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Old 04-10-2011, 06:07 PM
waltersm waltersm is offline
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Join Date: Mar 2010
Posts: 2
MOSFET Switching Circuit

Hi Harvey Thanks for the comments. I am not sure my inductor is behaving as just a resistor. Firstly it is constructed on a ferrite bead about 18mm outside dia and uses 100 strand Litz wire - measures 0.193mH and 0.5ohms.
My test meter checks at 20kHz so not too far from my current 10kHz op Freq.
With just this as the MOSFET's drain to supply load I see BEMF pulses of 1500V on my scope (good job the probes from China are cheap!).

In order to collect this BEMF I am using a separate coil and capacitor series tuned by adjustment to 10kHz. (about 5mH),t with a moveable Ferrite core. This is connected from Source to Drain via 0.07uF 2kV wkg cap. & from the mid point there are two 1kV diodes in series to a 1uF 3kV oil/paper cap. The DC measured is 2kV across this capacitor. This charges in very quick time 1-3 seconds hard to determine. I have at one time earlier had either 25uF @ 4kV or a string of 2000uF 450v El Caps to similar voltage. But a bit lower with the Electrolytics due to their leakage. As I am using my AVO meter on the 2.5kV range, and the needle does not move too rapidly any time. I am also seeing (at the Centre of the Tuned Circuit) approx 200v sine wave which I can maximise. The input current from the (12v nom) PSU minimises and the voltage measures 15v (about 16v no load). Input current checked at between 150 & 250mA varying with tuning.
I will report further after some comments please.
Yes very familiar with Antenna's (Google G3JVL)

Again many thanks for your references very interesting.

Best regards

Last edited by waltersm; 04-18-2011 at 07:31 PM.
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