H-bridge problem with TIP122.

Question

I tried to make H-bridge by TIP 122 transistor. I used 9V for a VCC. I used push button to control H-bridge, and it's fine. Then I used Arduino 2560 with two digital pins for driving H-bridge. But the output voltage was only 5V. And also, I tried with VCC= 15V, output still be 5V.

Could you please give me an advice? enter image description here

Read this answer and this question. There I present a link to a better BJT bridge that is able to handle 5A (with proper heatsinks) and does not allow the user to short the power supply (like Wouter implied). — Ricardo, Feb 27 '15 at 13:37

score 4 · Answer 1 · answered Feb 27 '15 at 13:24

4

The upper TIP122's are wired as emitter-followers, so their output voltage can never be higher than their input (base) voltage. (In fact it will be ~ 2 x 0.6V lower.)

When you fed the inputs with the full Vcc that was not much of a problem, but when you feed it with the 5V Arduino outputs you will get no more than 5V from the outputs.

One solution is to put a level shifter between the Arduino outputs and the H-bridge. But it will still be a crappy H-bridge. (Do you realize what will happen when you make BOTH inputs high?)

answered Feb 27 '15 at 13:24

Wouter van Ooijen

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I haven't tried with both HIGH inputs. Why output voltage cannot be higher than input voltage? – Than Feb 27 '15 at 15:43
a) don't, just try to reason what will happen. b) google and read "emitter follower" – Wouter van Ooijen Feb 27 '15 at 15:49

H-bridge problem with TIP122.

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