Crossing the H-Bridge.

1: What is H bridge?
2:The H bridge is the board that drives the motors. Tis something that actually drives the motors once you’ve programmed the microcontroller to do so.

1: does it do so by providing 12 V?
2: Hmmm.. nope. That 12 V that is needed for running the motor is given by the battery. You see, this is motor driver IC!

1: Okay, then it must have to do something with providing the main board with 5V. I see the jumper wires are connected to the main board!
2: Interesting observation! But, this work is done by the Regulator 7805. The last digit 5 in 7805 tells us what is the regulated output that you get. So, the next time you hear a Regulator IC, you know the output voltage of it just by hearing the name of it.

1: Cool! So why isn’t the power supply given directly to the Main Board?
2: You know, it becomes cumbersome to have 2 power supplies, 1 for the board and 1 for the motors. You just keep things simpler by using one power supply for both!

1: Yeah, that makes sense. But i think it has to do something with the IC on the H bridge. L293D it is. But i don’t see a point in it being there as the motor can already be running on the 12 V supply, plus the direction control and stopping and starting a motor can be done by the program itself. So, why on earth do we need this L293D?
2: Temme one thing,how would you make the Robot go front, then stop and then go back again?

1: Do you want me to write a program for this one?
2: … yeah. Why not?

1: Really??? Should i?
2: Never-mind, just tell me the algorithm.

1: So, you make one of the pins HIGH, then other one LOW, it goes front. then make both LOW LOW, it stops. Then LOW HIGH, it goes back.
2: Oh Right! So you don’t need any motor driver IC. I thought it will be used to change directions and all.


Next Day…
2: So you said , you can change the motor direction using the program. But there was something missing that struck me as i was thinking about the motors. You see, the main board’s output is only 5V, whereas you need 12V to run the motor. Also, you need to match the current ratings as well.
1: Oh yeah! I completely missed out on that.

2: By the way,did you know that ‘D’ L293D stands for the presence of a diode for each output.
1: Oh! what does a Diode have to do? Anyway, the required current and voltage will be sent by the IC, as you said.

2: When the change of direction of current happens in a motor, it opposes this change. This results in a back current. You don’t want the IC to starting smoking and all, so you use the IC with a diode in it.
1: IC…I see.  But why the name H- Bridge?

2: You see, this thing acts like bridge for the motor, which would otherwise fall into the abyss of doing-nothing-terrifically-useful-ness.

1: How does it act like a bridge actually?


2: See, suppose there are two channels of water flowing parallel to each other. And you bridge the gap between them.  Now the water can flow in any direction in this newly formed channel. But the direction now depends on which side is on higher level and which is on the lower level.

1:Ok… but where  does the motor supposed to be connected in this scenario?


2: Motor terminals are nothing but the bridge connections.   And there are transistors which will act as switches so that the current flow direction is controlled.

1: Something like a Full wave rectifier circuit?


2: Almost, but here transistors are used in ON/OFF mode. 4 transistors would be used for 1 motor.  And 2 of them will be on the opposite side of the bridge so that the current is flowing in one of the direction. And when the polarity changes, the other two transistors switch on. And, that the motor starts moving in the opposite direction.

1: Okay.. Now I know!

This is how a H- Bridge looks like!


P.S. : The location of the H-Bridge is here.




One Comment

  1. Aravind Krishnan
    Jul 21, 2011

    Really nice narration to explain the Technical details in a fun way!

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