Electronics > Projects, Designs, and Technical Stuff
Jet engine Ignition system Build with LT375 / TL494 / ZVS driver Lockup
Amper:
Hi,
i have another one of my stupid little problems.
At im building the ignition system for my jet engine and basically everything is fine and dandy.
The high voltage part is a 1uF 2kV foil cap thats discharges through a GDT into the spark plug, the cap is charges from an old ccfl inverter transformer via a normal rectifier which works great. The power required is roundabout 25-50W @ 24V input. For this reason and the small form factor i choose to use the old zvs circuit exactly in the way shown in the picture with a few variation on values and parts types.
This essentially works great from the current limited power supply but occasionally the oscillator locks up especially at startup or after some ignitions because they are violent enough to disrupt everything.
Reducing the current to allow steady operation in this moment i could measure that both gates of the fets are stuck to 2V (roughly threshold voltage) and the supply voltage drops over the fets coming from the transformer.
I tried adding asymmetry to the system to stop it from having stable states but this only made things worse. Transformer impedance and resonant frequency also seem to have some effect on the likelihood of a lock.
My Problem is that the two ignition boxes will later be supplied directly by 24V by a self resetting fuse but they will not be monitored. This means when the engine is in startup and the ignition should fail the ECU will keep doing its thing and inject kerosene which will lead to a hell of a mess and a large cloud of fuel mist in the air. Its wont hurt anyone but at 2l/min rate of fuel i dont want this to be a regular thing even in testing.
Did someone see the same problem in the past? Is there any known reason and or solution to it?
An obvious one would be to just go for a tl494 but space is limited and im not sure if i can cram all of it into the tiny box i have available let alone shield the driver from the intense EMI of the ignition just cm away.
cheers!
https://youtu.be/lPanMl8RUJk
https://youtu.be/3oVrPL6lryI
fourtytwo42:
To me it looks too symmetrical! In other words something needs to bias it so that one fet is more likely to turn on at start.
Amper:
wtf is wrong with this forum...
--- Quote ---I tried adding asymmetry to the system to stop it from having stable states but this only made things worse. Transformer impedance and resonant frequency also seem to have some effect on the likelihood of a lock.[/qoutoe]
--- End quote ---
alsetalokin4017:
--- Quote from: fourtytwo42 on January 28, 2019, 05:11:36 pm ---To me it looks too symmetrical! In other words something needs to bias it so that one fet is more likely to turn on at start.
--- End quote ---
I have a lot of experience with that particular circuit. I've even got multiple copies of that same schematic, along with some variations. It can be used for wireless power transmission, flyback transformer driving, induction heating, etc. It's a Royer oscillator, aka Mazilli, and a lot of the "alternative energy" aficionados like it.
I think the answer is just the opposite: the circuit, especially the power section, has to be as symmetrical as possible so that one phase doesn't want to dominate and prevent oscillation by turning on hard and blowing up its mosfet or both of them.
When I first started playing with this circuit I would apply power slowly by ramping up the input voltage. This was fatal: oscillations often wouldn't start and a mosfet would quickly blow. I found that power has to be applied abruptly, by switching on hard, and the power supply has to not sag much when the connection is made.
The "must be good" capacitor in the schematic sets the resonant frequency by forming the LC circuit with the transformer primary. This capacitor will get hot, it must be a good quality poly film capacitor and in my own experience a stack of parallel caps to build up to the correct capacitance works better than a single large capacitor. In fact this capacitor is critical.
You must also realize that there will be very high currents in the LC part of this circuit. Lots of reactive power.
coppercone2:
--- Quote from: Amper on January 28, 2019, 05:39:51 pm ---wtf is wrong with this forum...
--- Quote ---I tried adding asymmetry to the system to stop it from having stable states but this only made things worse. Transformer impedance and resonant frequency also seem to have some effect on the likelihood of a lock.[/qoutoe]
--- End quote ---
This made me laugh.
--- End quote ---
Navigation
[0] Message Index
[#] Next page
Go to full version