Based on the above schematics simply switch motor voltage off, change direction, then motor voltage back on. 10 illustrates the use of a motor power switch to turn off a generic h-bridge circuit. This can be a particular risk with high speed motor direction change or using pulse-width modulation to control motor speed.įigure 10 MOSFET H-Bridge motor control with motor power on-off control.įig. I've used this circuit without problem, but we can't ignore this problem. This creates power supply overload and possible damage to MOSFETs. This is a condition where a transistor, let's say Q2, has not switched fully off as Q1 is switched on. Now I'll address the problem of shoot through. 8 illustrates stop mode - the motor is simply turned off as Q2-Q4 are turned on while Q1-Q3 are turned off.įigure 9 MOSFET H-Bridge motor control with shoot-through. This results in counter-clockwise or reversal of rotation of the motor.įigure 8 MOSFET H-Bridge motor control in stop mode.įig. Q1 and Q4 provide a current path for the motor, but in the opposite direction for the current flow. Here we turned off opto-coupler OC1 (turns Q1 on and Q2 off) and turned on OC2. The motor will rotate I'll say clockwise.įigure 7 MOSFET H-Bridge motor control counter-clockwise rotation. This creates a current path for the motor through Q2 and Q3. When a HIGH turns on opto-coupler OC1, Q1 is turned off while Q3 is turned on. Counter EMF from the motor motion acts to break the motor direction of rotation.įigure 6 MOSFET H-Bridge motor control clockwise rotation. In break mode both sides of the motor are grounded through the lower N-channel MOSFETs. See the spec sheet ta8050p.pdf.įigure 5 MOSFET H-Bridge motor control in break mode. This was based on a Toshiba TA8050P H-bridge. 3 illustrates a common ground between microcontroller control circuits and the h-bridge common ground.įigure 4 MOSFET H-Bridge control truth table. 2 illustrates how the use of opto-couplers OC1 and OC2 allows electrical isolation of motor voltage from the microcontroller circuits.įigure 3 MOSFET H-Bridge with motor voltage common with control circuit.įig. See the following two larger size diagrams: H-Bridge Separate Power Supply Common and H-Bridge Power Supply Common.įigure 2 MOSFET H-Bridge with motor voltage isolation. LM317 High Power Constant Current Source Circuitīecause I'm using opto-isolators the motor supply common can be separate from the microcontroller common.When it is working then move up to heavy motor. In building the following circuits I suggest using a current limiter on the power supply and a small motor. The IRF4905 is a P-channel device rated at 55V and a RDS(on) of 0.02 Ohms max. The IRFZ44N is an N-channel device rated at 55V and RDS(on) resistance of 0.032 Ohms max. I found two MOSFETs that work at 3.3-volts. Many micro-controllers today are using 3.3-volt Vcc. It is just as easy to use combinations of MOSFETs, bipolar transistors, and even insulated gate bipolar transistors. This depends on motor voltage and current that determines the H-Bridge construction. Press both switches and both sides of the motor will be at +12 volts and won't run.Īny number of solid-state switches can be used for H-Bridges. Release SW1 and press SW2 and +12 volts is supplied to the '+' side of the motor while the negative side is grounded through SW1. The motor will rotate at full speed say counter-clockwise. If we press SW1 the NC contact opens and the NO closes supplying +12 volts to one side of the motor while the other side is still grounded through SW2. In its normal state, both motor connections are grounded through the switches. The normally closed (NC) contacts are grounded and normally open (NO) contacts are connected to +12 volts.Ī DC motor is connected between the two commons. In figure 1 we have a very basic H-bridge using two spring-loaded, single-pole, double-throw switches. One of the most common solid-state controls is known as the H-bridge. Reverse the voltage, the direction of rotation reverses. Their direction of rotation is dependant upon the polarity of the applied voltage. Permanent magnet DC motors have been around for many years and come in a variety of sizes and voltages.
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