Exciting a resonant tank with pulsed DC - help

[gyulasun]

Hi Dann,

Sorry for my rusty brain, it did not 'click' for the negative resistance behaviour possibility with the inverse supply polarity.
So I agree, that with this test the 7 transistors cannot be all bad, nevertheless you may wish to test the transistors
(unless you already did so) with a DMM as suggested here
https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07
And there remains the question what is the explanation for only the 30 mV difference
between the on and off states you found between the base-emitter of the transistor with the scope?
Now there is another question with the IGBT: what if its collector-emitter behaves similarly for the inverse polarity voltage?
Afterall, the output side of it is that of a bipolar transistor....

I would suggest a combination of two N channel MOSFETs for switching if you have two power MOSFETs with 200 -300V drain source breakdown voltage ratings for each. Called also an AC switch because AC current can also be switched with it.
The drawback is the need for using isolated gate-source drive circuit because there can be no common in and out points
directly connected. The source pins of the two MOSFETs are connected together and their gates too so you can get a switch between the
two drains and current can flow through them in both directions when the MOSFETs are on. And when they are off, then
no current can flow through any of their body diodes because they are mutually in blocking direction with respect to each other.

I attached such circuit which uses a 9V alkaline battery for feeding the control bias voltage via a reed switch to the common
gates and sources. This was used to short circuit a coil in a pulse motor.
With your opto coupler and the 9V alkaline you can build a driver circuit which is ground independent from the output drains
side where your coil will be with the 12V supply voltage. There are isolated driver ICs too.

[madddann]

Hi Forest!

Sorry for not posting the picture, I thought I described the circuit well enough... will post it tomorrow, THX!


Dann

[madddann]

Hi Gyula!

Wonderfull, this really looks like it should work. So I just have to put in the opto instead of the reed switch, the correct resistors and the 9v battery? I have some IRF740 mosfets... I'll put it together tomorrow and see what it does.


Dann

[gyulasun]

Yes, the opto output can directly replace the reed switch or the opto output can be connected across the gates and sources to discharge the gate - source capacitance whenever the opto is on so the MOSFETs can switch off. The series resistor R1 (1-2-4.7 kOhm or so) remains of course to limit current via the opto output from the 9V alkaline. Of course a wall plug-in DC supply can also be used with up to 15-18V DC output maximum, it is ground independent from your 12V supply for most types.

Gyula

[madddann]

Hi Gyula!

I saw jour post just now, but I already put together the circuit - it is exactly like you have drawn it. I used a 9V battery and a 4.7k resistor (with smaller resistance the opto was unable to sink the current fully). I first tried to switch a 10W 12V bulb on and off with the circuit. It worked, then I reversed the polarity on the 12v battery and it also worked. Then I connected it to the tank circuit and the voltage and current oscillations look good. Awesome! I did not test it for an extended time, I only hope it keeps working.

The next step is to make another channel with another identical LC tank with the second channel from arduino phase shifted by 180°.
I'll post again in the evening, now I have to do something else...

Thank you very much for all the help, it is much appreciated!


Dann