D14 Circuit Freq. Range?

[56K]

Hi All, first post here. I have the D14 updated circuit, but do not know the freq. range it can obtain. I am looking for 20K - 40K something range. It seems Stan Meyers used some pretty high freq. to obtain good results. If this circuit is to low can it be modified and how. Thanks for Help in advance.

[SeaMonkey]

For a very good explanation of how the 555 timer astable multivibrator works and how to tailor the frequency and duty cycle output to suit your needs, go here:

http://www.kpsec.freeuk.com/555timer.htm

[56K]

Thanks for reply. Do you happen to know the specs on the D14 circuit? I have yet to wind the torroidal transformer but I have the updated d14 as well as a hefty blocking/switching diode for the vic part. thanks again.

[SeaMonkey]

The timing component values for the High Frequency side of the circuit should enable tuning over a broad range in the tens of kilohertz range - including above the 42 KHz that you're looking for.

The 1N4007 diode is rather slow acting at those high frequencies, so it would be better to use a Fast Recovery Diode with less than 100 nanoseconds recovery time, in its place.

As Patrick points out, the Zener Diode array across the Power Mosfet isn't necessary unless you're doing a lot of tinkering. The Fast Recovery Diode mentioned above will offer protection as well.

Any Power MOSFET rated at 100 volts or more capable of 20 or more amperes of Drain Current will do nicely.

The Gate Drive resistive connection from Pin 3 of the 555 to the MOSFET Gate isn't the best. It would be better to eliminate the resistors and connect a small signal diode (1N914, 1N4148, etc.) from Pin 3 to the MOSFET Gate (Anode at Pin 3, Cathode at MOSFET Gate) with a small PNP Transistor (2N2907 or equiv.) added: Base to Pin 3 side of the Signal Diode, Emitter to Gate side of the Signal Diode, and Collector to Common where the Source of the MOSFET is connected. This will provide sufficient Drive to the MOSFET Gate to turn it on fully, and will accelerate the MOSFET turnoff time, sharpening the pulses up dramatically. It would also be good to add a 10 Kilohm resistor from the MOSFET Gate to Common to drain off any residual Gate charge when powered off.

The absolute best way to "tune" the pulse width is with an oscilloscope so that you can visually see the ON and OFF times of the pulses and avoid stressing the MOSFET. Trying to tune the pulse width without a scope will result in more burned out MOSFETs than otherwise, unless you keep your finger on the MOSFET tab and shut the circuit down immediately when it begins to run hot.

Best wishes on your experiments - you will succeed if you persevere.

[chemelec]

[56K]

[56K]

[chemelec]

[56K]