Instruction manual

OM2  5
Under most conditions C2 is not required. If you have a DC input JMP2 MUST
be installed which removes C2. If you are using an AC voltage input you may
want to install C2 but for most applications JMP2 may be installed which
removes C2. See the “Configuring Your OM2” section of this manual for more
information about how to figure out resistor and capacitor values and if you
need C2 for your specific requirements.
The Output Circuit
Moving on, the output circuit consists of the transistor inside optical coupler
OC1, transistors Q1 and Q2, along with resistors R3, R4, R5, R6, capacitor C2
and diodes D1 and D2. When either of OC1’s LED’s is turned on it causes it’s
internal transistor to turn on. An external voltage, +V, is required for the output
circuitry to operate. This voltage is applied to the OM2 on J3 and J4 and may
be any voltage up to 40 volts DC depending on your application. Again the
“Configuring Your OM2” section of this manual provides more information
about this.
A little bit of transistor theory is provided here to help with understanding the
OM2 operation. The base connection of a transistor is used to control the
current flow between the emitter and collector. A very small amount of current
flowing in the emitter/base circuit can control a very large current flow in the
emitter/collector circuit. With no emitter/base current flowing, the current in the
emitter/collector circuit is essentially turned off. If current is allowed to flow
between the emitter and base then a larger current is allowed to flow in the
emitter/collector. The base current can control a proportional amount of
collector current based on the gain of the transistor. For our purposes we’ll
just say that the collector/emitter current is either on when base current flows
or off when there is no base current.
With no input signal applied to the OM2 the output transistor in OC1 is off
which places a high level on the base of Q1 thru R3. Transistors don’t like a
lot of current flowing from emitter to base so resistor R3 limits the current. This
allows current to flow in the emitter/base circuit and Q1 is turned on allowing
current to flow in the emitter/collector thru R4. When a signal is applied to the
OM2 input the OC1 transistor turns on. It’s collector, OC1 pin 4, is essentially
connected to it’s emitter, pin 3 connecting it to ground which stops the emitter/
base current in Q1 and Q1 turns off. All that to say when an input is applied to
the OM2 the optical coupler transistor is turned on and transistor Q1 is turned
off making it’s collector high. With no input the optical coupler transistor is
turned off and Q1 is turned on placing it’s collector at a low or ground level.
Now look at the connection between Q1’s collector and Q2’s base thru R6.
When Q1 is turned off current can flow in the emitter/base of Q2 thru R4 and
R6. This turns on Q2 in the same way Q1 is turned on when it’s emitter/base
current flows which in turn places Q2’s collector at a low level. Turning on Q1