In previous post, I showed a schematic able to switch ON/OFF SCR. You may wonder how that is possible. Well here is the explanation.
The SCR I used is C106D, which is a sensitive gate thyristor. Those ones have ability to increase their holding current when their gate is negative biased like this:
SCR holding current increases reciprocal to the resistance – smaller resistor increases holding current more than a bigger resistor. In proposed schema
There is no such resistor as discreet element, but still there is a tiny resistance of the transistor’s collector-emitter junction. Because this resistance is very low, we increase holding current quite significantly. This on other side means, that there will be no enough holding current to keep SCR open when we remove triggering signal from the gate.
We have to be very careful not to burn the transistor, as we will expose it (even this will be for very short period) to current that could be higher than one it can sustain. We can use small resistor attached between the SCR gate and transistor’s collector, to eliminate this danger, but we should not add very high value resistor, because this will lower the holding current and we may lose the switching OFF ability.
Now when we send 1 (true) to transistor switch, it will open and thus cause a reverse current to flow from cathode (at that moment it will be more positive than the gate), through the gate, then sinking to the ground. As the transistor’s resistance is so small, it will make SCR holding current to rise significantly. When the current flowing through the load is below this holding current, the result is that SCR switches OFF.
For now, I will leave you to try and figure out the calculations. I am writing down all that in much more detail including the calculations and will publish it very soon inside next version of my Netduino tutorial (code files) at the forum and wiki.
Last thing to mention is that I successfully tested the schematic with different sensitive gate SCR – MCR22-6.