PS2621 datasheet(9/38 Pages) NEC

9 / 38 page

STABILIZING OUTPUT LEVELS

When an optocoupler is used with the base pin of its phototransistor open, the collector dark current (ICEO) 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 (ICEO) through the base-

emitter resistor (RBE).

Fig 2-5 shows the ICEO vs. TA characteristics of a PS2601 optocoupler.

RBE =

IF = 0

VCE = 80V (40V for the

PS2603) 2601

Solid line: PS2601

Dotted line: PS2603

10000

RBE = 1M

Ω

RBE =100 M

Ω

1000

100

0.1

- 20

100

RBE =1M

Ω

Ambient Temperature TA (

°C)

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 C1-2 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 (CCB), which causes a malfunction.

In Fig. 2-7, for example, an instantaneous base current flows through the collector-base capacitor (CCB) of the optocoupler. The

current is multiplied by hFE (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 CBE between the emitter and the base of the phototransistor. Graph 7-(b)

shows the waveforms. Fig. 2-8 shows the output-voltage vs. CBE characteristics.

CBE

Figure 2-6.

Figure 2-7.

PS2601 ICEO-TA Characteristics

Figure 2-5. ICEO vs. TA Characteristics

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