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Topic: Transistor H-bridge (Read 2925 times) previous topic - next topic


I made a Transistor H-bridge that looks like this  (with different components  of course ). First I had a large tip42 transistor plugged into my arduino with a 150 ohm resistor and connected to a 12 volt source. The motor spun fine there was no heat from anything. Now I set up my H-bridge and all the transistors get really hot. I have not changed anything except for adding
the H-bridge. Do H-bridges naturally cause heat? Or am I doing something wrong?


What are you doing with A & B?
Q3/Q4 will turn on/off okay with logic level signals.
Q1/Q2 need 12V on the Base to turn off tho, if they have 12V on the emitter. They are likely partially on all the time with a 5V high onth gate, and then full on when low.
You need another NPN in front of them with a pullup from the Q1/Q2 bases to pullup to 12V when the added NPN is not on.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Some observations:

- Bipolar transistors don't make very good h-bridges, they switch off too slowly and have too much voltage drop. These are both reasons why they get hot.
- If you run that circuit from 5v, you will only get 2v to 3v at the motor, because of voltage drop in the transistors. You can't run it from more than 5v without increasing the voltage used to drive the transistors too.
- You need to add commutation diodes across the transistors to absorb the back emf of the motor.
- You should be driving the transistors much harder, 4mA is not really enough base current. Reducing the 1K resistors to 220 ohms may help a little.
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