A Transistor Driven Flyback Transformer Page

Email: sixmhz@yahoo.com

Link to Homepage and index

Introduction

A very simple road to your basic high voltage generator is the transistor driven flyback transformer. This circuit uses a common flyback transformer (you probably have more than one in your house right now) to boost typical battery voltages up to the multi-kilovolt range. It will make a several thousand-degree continuous 2 cm plasma arc. This page is designed to help you get yours working as quickly and painlessly as possible.

The Circuit

As you can see this is a VERY simple circuit and when you have all the parts, it takes only a few moments to get working.

The Flyback Transformer

The main component of the system is the ferrite core flyback transformer. Flyback transformers can be found in any equipment with a Cathode Ray Tube (CRT) like televisions or computer monitors. There are many types of flybacks out there. The really old ones have disc shaped secondarys, removable primarys, no rectification and look like this:

Most of the ones you will find today will have a rectified output (DC) and the primary windings will not be removable as they are potted in with some kind of polymer. Any flyback will work in this circuit, but depending on the flyback, the high voltage output may be used for some applications and not others. For example, if you have an old flyback without rectified output or have removed the rectifier in a new flyback, the high voltage output will be radio frequency AC and will work for a plasma globe whereas a rectified output flyback will not. Although with an unrectified flyback you can drive a plasma globe, you cannot charge capacitors. But, with a rectified output you can charge capacitors. The flyback with the highest output voltage and longest arc that I have ever seen comes from large modern TV's. I have found two like this and bought another like it on ebay. I'm not sure why it has a green spot painted on it but it is a very distinguishing mark for a great flyback.

Windings

If you have an old flyback with removable windings, don't be in such a hurry to remove them and wind you own. If there are at least two sets of coils, you can use the original windings with no problems. It would probably work even BETTER than winding your own coils (I've never figured out why everyone says to wind your own???). If you don't have an old flyback and your windings are potted inside the flyback don't worry, cause that's where you want them too. I would actually prefer a flyback with the windings potted inside so they don't get damaged.

Locating the Coils and Coil Polarity

The flyback circuit diagram calls for two sets of coils: a primary coil and a feedback coil. The turn ratio is really not that critical so usually ANY two coils in the flyback primary will work. Locate two sets of coils on the horse-shoe-like configured pins by testing the flyback pins for continuity. Often times there are more than two pins connected to a single coil in the transformer. You will need to try the different positions to see which configuration works better for your application. After you have located two independent sets of coils on your flyback, hook them up to the circuit with any polarity. If you turn on the power any you don't hear a whine or hum, try reversing the polarity (switch the leads) of ONE of the coils. If nothing now, reverse the polarity of the other coil. If nothing now, reverse the polarity on the first one you switched again. So the take home message is trial and error. Flybacks can be very picky as to coil polarity because some of them have a rectifier built in them. So try each configuration (8 of them with two sets of coils) until it whines (Occasionally I'll find one that doesn't wine but VERY rarely). Any two sets of coils in a potted flyback should work, so don't try new coils until you're sure you've exhausted all possible combinations.

The high voltage will come out of the fat wire from the top of the flyback-usually connected to the CRT with a suction cup. You will not be able to locate the high voltage return pin with a multimeter. The only way to do it is to bring the high voltage line down to the pins and whichever one it arcs like mad to is the one yer looking for. Try to stay away from arcing to any of the pins used for coils. High voltage is not so good for your transistor or power supply.

Transistors

The transistor is the little guy who does all the work in this circuit. This dude gets real hot, so you should put a heat sink on him if you use him for more than a couple seconds. Transistor selection is VERY important especially if you are using a newer flyback or any non-disc shaped secondary. Typically the circuit calls for the 2N3055 power transistor. Every person that uses this transistor says that you can't use a new flyback because they don't work. Well they're right, but for the wrong reason. It's not the flyback that doesn't work, it's the transistor. The 2N3055 works well for the older disc-shaped flybacks but doesn't do jack for the new ones. A thousand volts or so is all you'll get out of a new flyback with a 2N3055. Well then Greg, what the hell am I supposed to do? I'll tell ya's what to do. Ere's a list of transistors I've tried on new flybacks and how they work:

Transistor NPN/PNP Vce (Collector-Emitter Voltage) Ic (Collector Current)

As you can see, only certain transistors work well with the new flybacks as most of them fry very quickly and cost you dollars :< I would recommend only two transistors, the NTE284, that costs about 10 bucks and the KSE13009 from fairchild. The KSE13009 is a NEW find for me that works very well and only costs 81 cents!!! You can get the KSE13009 from the fairchild semiconductors website. This transistor works well for the new flybacks, but if you wind your own coils it will burn out quickly.

Power Supply

When it comes to operating voltage this circuit doesn't really care. You can run it as low as 5V or as high as 50 with no modifications to the circuit. I have noticed that increasing voltage does not increase the current linearly as one would expect. This circuit seems to have a maximum current usage around 3 Amps or so, possibly due to inductor action in the transformer primary which actually limits the current rather than resistance. Also the output voltage seems fairly independent of the input voltage, which is also strange to me. If any one has any ideas I would like to hear them!

Circuit Operation-So you say...

I have heard and it seems very plausible to me (I am no electrical engineer) that the circuit works in the following manner: passing a current through the transistor to the main set of coil windings induces a field on the ferrite core which also induces a current in the feedback winding. The feedback current signals the transistor to stop conducting and the field on the ferrite core collapses, causing a high voltage pulse on the output side. After the feedback signal collapses, the transistor again conducts current to the primary windings.

This seems all nice and good, but it doesn't explain why some transistors work well and some don't. The transistors should only be switching the current at a fixed voltage off and on so as long as the transistor is fast enough, the output should be the same for every transistor. This is clearly not the case in my mind. In my experience, the output voltage is strictly dependent on the individual transistor, not even the specifications for the transistor. Many transistors are getting fried for no apparent reason and almost instantly. This circuit draws only 2-3 amps but transistors rated for 16 Amps get cooked like biscuits in a lava bath. So what's really going on with this circuit??

Ok, so now I think I have a *reasonable* explanation of what is happening with this flyback circuitry. Initially, the circuit operates as the simple feedback system. The transistor conducts, induces a voltage on the feedback coil and then stops conducting. When the transistor stops conducting, the induction in the primary coil causes the voltage to go up, way way up. So way way up that the voltage exceeds the breakdown voltage of the Vce on the transistor. This high voltage spike is now allowed to "escape" through the transistor and is brought back to nominal voltages. This high voltage spike I think is what is causing the high voltage output, not the nominal 12V input. The HV spike is also causing the transistors to get fried as their Vce voltage is being greatly exceeded. So the transistors that work well apparently are very "robust" and are handling this rapid breakdown of the Vce and are recovering with no permanent damage to the transistor. This is why only a few transistors work for this application. So to get a *nice* spark you your transistor needs to have a high Vce voltage, AND it needs to be special to not get fried by repeated over voltages.

Fun with Flybacks!

Radio Frequency Plasma Globe

To make a plasma globe you really need two things. A radio frequency (RF) high voltage power supply and a partially evacuated tube with some type of cool gas in it. The power supply I use is a flyback transformer taken from an old Mac computer monitor that was laying outside the chemistry building. The flyback was old enough that the rectifier "stick" stuck out of the side of the secondary windings and I was able to remove it with a hack-saw. The circuit is driven by a 2N3773 transistor making the voltage adequate for this application. The power supply I use is a large lab type (I found it in the trash) with four channels; I am using the 12VDC output for this project. The globe consists of a 25 Watt clear light bulb and lamp I bought at Goodwill for like two (2) bucks. The high voltage output is attached to the AC cord on the lamp and the high voltage return (minus side) is attached to ground (-DC). It puts out enough voltage so that when I put my hand close to the bulb the plasma goes to your fingers. With such small current at this voltage you can even touch the glass with only a tingling sensation to your fingers (no permanent damage) but don't bet your life on it. Here are photos of my plasma globe driver and the globe in action. You can also check out my plasma globe page here.

High Voltage Capacitor Charger

A cool use for flybacks with rectified outputs is charging high voltage capacitors. Now you can store up a nice high voltage charge on some caps and put them to use. You could use them in a coil gun or maybe even to shrink some quarters!

High Voltage/Current Power Supply for Tesla Coil

I am trying to see if I can push enough current into this flyback circuit to use as a tesla coil power supply. I intend to keep pushing a flyback until the coils melt or it explodes. So I've got myself a VARIAC now and I've wired up 6 2N3773 transistors in parallel with a HUGE heat sink. Hopefully soon I can get a flyback to explode!

Ignition System for Potato Gun

Need I say more?

Voltage Multiplication

You can double or triple or whatever to your flyback with a voltage multiplier. The multipliers only work on flybacks with unrectified outputs. I've heard that multipliers can be found in some televisions but I've never ever found one :< Once there was an auction for 5 of them on ebay but some bastard outbid me. Since you'll probably never find one, you can make your own easily if you can find the parts. Diodes and capacitors that can withstand 20 kV are kind of hard to come by. Here's the schematic for the voltage multiplier, which will work for any AC power.

WARNING: THIS PAGE IS PROTECTED BY MATT'S RABID MOOD CHANGING MONKEY!!

DISCLAIMER

The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise arising from the use or misuse of the information contained herein are entirely the responsibility of the user, have a nice day!

Last updated: 02/20/05
Copyright 2002-2005, Greg Miller
http://www.angelfire.com/80s/sixmhz/flyback.html