Instruction manual

OM2  4
Introduction
The OM2, Optically Isolated Interface, provides an optically isolated interface
between two devices. It can be used to interface nearly any AC or DC signal
to a logic level signal. It can also be used to operate a relay upon detection
of an input signal. The really great thing about the OM2 is the input and
output are totally isolated from each other. Virtually any voltage input level
above 1.5 volts can be used as long as long as the input current is limited to
a maximum of 60mA. The input can be a simple DC level or an AC signal
with a frequency up to 250KHz. (That’s 250,000 cycles per second for the
old pre-Hertz folks.) The input and output are isolated from each other to
5000 volts AC(rms) difference between the input and output connections.
The output can drive up to 150mA at up to 40VDC. It even provides active
low and high outputs when the input is activated.
Theory of Operation
The heart of the OM2 is an optical coupler. An optical coupler contains an
infrared LED and a photo transistor. When the LED is turned on it causes
the photo transistor to turn on. The coupler used in the OM2 actually has two
LED’s on the input. These LED’s are connected in parallel with each other in
opposite directions. This makes it able to operate using either a DC or AC
input. The DC polarity of the input doesn’t matter since one of the LED’s will
be turned on when an input signal is applied regardless of polarity. We’ll
take a look at the schematic now and go thru the entire circuit operation.
The Input Circuit
On the left side of the schematic you will see J1 which is the input
connection. The input signal is applied to J1’s two pins. Starting at pin 2 of
J1 you will see resistors R1, R2 and capacitor C2 connected in series with
each other. These components act to limit the current supplied to the LED’s
in OC1 pins 1 and 2. The LED’s in OC1 are just like any other LED in that
the forward current must be limited, in this case to a maximum of 60mA.
Why are there two resistors and a capacitor you ask. Well we tried to give
you lots of options to configure the OM2 to meet your requirements. The
values shown for R1 and R2 are such that you can use a standard 110VAC
or 220VAC signal to trigger the output. Since the original concept for the
OM2 was to sense presence or absence of power on an AC power line it
made sense to provide a simple way to select the input voltage by either
having both resistors in place for 220VAC or to remove R2, by installing
JMP1, to change to 110VAC. Notice they are both the same value and
220VAC is twice 110VAC so the current is changed by a factor of 2 by
adding or removing one of the resistors. The “Configuring Your OM2”section
of this manual explains how to set up the input of the OM2 for your
application+. But what’s C2 there for?