SCR triggering and ground loop issues

Question

I am trying to trigger an scr/thyristor from a 0-5V TTL pulse (channel 1 on the oscilloscope output). The schematic can be viewed below. I am using the TTL to pull the 2n2222 transistor 'on' which than triggers the p-chan. mosfet allowing current to flow to the gate of the scr. Both the mosfet and transistor are biased by a floating 5 volts power supply from the 'wall'. To test that the scr is open, I have added another floating 16 volt power supply with resistor to measure a voltage drop across. The problem is that when I have the two grounds of the power supply's isolated I get ringing and wild oscillations (seen in figure 1). When the two grounds are reference to each other (seen in figure 2) the output of the scr than goes from low to high, and I would expect the voltage to go high to low.

Note that the channels 1,2,3 correspond to the measurement points 1,2,3 in the schematic and ground#1,2 refer to the different grounds of the power supplys. Any advice would be awesome thanks!

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

With separate grounds, there is no guarantee that a pulse generated with respect to the first ground will have any effect on a circuit referenced to the other. There are at least two ways to handle this. The most obvious is to use an optocoupler to transfer the pulse to the SCR. The other, older, technique is to use a pulse transformer.

^{simulate this circuit – Schematic created using CircuitLab}

You will need to research optocouplers, since some are too slow for your application. Also, if your grounds really get to swinging wildly, a given optocoupler has a certain rated isolation voltage which you need to respect. Given that you have not destroyed your scope, your grounds are probably not that far apart, but it's something to think about.

As a side note, are you sure you need a 33 ohm resistor to the gate of your MOSFET? It pulls 150 mA when the 2N2222 is on, and that's a good deal of current. It's entirely possible that you do it to pull up the gate quickly during turn-off, but it still seems excessive.