I'm building an H-bridge with complementary Darlingtons, TIP122 and TIP127, as they are cheap and the nearby shop has them (i know about the high power dissipation, it doesn't matter), and i found trouble with the high side driver.
I tried to use a 2N2222 as the driving circuit to turn on and off the TIP127, and as far as EWB tells me, my idea should work, at least in theory...
I am building this sample circuit to test the current absorbed by the 2N2222 to see if it's within range of the microcontroller's GPIO i have available (the STM32F205).
In the real circuit, if i use the depicted resistor values, i can't seem to turn off the darlington, and i suspect the issue to be the 2N2222 leakage current in its off state, but i'm not sure.
I swapped the R4 pull-up resistor and the R2 resistor with 5k trimmers, and i managed to find a spot where the TIP127 does actually turn off; but then when switching again S1, it doesn't turn on anymore :|
What did i forget to take into account?
The circuit i built was behaving differently from what i predicted, so i thought there was some design mistake due to inexperience. My question was if an erroneous calculation of the resistor values (or some other error) in a circuit as the one shown, could lead to such an unforseen behaviour.
In the end, i was working with the wrong transistor (or the wrong datasheet, should you switch point of view), hence the question itself is rather pointless... Took me someone from the other side of the globe to make me take a closer look, so, my bad.