BJT H-bridge Implementation

Posted on Feb 4, 2014

Because these transistors are a complementary pair, the circuit using them is very symmetrical. The upper right and upper left corners of the H-bridge are called `sources. ` This name originates from their function which is to be a source of current for the load (in our case the motor). Referring to the schematic on the right, you can see that there is a 10K resistor between the base

BJT H-bridge Implementation
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of the TIP107 and the positive terminal of the battery. This is a pull-up resistor that insures that the transistor is "off" (or not sourcing any current) when the switch is open. The 1K resistor that is connected in series with the switch is used to limit the current coming out of the base when the base is grounded by the switch. So in this case when the switch is closed, the emitter-base junction becomes forward biased and current flows "out" through the base. If the battery is 12V and the voltage drop of the Base-Emitter junction is. 7V, then the current flow in the base will be -11. 3mA (negative because it is flowing "out"). Given the Hfe specs for this transistor, this level of base current will completely turn "on" this transistor allowing the load to consume as much current as it needs. Conversely, the schematic on the left is called a "sink. " Its name is derived from this circuit`s function which is to provide a place for current to go once it has passed through the load. The lower left and lower right corners of the H-bridge are implemented as sink circuits. In the sink circuit the 10K resistor connects the base of the transistor to ground, which forces the transistor off when the switch is open. When the switch is closed, current is injected into the base through the now forward biased base-emitter junction. Again the amount of current is nearly identical to the source configuration so an equal amount of...

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