (the one I tried hard clamped around 64v and got very hot fast).

The side effect of a large resonant capacitor and less coil turns is it'll make the arcs smaller, but at-least it'll save the transistor until you can find a more suitable one.

Got any other transistors laying around? Oddly some of them found in audio amplifier output stages seem to work well for this where's others designed for switch-mode power supplies perform poorly, MJ15003 (designed for audio amps) worked remarkably well and didn't get that warm. 2n3055 gets hot even if the voltage spikes are kept in check, when compared on an oscilloscope the 2n3055 took ages to turn off in this circuit whilst the MJ15003 was really fast.

There are ways of making the slow transistors work better and heat less, but it requires additional components and experimentation.

I know this Instructable has 2n3055 in the URL, but I wrote it around 10 years ago when I was just starting out. During lockdown I've come back and attempted to tidy up and correct mistakes.

peak measure.png

Reply 3 months ago

How do I determine the oscillating frequency here?


Reply 3 months ago

I am no expert but this is my understanding of how it works. I have managed to build one with two transitors and it works great.
The circuit is self resonating. as current is switched on, it flows throught the primary and induces a current in the secondary. It also induced current in the feedback coil in the opposite direction. This causes the transistor to turn off and collapse the current in the primary and consequently, in the feedback. This allows current from the PSU to flow once again and the cycle repeats. Depending on the transistor you use, switching speeds can be different.
Just make sure the coils are would in opposite directions or it wont work.


Reply 3 months ago

Yep, changing the number of turns on the primary and altering the feedback coil in proportion to match has the most effect. More turns = lower operating frequency = more energy stored in the core per cycle.

The transistor selection has a lot more going on than the headline specs such as max voltage and current ratings, some transistors have nasty turn off delay in this circuit which ends up turning a large chunk of the input power to heat and limiting the output from the flyback. 2n3055 and MJE13007 behaved like this whilst MJ15003 worked great, the differences could be seen on an oscilloscope at turn off plus the transistor didn't heat as much and sparks were bigger for a given input power. Unfortunately many transistors don't publish this data in the datasheets, but MJ15003 was designed for audio applications.

This circuit has been around since the 70's and 2n3055 became synonymous with it as it was widely available at the time, really old versions of the 2n3055 used to perform much better as they were manufactured using a different process, modern ones don't do well with this circuit.

As well as the diode and capacitor across the collector to emitter there needs to be something protecting the base to emitter, many transistor can take about 5v reverse before damage starts occurring. In my testing this manifested as the gain permanently reducing. A diode, TVS or Zener to clip emmiter to base voltages under the breakdown level will do.


3 months ago

Please how do I determine the oscillating frequency of this circuit


Reply 3 months ago

Its set by the core and number of primary and feedback turns mostly, transistors gain probably has an effect too.


1 year ago

Coils should be made in oppposite directions.


Reply 6 months ago

Yeah, I just noticed that too. Shame teenage me didn't all those years ago! I've updated the schematic now.


Question 1 year ago

My transformer war working, then i added more primary winding, and then, i turn it on again, i draw an arc and, it just stop arcing, the pitched noise stopped and the primary get very hot
short circuit?
please help


Reply 6 months ago

Hi, I haven't been active here in years. Please see the updated schematic and section on how to measure peak transistor voltage. Sorry it blew a transistor for you, but I originally published this instructable as a teen and didn't know better.


Answer 1 year ago

ok the transistor just blew up
but this happen my 2 other times, i'm gonna run out of transistors
why is this happening? my diode is good


Question 8 months ago on Step 7

ive built a very similar jacobs ladder using a different video, in which 2 wires are connected to the standard pins of the flyback transformer (pic attached to explain)
there is then a single cable of roughly 6 windings around the core and it shown to be accross the middle of said core, whereas you have 2 seperate windings that seem to be left and right of the middle of the core.
(the different transistor, lack of diode and windings are the only differences in the circuits)
yours also seems to have bigger arcs and i was wondering if you could explain why
so i could improve mine?
i am using a 12v power supply and a s2055N transistor.
my circuit does not have no appear to need a diode but would adding one help?
any other way i can increase output voltage like adding a capacitor?


Reply 6 months ago

Hi, sorry I haven't been active in this scene for a very long time but just noticed your comment when I logged in for the first time in years.

Your transistor appears to have that diode built in, it's also a horizontal output transistor from the TV and can be finicky to oscillate with this driver. Winding external primary and secondary coils often works better with lower voltages due to the lower inductances, number of turns can be a bit experimental and some transistors work best with more or less.

How big are the arcs you're getting? The flyback in that video had something like 20 primary turns and 7 feedback. This allowed the peak primary voltage to reach higher levels and thus give more voltage on the output.

Before doing this please see the updated schematic with the capacitor and method to measure peak collector-emitter voltage on the going further page, I know the instructable is a bit messy and inconsistent now and I'm thinking about updating it during the coronavirus lockdown period. Its important the correct capacitor value is selected to prevent the transistor from being destroyed by overvoltage.


Question 6 months ago on Step 5

Can I use a 1358 npn transistor in places a 305 5 and will the deflection transistor will that affect well I have to change up resistors and everything else or will it work


Reply 6 months ago

Hi, the 2SA1358 is only rated for 1 amp of collector current and this circuit often draws around 2-3 amps average with peaks much higher. It's also a PNP part (althought that's not a problem if the circuit is adjusted).

Are there any other transistors available to you?


Answer 10 months ago

No its too powerfull .if you want to build a safe shocker use mosquito bat citcuit
Just change the the battery to 4v 1amps lead battery and put a heatsink to the transistor.


Answer 1 year ago

If you want to kill a person, yes


5 years ago on Step 4

what happen if i use 2 or more transistor or greater then 2n3055 and more resistors of same value that you suggested us and a more powerful it sufficient to drive out 30kv. if not then can you please suggest us what to do for driving 30kv output from a fly back transformer of a monitor.

one more question ???

can i use transistors ,diodes ,capacitors ,resistors etc from my monitors circuit board.