Tri-state H-Bridge motor control uses optocouplers on output
Fig. 1 Tri-state H-Bridge motor control uses optocouplers.

Tri-State H-Bridge Uses Optocouplers


YouTube Video Opto-Coupler Interfacing H-Bridge Circuits

By adding optocouplers to a tri-state switch we isolate 3.3 to 15 volt logic from high voltage output transistors.

An optocoupler operates at a higher voltage than a MOSFET transistor. The gate-source voltage for a MOSFET is often 20 volts.

The Optocoupler coupled with a Darlington transistor allows the use of higher voltages than a p-channel MOSFET. This is limited by the collector-emitter voltage and collector current of the optocoupler.

For information on the digital gates see Review Connecting Digital Logic and Transistors. It also has the 7407 interface circuit not shown.

The circuit example in Fig. 1 does two things. Q2 bipolar transistor was replaced with a N-channel power MOSFET.

To get above the 5-volt limit created by the 5-volt digital logic, I've used an optocoupler. This consists an LED infrared emitter and in this case a NPN photo transistor.

When the optocoupler transistor is switched on current flows though Rb1 turning on the TIP120 Darlington transistor.

+Vcc is the motor voltage limited by the collector-emitter voltage of the optocoupler. Rb1 is calculated to limit base current of Q1 and optocoupler circuit. 100 Ohms works for 12 volts.

A higher motor voltage can be used but must be kept separate from the CMOS logic voltage limited to 15 volts.

PC817 optocoupler pin connections.
Fig. 2

Fig. 2 is the PC817 optocoupler. I bought 50 for $5 from Ebay. It has a transistor voltage limit of 80-volts and a collector current of 30mA.

The formula to calculate Rb1 is +Vcc - 2V / 25mA. I chose 25mA to stay under the 30mA limit of the optocoupler transistor. Q1 is a TIP120 NPN Darlington with a gain of 1000+.

See Tutorial Using TIP120 and TIP125 Power Darlington Transistors.

For the power MOSFET Q2 see:

N-Channel Power MOSFET Switching Tutorial.



4N25, 4N26, 4N27, 4N28 outline.
Fig. 3

Fig. 3 shows the outline of the 4N25, 4N26, 4N27, 4N28 type optocouplers. The transistor has a collector-emitter breakdown voltage of 70V and a maximum current of 50mA.

For more information on optocoupler circuits see:

Truth table for tri-state non-inverting switch.
Fig. 4

Fig. 4 is the truth table for Fig. 1.

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