Hardware Reference
In-Depth Information
Figure 9.20
Typical waveforms produced by the circuit of Figure 9.19
Figure 9.21
Optically isolated digital input for DC signals
500 V to 3 kV and switching rates may be up to 300 kHz, or so. High-voltage
opto-isolators are available which will work reliably at voltages of up to 10 kV.
A typical single-channel optically isolated input arrangement is depicted
in Figure 9.21. The external diode protects the infra-red emitting LED from
inadvertent reversal of the input polarity and the value of the series resistor
should be selected from the following table:
Input voltage (V)
Series resistor (R)
range (DC)
(all 0.25 W)
3to4
330
4to5
560
680
5to6
6to8
1k
1.5k
8to11
11 to 15
2.2 k
15 to 30
3.9 k
Bipolar optoisolators are available that will operate from inputs of either
polarity. Such devices are useful when the input polarity is unknown but they can
be unsuitable for general AC applications as the output will go high momentarily
whenever an applied AC signal passes through zero volts. Figure 9.22 shows a
