OptoIsolation:

Optoisolation of microcontrollers is essential in cases where a microcontroller needs to supply signals to a controller which controls an inductive load such as a motor. Back EMF spikes from an inductive load can easily glitch, or destroy a microcontroller (as many students have unfortunately experienced firsthand). Back EMF spikes typically manifest themselves as very short duration spikes which may or may not contain enough energy to actually destroy a microcontroller. Nevertheless, these spikes (which can easily approach 100V amplitudes) can easily glitch a microcontroller, even if transient suppression techniques such as generous use of decoupling capacitors, zener diodes etc. are used. Optocoupling should be thought of as absolutely essential if you are using the new 68HC12 series microcontroller boards, as the 68HC12 chips contain many many (tiny) I/O pins, and are surface mounted to the boards! Thus if you zap your board, chances are you are going to end up buying an entire new board.

Students invariably tend to use whatever is available at Radio Shack, which if memory serves me correct, is an optoisolator with a phototransistor output. Although these will perform satisfactorily under most conditions - if used correctly, students should be aware of newer developments in the optoisolator arena. In particular, consider the 6N136 and 6N137 (see www.hp.com for data sheets).

Hewlett-Packard Optisolator Products - Data Sheets

The 6N136 touts a 1MHz bandwidth (by virtue of a PIN photodiode which drives an internal NPN transistor). Also consider the 6N137, which has a TTL/CMOS compatible output and touts a whopping 10MHz bandwidth. These optoisolators are readily available from Jameco Electronics.


Typical Optoisolator Circuits (left is an inverting, right is noninverting circuit)


Note that all the above transistors are configured in common emitter mode as opposed to voltage follower mode. The common emitter configuration generally assures a faster response. The transistor used to buffer the optoisolator, in my mind, is ESSENTIAL. Although students have gotten away, in the past, with driving these LEDs directly from a 68HC11 output port, the Motorola manual clearly indicates that these output ports are not designed to sink more that about 1.5mA of current tops! Trying to directly drive the LEDs will no doubt give your optoisolators slow response (large rise times) and may eventually burn out the output ports on the 68HC11 !

Remember this - do not source the output side of the optoisolator with the same power supply (+5VDC) used to supply the microcontroller! To do so is to completely defeat the objective of the optoisolation - as it allows back EMF spikes to propagate back through the power supply to the microcontroller.

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