H-bridge over 6v dc engine

[tonyeson]

Hello, this is my first post here, and also one of my first project involving building some simple circuit's so plz forgive me if i'm sounding stupid, or are missing something obvious (bco in this area I probably am).

So I need to be able to control the way a motor is running, to be able to do this I build a H-bridge out of 4 npn transistors (BC337-40), some resistors and two switches, circuit in the image (sry for the image quality, my paint skills are limited)

The problems that I'm having is that the engine don't seem run at it's full strength and the transistors are getting very hot.
I have been doing some experimenting and had some "strange behaviors". at first i chose every resistor to 10k, this didn't work and the engine did never even run, then I changed them to 5k resistors, now the engine started to run, but it where rly weak, so i changed the resistors to ~ 1k, here the engine where running better, but still not as good as I had hoped. at this point the transistors also started to get hotter. I now changed the resistors to ~800R for R4 and R2 and ~ 450R for R1 and R3. at this setting the engine is running far better than before (but still not that good), but now the transistors where getting rly rly hot (R4 and R2 are getting hottest). I made one last test where i completely removed R1 and R3, this resulted in the engine running strongly for about 10 sec, then T2 stopped working, I fried it.

now to my questions:
Why is the engine running better whit less resistance to the base of the transistors?
why is the transistors getting burning hot at low resistance settings?
what resistors values would be best to use in this scenario? (i do not know the resistance of the engine)
is my H-bridge badly designed? if so why?

 

[Audioguru]

The maximum allowed current for a BC337 is only 500mA. How much current does your "engine" use?
For a collector current of 500mA then the base current must be 50mA then the voltage lost across the bottom transistors is 0.7V so they each heat with about 0.4W which is OK but near the maximum allowed of 0.625W.

50mA of base current is provided by a resistor for the bottom transistors that is (4.5V - 1V)/50mA= 70 ohms so your calculations were wrong and the transistors were heaters instead of switches.

The upper transistors are emitter-followers so they do not need base resistors but they have a voltage loss of about 1V. Then the engine gets only 4.5V - 0.7V - 1V= 2.8V when the battery is brand new and the current is only 500mA.

You should use N-channel Mosfets for the bottom and P-channel Mosfets for the top for a very low voltage loss and the Mosfets must be "logic level". The Mosfets will barely get warm.

 

[hevans1944]

You can purchase a commercial integrated circuit with H-bridges inside and all the details of how to keep them from "frying" are described in this example datasheet.

Is there some compelling reason, other than self-education, why you are trying to build rather than buy this particular wheel?

 

[tonyeson]

thank you Audiguru, you answer helped me out a lot! the engine is now running with good power and everything is finally working as expected!.

My reason for building my own H-bridge is mostly that I like to learn, but also that I happened to already posses the components needed for the bridge