The 555 Driver Basic
Flyback transformers are found in monitors and TVs, sometimes known as Line OutPut Transformers, or just LOPT. They are used for generating HV for the CRT. Their resonant frequency is between 15 – 100 kHz, which is the frequency they are designed to be run at. Running them at this frequency can be done in many ways. One way is by using the 555 timer IC. This simple driver circuit is quite efficient if tuned correctly. It is set to run between 17 – 50 kHz.
This is a pretty standard 555 astable design. All parts except the timer are non-critical. Input power should be 12-16 volts, the current draw is a few amps. The circuit is quite reliable.
Winding your own Primary
Winding your own primary is recommended because you don’t have to worry about finding the built-in primary, and you can tune the flyback to the operating frequency and voltage. The primary must be wound directly onto the ferrite core. The wire gauge must be adjusted according to input power, if it’s too thin it will melt. The number of turns varies, and is determined by operating voltage, on time and core cross-sectional area. 5 to 10 turns should be right for this driver. If the mosfet heats add more turns, remove turns to increase power.
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. The primary can be found by measuring resistance. It should be around 1 ohm. These pins are usually beside each other. Some flybacks may have several “primaries” so just experiment.
If this isn’t clear just wind your own, it is well worth the extra trouble.
Spark at 5v. Driven by the basic 555 driver.
Same specs as previous picture.
Of course something can go wrong, so if you’re unlucky check these points.
- If you hear a high pitched whine but you don’t have any HV, you’re close. This is caused by a false primary. Or try switching the primary leads, since the flyback is a DC-DC converter, and “half-wave” rectified. A single diode rectifies the flyback pulses, so polarity is important. If the output is weak, it’s also caused by having the primary hooked up the wrong way.
- 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 it’s alive. If you’re getting a frequency out of the circuit, you are probably not connected to the correct primary. Wind your own if it is to difficult to find.
- Always check that the 555 is still alive and well.
555 Driver MKII
One and a half years later I decided to try this again, but this time I knew what I was doing. I whipped up this new 555 driver, which works quite well.
The max voltage is 50V, due to flyback spikes. If you want to power it from an even higher voltage, use a snubber or higher voltage mosfet instead. One day I’ll run this thing off rectified mains, the sparks should be incredible!
24v input 3cm+ sparks.
Fat, juicy spark.
70kV from 50 volt supply
Tired of measly 2 cm sparks? Want more power? Try the Mazzilli ZVS flyback driver! This thing will push a few 100W through the flyback if you let it. The circuit is has very few parts, which means less can go wrong, and more power can be put through it without killing weak ICs. The arcs produced by this driver are very hot, the copper ground wire goes red quickly, and anything brought close to the arc in incinerated. Primary should be between 5 + 5 and 20 + 20 turns, with a center tap, all wound in the same direction. The amount of turns depends on supply voltage. 5+5 for 12V, and work up from there. The most I’ve heard of people pushing through the Mazzilli driver is 100V. As you can imagine, the arcs were insane!
Circuit design by Vladimiro Mazzilli.
Arc at 25V supply voltage, actually the voltage was 50V provided by my MOT PSU, but it dropped to 25V when pulling an arc. I should have used thicker wire!
Off-Line Flyback Driver (The wall’s the limit)
Running power through multiple supplies just to power a transformer seems a bit absurd, so I though it was time for a direct mains powered flyback driver. Ironically you need an auxiliary supply to power the PWM section, there’s always a catch.
This driver is capable of pumping some serious power. The frequency and number of turns are a matter of tweaking, but the values shown will work fine. I can run flyback transformers for an unlimited amount of time at 100 kHz and 30 primary turns. Since you won’t be running the flyback transformer is flyback mode anymore you’ll need to remove the air gap. Remove the metal bracket, and pull the core halves out. In-between the core halves is the spacer which should be removed. It is not necessary however. The duty cycle pot will vary power. You may want to scope the GDT to make sure the PWM is working before turning down the duty cycle. Homemade GDTs don’t always perform well with PWM.
Entire driver assembled in case.
Safety!!!One thing that surprised me when I was a newb was finding out how dangerous just looking at arcs and sparks can be. The arcs are gas heated to thousands of degrees, and staring at them is almost as dangerous as staring at an arc welder. Notice how everyone uses a mask when welding? Dark sunglasses are recommended if you want to stare at the arc. Also notice how purple the sparks are? They radiate lots of UV light, so use UV-protective sunglasses and be wary of sunburns. Also don't wield the hot HV wire barehanded, but use a "chicken-stick" or non-conductive rod to move the HV or ground wires. The HV from the ZVS inverter and offline flyback driver has enough current to kill, so be careful.
The simple 555 flyback generated arcs are a lot less hazardous and a shock is not lethal at low driving voltages, unlike the ZVS driver. So if this is your first HV project, stick to the lower power projects first.