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).

switch off switch on

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?

EDIT, clarification:
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.


closed as unclear what you're asking by Sunnyskyguy EE75, Andy aka, laptop2d, RoyC, Sparky256 Jan 28 '18 at 23:13

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    \$\begingroup\$ Have you measured Q2's Vce in the supposed 'off' state? Are you certain of its pinout (I've seen some variations ...). \$\endgroup\$ – brhans Jan 25 '18 at 14:27
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    \$\begingroup\$ If the leakage of the 2N2222 is an issue then for sure it is broken or not connected as you think it is. Take a multimeter and measure all points to see if the voltages make any sense. This is an easy enough circuit to debug like that. \$\endgroup\$ – Bimpelrekkie Jan 25 '18 at 14:39
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    \$\begingroup\$ R4 is not required for those TIPs unless you need it smaller than ~8K \$\endgroup\$ – Trevor_G Jan 25 '18 at 14:43
  • \$\begingroup\$ @brhans Yes, 7.46V (with Vbe[Q2]=0): this, plus 2.91V on R2 totals 10.37V, making Vbe[Q1]=-1.63V, which apparently is enough to turn on Q1; the datasheet for Q1 specifices only Vbe_on_MAX as -2.5V.... Which maybe means Q2 is not fully saturated? \$\endgroup\$ – Jack Jan 25 '18 at 14:46
  • \$\begingroup\$ 2.92mA through R2 when Q2 base is at zero volts? That aint right. \$\endgroup\$ – Trevor_G Jan 25 '18 at 14:51

Now i feel utterly stupid... The guy at the counter gave me different transistors from what i requested, though he didn't bother to point that out: turns out i currently own the P2222A, not the 2N2222, which the datasheet clearly shows has a reversed pinout.

Thanks to @brhans for the precision in the comment and everyone else who helped me figure this out.

  • \$\begingroup\$ With R2=1k (10mA ) Q2 can be driven to 3Adc with Vce=2V .. i.e. R1= 3.3 Ohms 36W lightbulb. \$\endgroup\$ – Sunnyskyguy EE75 Jan 25 '18 at 15:10
  • \$\begingroup\$ Would be worth deleting the whole question as it's got no long-term value for other visitors and would be a distraction to those searching for answers. Thanks. \$\endgroup\$ – TonyM Jan 25 '18 at 15:12
  • \$\begingroup\$ @TonyM how do i do that? \$\endgroup\$ – Jack Jan 25 '18 at 15:22
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    \$\begingroup\$ @TonyM tried but can't, here's why, the community or the moderators will remove this. Thanks for the help :) \$\endgroup\$ – Jack Jan 25 '18 at 15:41
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    \$\begingroup\$ @TonyM I beg to differ [so might the 4x upvoters too]. This answer may be useful for somebody who is struggling with a P2222A without realizing that it has got an unexpected pinout. \$\endgroup\$ – Nick Alexeev Jan 26 '18 at 2:57

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