mains slayer exciter HELP!!!

[Jun]

Hi everyone.
I constructed the mains powered slayer exciter from sketchy Russian schematic, but there is problem with oscillating.
I am using 7 turns primary, 600 turns secondary, ballast which passes 2A at 230V, IRFP460 and two 12V zener diodes to clamp the gate voltage below 12V. I am also using potentiometer instead of R1 to adjust for different input voltages.
However, when I probe the gate-source voltage of the mosfet, the gate voltage is clamped below 5V which is just enough to turn on mosfet and heats up, but cannot start the oscillation.
I would like to know why the gate voltage is below 5V, and why it cannot start up the oscillation.
I would also like to know the purpose of the 1uF capacitor. Is it a resonant cap or does it help to start the oscillation.
Thank you in advance and any help is appreciated.

[Da_Stier]

Hi,

interesting circuit, amazingly simple for sure.
Can you show your exact circuit with the parts you used?

I guess you have a zener diode in each direction to GND?
This would explain why the gate voltage can't go up, since a Zener diode is just a normal diode in reverse.


Greetings,
Michael

[davekni]

Have you measured your zener diodes to verify they are 12V and not 5V?  (If you do get it to work, the original part is a 12V bidirectional TVS diode capable of 5W dissipation.  Typical small zener diodes may fry in this use.)

The only other possibility that comes to mind is a scoping artifact.  If that scope has a mode similar to Tek's "Hi-Res" mode, it may be filtering out all the oscillation at 5ms/div.  (That mode averages all the 1GHz samples within each longer sample interval for slower sweeps.  Very useful for precise low-frequency measurements, as it hides high-frequency signals and noise.)

[Jun]

The spec is:
50 ohm 230mH inductor as ballast
10A fuse
MUR460 diode
1uF 275V santan polypropylene cap
10k resistor
12V zener
1k potentiometer
IRFP460
7 turn primary (wire from inverter microwave transformer)
600 turn secondary 26awg on 3 inch PVC
aluminium ducted topload
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I originally had tested 16V zener diodes but they also experienced the exact same problem.
It is not resonating at all as I would be able to the output in the secondary if it did. The mosfet is just conducting 50hz current and heats up little bit.

[davekni]

I suggest disconnecting L1 so that no power is drawn.  Then scope the gate waveform with the 1k POT at maximum (1k).  Presuming you still get 5V max, next disconnect the IRFP460 gate.  If the voltage rises, then the IRFP460 gate has been damaged, likely due to ESD before assembly or some other reason for excess voltage.  If voltage is still 5V with gate disconnected, next disconnect zener diodes.  At this point it is only a resistor divider, 10k to 1k, so must go to 1/11th of line voltage.  If not, then there is an issue with your scope or ...  If voltage does rise, then the zener diodes are bad.

[Jun]

I think I figured out the problem. After I dismantle everything and measured the fres using normal slayer exciter, the resonant frequency was around 2MHz which might be too high for stable oscillation. I can try to reduce fres or make HFSSTC instead.

[davekni]

2MHz is on the high side, but might be possible with that circuit.  That still doesn't explain why the gate voltage limited at 5V, unless there was some 2MHz oscillation that didn't show up on the scope.

[Magneticitist]

I read some comments the other day on Teslista555's new videos that brought to mind something I hadn't actually thought of before. One person was saying their gates seemed always clamped to 5v because of the zener (12v zeners) capacitance at around 1mhz. This was a shared sentiment by Teslista555 as to why he went without them and chose to protect the gate with a small filter.