DC Motors and H-Bridge

DC Motors and H-Bridge

The DC motors we ordered are rated as 6 Volt motors with a stall current of 5 Amps.These motors were chosen because they provide a stall torque of 376 ounce-inches and an maximum RPM of 294.

We found that the motors draw approximately 3.8 Amps of stall current with 5 Volts passing through them, giving them an internal resistance of about 1.3 Ohms. The current drawn by these motors are too much for the H-bridges provided, so we had to come up with a H-bridge design.

We found an H-Bridge design on the following website:
 http://www.talkingelectronics.com/projects/H-Bridge/H-Bridge-1.html

This model only supplied 3 Amps and we did not have the correct components to design this, so instead, we used the design structure to implement a new design using our own components. We could not get the top design working with the NTE2328's and NTE2329's, so instead we implemented the following design instead using the TIP 122's (NPN)and the NTE2328 (PNP):

This design powered the motor without a problem, but the issue with this new design was that in order to power the motors off, the input transistors essentially connected the base of the PNP's to ground, but the 10 Ohm resistor connected 5Volts to Ground. This made the design too unreliable, since the resistor was too hot. Experimentally, changing the resistor value to a higher value would make the motors slow down because it caused less voltage across the motor. To fix this, we changed out the PNP transistors with NTE2325's and used the same design as the one found from the website. 

Unfortunately, these transistors have a high Vce drop, so the motors were slow. The large voltage drop may have been due to the resistor values chosen, especially since they were not changed. We were going to calculate appropriate values, but luckily an  older H-bridge model was found and provided by Gabe.