Opto-Isolated Transistor Drivers for Micro-Controllers
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.
On the left is basic opto-coupler or opto-isolator. The voltage level at point Vo is at VCC with S1 open. When S1 is closed the infra-red light from the light emitting diode D1 falls on the base of Q1 causing it to conduct to ground like a switch. The voltage at point Vo drops to zero.
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.
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.
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.
Here we switch a N-channel MOSFET to "sink" the load. R3 can be 100K ohms.
In these two examples we sever the 24 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.