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Old 04-06-2011, 08:54 PM
waltersm waltersm is offline
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Join Date: Mar 2010
Posts: 2
MOSFET Switched Circuit

Hi My experience is from circuits very similar but not setting out to heat the load! My circuit is very similar but there is an inductor (intended) in place of the resistor in Rosemary's circuit. The MOSFET various types from 600v to 1500v internally limited devices. My driving circuit has a PIC and a MOSFET
gate driver. Does the same job.

In one case (1) I use a pulse width modulator PWM controlled by a HEX switch setting the mark period. Tests on the related circuit is usually limited to 50% M/S maximum. PWM is @1.6KHz.

In the other case (2) a square wave is frequency set to that desired by programming the PIC. Currently using 10KHz.

Both have been designed after making several Adam/Bedini type motors.

The main observation was initially concerning the maximising of the BEMF amplitude collected in a capacitor (many values). From the driving coils initially the after realising no moving parts were needed - the switched coil BEMF was harvested.
I soon realised the value of the BEMF was being limited (as seen by a flat top on the pulses seen at MOSFET switch off - when viewed on an Oscilloscope. In short the value reached was mainly limited by the internal MOSFET diode.
The highest value I have purchased at an affordable price is 1500V. Using this I charge a suitable capacitor to 2000V.

Back to the similarity to the Rosemary Circuit - I have experienced the inductor used to generate the BEMF getting very hot where it should not.
The lower the inductance used the greater the heating effect is. I have resorted to using a large ferrite bead as a former as this allows for heat sinking by mounting it on the box/case surface. I can also reduce the heat by
increasing the inductance but the power of the unit is reduced. This was observed by monitoring the amplitude of the current pulses seen when recharging a car battery. A small value (1ohm) was put in series and scoped the voltage I=E/R. I was seeing greater than 10A pulses at either 1.6 or 10KHz rate depending on the drive used.

Currently not tried to maximise the heat as yet - hence the interest in your tests.
This might not be the thread to outline my observations but I am finding a common link in many of the reported experiments that are aiming at different ends!!
If this is in the wrong place please let me know as more observations and feedback are of great interest.
As a ex large company Electronics Engineer I miss the discussions of old.

Mike

Ham call G3JVL
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