H-Bridge motor control with IGBTs and bipolar transistors.
Fig. 1 H-Bridge motor control with IGBTs and bipolar transistors.

Common Collector Opto-Isolated Bipolar Transistor Switches

by Lewis Loflin

Earlier I presented several circuits related to a tri-state H-bridge motor controls. We used MOSFETs and CMOS integrated circuits.

The single largest headache was trying to use P-channel MOSFETs.

Also many microcontrollers produce insufficient output voltage to drive many MOSFETs.

For more on this see Basic Digital Circuits to H-Bridge Motor Controls.

Here I'm concerned with the "high-side" transistor driver circuits. They are Xsistor1, Xsistor2 shown in Fig. 1 above.

This with an opto-coupler the solved many problems. The culprit was P-channel gate-source MOSFET voltage limitations. This is usually 20V.

Much of that is covered in From Basic Digital Circuits to H-Bridge Motor Controls.

See this sample schematic and note Q1: Non-Inverting Tri-State switch. The use of a CD4011 allows easy use with 5-volt or 3.3-volt microcontroller. This also works with the low-current 3.3-volt output on a Raspberry Pi GPIO. This assumes +Vcc is 3.3 or 5 volts.

Inverting Tri-State switch.
Fig. 2 Inverting Tri-State switch.

Note Fig. 2. I replaced Q1 p-channel MOSFET by a TIP120 NPN Darlington transistor.

I also used a 4N25 type optocoupler. This allows any motor voltage up to 55-volts. The design goal was 48-volts.

If the motor voltage is 24-volts a 4N25, etc. optocoupler is fine. For 48-volts use PC817 or similar optocoupler.

Here again I'm only concerned with Q1 and its associated optocoupler.

Generic high-side opto-coupler driver.
Fig. 3 generic opto-isolated high-side switch.

In Fig. 3 illustrates my generic device symbol.

The circuit incorporates some form of optocoupler and a Darlington bipolar output transistor.

In the series of tests +Vcc consisted of 2, 12-volt batteries in series.

When completely charged the output voltage was ~26.5-volts.

The load is 5-Ohms. The output Darlington transistor is aMJE100005 with these values.

The collector-emitter voltage drops ranged around 2-volts.

Also listed was the parts used in the tests.

Example opto-isolated high side transistor drivers.
Fig. 4 Example opto-isolated high side transistor drivers.

Fig. 4 are the two circuits I used. The rest of this concerns the MJE10005 that was used in Fig. 1 and associated videos.

MJE100005 opto-coupler driver.
Fig. 5 MJE100005 opto-coupler driver.

Fig. 5 is the exact circuit used in Xsisotr1 and Xsistor2.

With a 5-Ohm load and +Vcc of 26.5-volts. The results of the test was interesting.

The circuit delivered a lot of current with excessive heat.

The collector-emitter voltage of Q4 must be at least 2.4-volts. The voltage Vbe supplies base current to Q2 through Q1.

The base-emitter voltage drops of the TIP41A (0.6V) and Q2 (1.6V) leaves 0.2V to produce base current for Q2.

In fact I removed Q1 and connected the emitter of U1 to the base of Q2 operation was almost identical.

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