Example opto-iolators with 4N25.

Opto-Isolated Transistor Driver Circuits for Micro-Controllers

by Lewis Loflin

In part 1 we looked at a number of bipolar transistor and MOSFET driver circuits. They had a flaw of having to be electrically connected to low voltage digital circuits. See Part 1.

With opto-isolators we can sever this connection including the use of higher-voltage power supplies totally isolated from the low-voltage digital circuits. In fact we can even change the polarity of the higher voltage supplies without regard to the digital circuit common negative grounds if needed.

Q1 is a NPN bipolar photo transistor and both the LED and transistor are often one physical device. Again an important note is the output has no electrical connection to the input and can isolate several thousand volts. Also see More sample circuits.

Internal opto-coupler circuit different detectors.

While most opto-isolators use a LED infrared emitter the detector can include photo SCRs, Darlington transistors, and even digital circuits.

Typical low-power opto-coupler circuit.

The output transistor of a 4N25 is still a low power device, so we must use it to drive higher power components for greater loads. This is usually higher power bi-polar transistors or power MOSFETs.

Opto isolator driving a TIP120 NPN Darlington transisotr.

Here we drive a NPN Darlington power transistor. With a gain of 1000 very little base current is needed to be switched though the 4N25. The TIP120 has a maximum base current (Ib) of 150 mA while the 4N25 has a maximum collector current of 150 mA. R2 can be 5600-10,000 ohms.

Opto-isolator driving a TIP125 PNP Darlington transistor.

Same as above except we use a PNP Darlington. In this case the base of the TIP125 is switched to ground though a 1000 ohm resistor. Note the 24-volt supply is electrically isolated from the digital circuits.

transistor driver circuit

Here we switch a N-channel MOSFET to "sink" the load.

transistor driver circuit

Once again we sever the 12-volt supply totally from the digital circuits. In fact we could have done this with all of the circuits on this page. Here we switch on a P-channel power MOSFET to "source" the load. R6 = 10K and BATT2 is limited to 30 volts because of the 4N25 or the Vgs rating for the MOSFET, which ever is smaller.