High Current Tri-State MOSFET Driver Circuit
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This will serve as an introduction to Tri-State power driver circuits. This is oriented towards H-Bridge motor controls.
The circuit reviewed here is not tri-state, but high-low output depending on the input. The input will be not inverted.
The ultimate goal is the H-Bridge shown as hb_big1.jpg
Fig. 1b uses an open collector 75LS06 with a pull up resistor. See Review Connecting Digital Logic and Transistors
HIGH is +5V and LOW is 0V unless specified otherwise. This assumes a 5-volt microcontroller.
The concept here is to drive heavier loads from a microcontroller I/O pin. Most are limited to 25-40mA This circuit can source-sink several amps.
Fig. 2 circuit is non-inverting. HIGH in is HIGH out. LOW in is LOW out. When DIN is HIGH the output of the 7404 goes LOW. This turns on Q1 a P-channel MOSFET power transistor, turns off Q2 a N-channel MOSFET.
A load such as an LED connected between Y and ground would turn on.
If DIN is LOW The output of the 7404 goes HIGH. Q1 is turned off, Q2 is turned on. This is the "sink" mode. A load such as an LED connected between Y and +5V will turn on.
Fig. 3 Sink current demo circuit.
Fig 4 Source current demo circuit.
Fig 5 High voltage buffer circuit using ULN2003 and pull up resistor.
Here I use two identical circuits each connected to one side of a DC motor. Two inputs can forward or reverse the motor rotation. Note the truth table. Only when the inputs A and B differ does the motor operate.
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