H bridge and MCU

Hello,
I'm building my first robot and i made the H bridge for controlling my motors, and connected the H bridge directly to the MCU and made the right code to drive the motor
the problem is the wheels works fine in the air but when i put them on the ground they stop working that when i connect the H bridge to the MCU
but when I connect the H bridge to the power supply directly The motors run very fast on the ground and the robot works fine
So do I have a problem in connecting the H bridge directly to the MCU
this is my H bridge circuit
**broken link removed**

please can anyone help?

Likely your transistors aren't being turned fully on by the MCU pins due to current requirements for the transistors vs your base resistor; in order to figure it out, you would need to know how much current a pin on your MCU can source, and what transistors you are using (so you can reference the base current requirements). Knowing this information, you can figure out what your base resistor should actually be - I have a feeling that 1K is too large. You could try to reduce it to something smaller (say 470 ohm - don't go below 220 ohm), but it is a tricky thing to do which, without knowing the above information, you can blow the pin on the MCU. There are a ton of references on the internet about how to calculate the proper size of the base resistor, so you might check into those (there's also a ton of info on how to design transistor h-bridges, as well). Finally, I hope this is just for a learning experience; ultimately it is better to purchase an H-bridge IC for motor control (or in the case of large motors, an entire motor controller) than attempt to build it yourself; you'll save time, money, and frustration in the long run.

actually now i discovered that the output of the H bridge when connecting to the motor is very low about 3 v
i don't know what is the problem
on the other hand the output voltage with no load is 6 v

Those values may be a little high for your resistors.

You also have to remember that the transistors themselves have some voltage drop. If you're starting with 6 volts, 3 volts to the motor would be a normal drop. I would up your voltage, so it's higher at the motor.

I'm confused by your comment "when I connect the H bridge to the power supply directly The motors run very fast on the ground and the robot works fine" The top of the H Bridge should go to the power supply, the bottom to ground. Is that how it is??

shico90 said:

actually now i discovered that the output of the H bridge when connecting to the motor is very low about 3 v
i don't know what is the problem
on the other hand the output voltage with no load is 6 v

Click to expand...

EDIT: Could be a problem with the hbridge. On the transistor hbridge I made a NPN transistor drove the base of the PNP. You can not properly turn on the PNP with a logic level (5V) signal. I think it is because the voltage to the base of a PNP has to be greater then (or maybe just equal to) the voltage on the emitter collector. The analog guys can sort this out if needed.

Often, if the voltage drops under load the power supply can not provide enough current. What is the power supply ?

Also you should not expect to see 6V on the output of the hbridge because each transistor has a voltage drop. You should see 2 transistor drops at the hbridge output. As suggested earlier up the input voltage.