Rockwell Automation 1771-VHSC , D17716.5.74 Very High S User Manual
Page 60
Application Considerations
C–3
The optical isolator manufacturer recommends a maximum of 8mA
to flow through the photodiode. This current could be exceeded in
the 24V position. To obtain this limit, a dc shunt circuit is included,
consisting of D1, Q2, R97 and R98. If the photodiode current
exceeds about 8mA, the drop across R97-R98 will be sufficient to
turn Q2 on, and any excess current will be shunted through D1 and
Q2 instead of through the photodiode.
If the driving device is a standard 5V differential line driver, D2 and
D3 provide a path for reverse current when the field wiring arm
terminal 1 is logic low and terminal 2 is logic high. The combined
drop is about the same at the photodiode (about 1.4V). The circuit
appears more symmetrical, or balanced, to the driver as opposed to
just one diode.
Detailed Circuit Analysis
In the example above, we used a constant 2.0V drop across the
photodiode and R97-R98. To calculate the true photodiode current,
consider the photodiode, D1, Q2, R97 and R98 as one circuit. The
voltage drop across D1 and Q2 will always be equal to the drop
across the photodiode and R97-R98. We will call this V
drop
.
First, consider the minimum requirement of I
f
= 5mA.The V
f
curves
for this photodiode will typically have a 1.5V drop With 5mA
current, R97 and R98 will drop (80.4 ohms x 5mA) = 0.40V. Thus, at
5mA,
V
drop
= (1.5V + 0.40V) = 1.90V.
Now let’s see what happens when I
f
goes to 8mA or above. With the
temperature about half way between 25 and 70
o
C, V
f
becomes about
1.5V. R97-R98 will now drop 0.64V (80.4 ohms x 8mA). That
means:
V
drop
= 1.5V + 0.64V = 2.14V.
The V
be
of Q2 is now sufficient to start to turn Q2 on. If the current
through the photodiode increases to 9mA, V
be
becomes 0.72V and
Q2 is fully on. Any additional current (supplied by a 24V applied
input) will be shunted away from the photodiode and dissipated in
Q2 and D1.
Thus, Vdrop will never exceed about 2.52V regardless of the applied
voltage. In addition, it will never be less than 1.7V if the minimum
of 5mA is flowing. Although there are some minor temperature
effects on the photodiode drop, you can expect the value Vdrop to be
relatively linear from about 1.9V to 2.14V as the current increases
from 5mA to 8mA.
Why is this important? Let’s look at the 5V differential line driver
example below.