So I had a CRT monitor that I was meant to throw away years ago hanging around, so I decided to open it up to see what goodies were inside.
I soon discovered that my CRT monitor contained a pretty looking fly back transformer! Which I quickly unsoldered from the board.
SOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoooo... I wanted to see if I could control this FBT with the digital pins on an arduino.
I've done a bit of research and it turns out a FBT likes to be pulsed with DC input at approx 22kHz.
My question is on an arduino nano, am I able to pulse the digital pins that fast? Also would a NPN transistor be able to pulse say 30v at 22kHz safely to run the FBT?
I'm using the stock primary and secondary coils... What would be the expected output? Around the 2kV range?
First, be aware that what you are proposing to do is dangerous. Those transformers and the associated voltage multipliers connected to the output are designed to produce around 25kV to drive the CRT.
Line output transformers would typically be driven by an NPN transistor (or possibly a mosfet) with a high voltage rating. The transistor would be turned on for the majority of the 40us or so cycle time. There would be a flywheel diode connected across the transistor. Supply voltage to the transformer/transistor combination is typically around 100 to 180 volts, and the peak voltage seen by the transistor or mosfet may reach 1kV or more.
These transformers are designed to work with the CRT horizontal deflector coils connected to them. With the coils disconnected, the transformer may behave rather differently.
The cycle time of around 40us is too fast to be generated by an Arduino - apart from anything else, the timer interrupt service routine would disturb the timings.
If you want to experiment with it, I would suggest driving it from a much lower voltage supply (say 12V) using a power mosfet such as STP40NF10L. This may give you up to 2kV from the voltage multiplier. These mosfets include the recovery diode and are avalanche rated, so they will absorb the high voltage peak if necessary. Driven from the lower voltage, you can also get away with a longer cycle time, maybe up to 200us on and 200us off. So you might be able to drive it from an Arduino.
Well the idea was to make a small Jacob's ladder, just for experimentation.
As I said in the OP, I was thinking of running it at 30V, but if you think 12V would be more appropriate, in terms of timing and sticking to the arduinos limits, I'm fine with that.
What is the point of a diode across the transistor? Is this to prevent reverse voltage spikes? If so, wouldn't it only be necessary to put a diode with say 2000-3000V reverse protection in series with the transistor going to the transformer?
What would vary if I changed the pulse time? Just voltage output?
The diode is to prevent voltage spikes from killing the transister when the inductor changes state.
You can use a 555 timer & set it for whatever frequency you want in free-running mode, use PWM with lowpass filter into 555 pin 5 I think to modify the frequency.
If you are taking the transformer output and filtering it to look like DC, then changing pulse widths will alter the average output level, changing frequency alone will just affect the smoothness of the output.
Let me repeat that what you are trying to do is dangerous, because of the high voltages that are produced. What would you do with 2Kv anyway?
The transformer normally incorporates a "jacob's ladder" type voltage multiplier already. The output from that is the high voltage cable that was connected to the CRT via a cavity connector.
The way line output circuits are designed to work is:
during the second half of the scan, the transistor conducts and the deflector coil current increases more or less linearly.
the transistor turns off. The deflector coils and associated capacitance respond by trying to "ring" at 100-200 kHz or even higher.
the first half cycle of the ringing produces the high voltage pulse that appears at the transistor collector, is stepped up by the transformer, and rectified/multiplied to produce the high voltage supply to the CRT. The current in the deflector coils reverses.
the second half cycle of the ringing would drive the transistor collector negative, but is clamped by the diode. So the remaining energy in the deflector coils is returned to the supply, and the current in the deflector coils decreases more or less linearly. This is the first half of the scan. By the time this energy has run out, the transistor is conducting again.
You should be able to convince a 328 to cough up a 22khz PWM output pretty easily, failing that an ATtiny85 can get to at least 2MHz PWM output frequencies, they're built for high speed PWM after all. (I got a nice 1MHz signal out at 8MHz clock speed, double that to 16MHz and the output doubles as well.
If so, wouldn't it only be necessary to put a diode with say 2000-3000V reverse protection in series with the transistor going to the transformer?
No the diode only needs to conduct in the forward direction, it doesn't need to withstand a reverse EMF. It only needs a reverse rating greater then the drive voltage.
Yes I know its dangerous... I'm not an idiot... "Ohh pretty lightning, I wonder what it tastes like?"
Ok, I'm starting to understand. I had a look at a few schematics and it seems pretty obvious I need some other components other then a DC power supply and a transistor. I was under the impression that a non ballasted load was fine for FBT, as seen from many videos on youtube with the use of a MOT. I might start off with a 555 timer chip as they are relatively cheap and easy to work with. I will then couple that with an arduino for on the fly changes.
I also did a bit more research on PWM and it looks like transistors are fast acting where as the delay would be from the inductor coil which makes sense in that if you increase the frequency, it will just bring the peaks and troughs of the PWM signal closer together and ultimately make a flat line.
If a Jacobs ladder is desired, an oil burner transformer works nice. 10kV for the old magnetic type, and 15-20kV square wave ] for a new electronic one. Just go to the dump or an oil burner service company and get an old one.
for a new electronic one. Just go to the dump or an oil burner service company and get an old one.