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.
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).
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.