STABILIZING OUTPUT LEVELS
When an optocoupler is used with the base pin of its phototransistor open, the collector dark current (I
CEO
) flows as a base
current. The current is amplified as a collector current and could make the output level of the phototransistor unstable. To
eliminate this unwanted base current and make the output level stable, flow the collector dark current (I
CEO
) through the base-
emitter resistor (R
BE
).
Fig 2-5 shows the I
CEO
vs. T
A
characteristics of a PS2601 optocoupler.
PS2601 I
CEO
-T
A
Characteristics
10000
Collector Dark Current I
CEO
(nA)
1000
I
F
= 0
V
CE
= 80V (40V for the
PS2603) 2601
Solid line: PS2601
Dotted line: PS2603
R
BE
=
100
8
10
R
BE
=1MΩ
1
R
BE
= 1MΩ
R
BE
=100 MΩ
0.1
- 20
0
20
40
60
80
100
Ambient Temperature T
A
(°C)
Figure 2-5. I
CEO
vs. T
A
Characteristics
ELIMINATION OF INDUCED NOISE
Generally, machine-controlling equipment generates induced noise which may cause malfunctions. This unwanted noise in input
signals can be isolated by means of optocouplers. However, if the noise is too strong, it may be switched to the output through
the input-output capacitance C
1-2
of the optocoupler. This unwanted noise in the output can be removed in the following manner.
Insert a capacitor (preferably 100 pF) between the base and the emitter of the phototransistor of the optocoupler. This capacitor
delays response and suppresses malfunctions.
Graph 6-(a) to 6-(d) show how an external noise (surge voltage of 1000 V/µs at rise time) is eliminated as the capacitance of the
base-emitter capacitor.
A fluctuation in the collector-emitter voltage caused by the on/off operation of a power switch at the output of the optocoupler
causes a base current to flow through the collector-base capacitor (C
CB
), which causes a malfunction.
In Fig. 2-7, for example, an instantaneous base current flows through the collector-base capacitor (C
CB
) of the optocoupler. The
current is multiplied by h
FE
(as a collector current) and causes an output voltage on both ends of the load resistance. It seems as
if an input signal was applied to the optocoupler. Graph 7-(a) shows the waveforms. This unwanted instantaneous induction
current can be eliminated by inserting a capacitor C
BE
between the emitter and the base of the phototransistor. Graph 7-(b)
shows the waveforms. Fig. 2-8 shows the output-voltage vs. C
BE
characteristics.
V
o
C
BE
R
L
Figure 2-6.
Figure 2-7.
8
NEC [ NEC ]
