Exciting a resonant tank with pulsed DC - help

[madddann]

Hello!

I have a circuit that works in simulation, but not on the bench - at least not as expected.
My goal Is to make it work right, hopefully with your help.

This is the circuit simulated (with the desired voltage and current waveforms): https://postimg.cc/image/mpvl3w7md/

On my bench, the signal generator is an arduino nano completely isolated from the main circuit (powered by it's own battery) and is set to 25% ON duty cycle, the optocoupler is an LTV817 driving the transistor E13007-2. The coil is the size of a small contactor coil (36 Ohms) on  i'ts laminated core about 80mm x 12mm x 9mm. The capacitor is 220 nF poly. For powering the transistor + tank circuit i use a 12V automotive starter battery.
This circuit (on my bench) generates the waveforms that look more like this: https://postimg.cc/image/lau0fawad/

After the initial 25% ON time pulse, the voltage drops to negative (the cap fills up with the backspike from the coil) but when the voltage climbs back up to about +21V, the waveform is cut flat for some time and then small oscillations continue until the ON pulse again.
Where the waveform is cut, it looks like something is coducting the current - maybe the transistor is turning on again for a brief time, but how is this possible what is causung this? ... negative resistance of the transistor?

Thx to anyone willing to help!

Dann

[gyulasun]

Hi Dann,

Have you checked the switching waveform with the scope whether it resembles the waveform coming from the signal generator?
I mean how well the opto coupler transfers the rectangular pulse to the base-emitter for correct switching.
It would help to see the bench scope shot on the base and the collector too, versus the common negative ground (if you have a 2 channel scope).  Maybe you can make a photo from the display (when upload, maintain pixel size not higher than 900 horizontally, otherwise we have scroll back and forth to read the text lines) 

I see you use a 1 kOhm resistor between the base and emitter in your 2nd schema: does it make a difference?  (it may help in a quicker switch-off of the transistor)  Maybe you could change its value between 470 to 1000 Ohm?

Gyula

[madddann]

HI Gyula!

Thanks for posting and reminding me of what I should have done in the first place.

At first I was using a N-channel mosfet with the second scheme from previous post (with the resistor), but I thought that the mosfet's body diode was causing the cut-off wave (the simulations that I did were indicating that), this is why i switched to NPN transistor E13007-2. Now I'm not sure anymore what is causing what...
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The resistor in the circuit does make a difference, but I think the optocoupler may not be capable to deliver enough current to switch the npn transistor as it should be switched. Perhaps an alternate way of driving the transistor should be implemented - am I on the right track here?

Here are the scopeshots that I did today: without resistor, with 1k resistor and with 470 Ohm resistor, all with voltage + coil current at the LC tank and collector emitter (VCE) + base emitter (VBE) voltages at the npn transistor (E13007-2).
See scopeshots in this gallery - pictures from 1 to 7: https://postimg.cc/gallery/1mu9z7pi/
Read the text below each picture when you open it, to see what is what.
Picture 1.2 is just a detailed view of  picture 1.1 (the top and bottom of the waveforms in 1.2 are cut off)

Anyone that knows what is actually happening here please post your thoughts.
The goal is to keep the oscillations at max, so anyone that may know how to do that is welcome to present their circuit here.

THX!

Dann

[gyulasun]

Hi Dann,

Thanks for the scope shots. Yes, the 1 kOhm resistor is a bit better instead of a 470 Ohm between the base-emitter from the base-emitter driving signal point of view but maybe the overal driving is still not ok.

If you agree, please do the following: connect the collector of the opto output directly to the base of the transistor where the upper 1 kOhm resistor is also connected, this latter is coming from the +12V directly as before.  And connect the emitter output of the opto to the emitter of the switching transistor i.e. to the negative rail. You may wish to check operation with or without another 1 kOhm connected also across the base-emitter. 
This way the opto output will quasi short circuit the base-emitter of the transistor whenever the opto input receives an input pulse from the Arduino.  In this new opto drive connection the opto will invert the input pulse as shown in the bottom right Figure of this data sheet, page 5: http://www.mouser.com/ds/2/239/LTV-817-827-847-647.pdf  but I do not think this control signal phase inversion matters at all in this setup.  The R_L resistor shown there will be your upper 1 kOhm, ok?

By connecting the opto like that, the control pulse between the base-emitter of the transistor would resemble the most to the signal coming from the Arduino if that is a decent rectangular pulse. 
If the control pulse is okay, then we can continue with the collector side.

Gyula

[forest]

Can you post a picture of your physical circuit ?