Instruction manual

OM2  19
As described earlier a capacitor will block the flow of a DC current but will
appear to conduct and AC current. However it is not an absolute short circuit
to AC and actually changes its resistance with changes in the frequency of the
signal. In an AC circuit resistance is call impedance and can be plugged into
Ohm’s Law in place of the resistance values. To calculate the impedance of a
capacitor, we’ll call it ‘Zc’, of a capacitor the formula is:
Zc = 1 / {(2 * pi) * f * C}
Zc = 1 / 6.28 * f * C
Zc = Impedance of capacitor in ohms
f = frequency of the AC signal in Hertz
C = capacitance in Farads
pi = 3.14 (approx)
Notice that as the value of a capacitor or the signal frequency is increased the
impedance of the capacitor decreases. So then if the frequency is zero Hertz
the impedance of the capacitor is infinity since 1 divided by 0 is infinity. Hence
a capacitor blocks DC current since the frequency of a DC current is zero
Hertz.
To calculate the input resistance with an AC input signal with C2 in the circuit
all you need to do is add the impedance of the capacitor to the values of R1
and R2 and calculate the current. So the total resistance, Rt, becomes:
Rt = R1 + R2 + Zc
Insert the total resistance into the formula for current and you have:
I = (Vin - 1.25) / Rt = ( Vin – 1.25) / (R1 + R2 + Zc)
Output Configuration
The OM2 output is easily configured. The main considerations are the output
current, output voltage, and if the output needs to go low(active low) or high
(active high) when the input is activated by turning on an input LED. Start by
determining the output voltage level required. The outputs are capable of
supplying up to 150mA with a maximum voltage of 40 volts.
Now determine if you need an active high or active low output level. This will
be entirely dependant on your specific circuit requirements based on the type
of device you are connecting.
If an inductive load such as a relay is being used then diodes D1 and D2
MUST be installed. Otherwise they are optional. For most applications they
should not cause any problems. They may be removed if the load is not