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Explanations
2.
How to Use Photocouplers
2.1
LED Control Circuits
2.1.1
DC Drive
Figure 8.2.1 shows an example of controlling LED drive current by switching the power supply on and off. In this case, the resistor R is R= VIN − VF IF
IF R IF VIN VF
VIN R IF VF
VF
VIN
Figure 8.2.1 Simple Drive Circuit for an LED
For example, when IF = 10 mA, VF (max) = 1.35 V, and VIN = 5 V, R = (5 − 1.35) V 10 mA = Ω 365
Therefore, the resistor should be selected as R = 360 Ω. Assuming that VF = 0.9 V due to its fluctuation or temperature dependence, the value of IF is 11.4 mA.
2.1.2
Reverse Voltage Protection
When a reverse surge voltage may be applied to a light emitting diode, a Si diode (for example, 1SS348) should be connected in reverse parallel with the light emitting diode, so that the reverse surge voltage bypasses the LED.
R Si Diode
Figure 8.2.2 Protection from Reverse Voltage by Silicon Diode
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2.1.3 Threshold Voltage
Explanations
When the input voltage is not absolutely zero or some unnecessary steady current flow is in a data transmission line, the threshold voltage of the LED should be raised up to a certain level by connecting a resistor in parallel with the light-emitting diode. (Figure 8.2.3)
R VIN RS
Figure 8.2.3 Threshold Voltage
If the forward voltage of the LED in the zero-light-emission state VT, the OFF-level input voltage VIN (OFF), and the OFF-level input current IIN (OFF) are given by VIN (OFF) = VT + R = (1 + IIN (OFF) = VT RS R RS VT RS ) VT
Then in the case of the Toshiba IRED, the value of VT is 0.5 V.
2.1.4
Driving by Transistor or IC
In Figure 8.2.4 are shown examples of using an LED for driving circuits by utilizing a transistor or IC.
VCC IF R IF VCC
R
R=
VCC − VF − VCE (sat) IF
R=
VCC − VF − VOL IF
Figure 8.2.4 Driving by Transistor or IC
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Explanations
2.4
Interface Circuit between TTLs Using a Phototransistor Coupler
A circuit using a DIP 4 pin photocoupler as an interface between TTLs is shown in Figure 8.2.22. In order to assure positive ON/OFF operation of the TTL, the LED current IF should be set to satisfy IOL which is determined by RC and IIL. Example of Design Specifications Operating temperature: 0 to 70°C Data transmission rate: 5 kbit/s Supply voltage: VCC = 5 V ± 5% Operating life: 20 years (170,000 hours) System working ratio: 50% Specifications of products required for designing interface circuits are shown in Table 8.2.1.
R IF VCC TLP521-1 VCC II IC RC IIL IC > II + IIL
Figure 8.2.22
Interface Circuit between TTLs Using a 4 pin Photocoupler
Table 8.2.1 Principal Characteristics of the TLP521-1
Item Forward voltage Collector to emitter Breakdown voltage Emitter to collector Breakdown voltage Collector dark current V (BR) ECO V (BR) CEO Symbol VF Test Condition (Ta = 25°C) IF = 10 mA IC = 0.5 mA IE = 0.1 mA IF = 0, VCE = 24 V IF = 0, VCE = 24 V, Ta = 85°C A rank Current transfer ratio CTR (IC/IF) IF = 5 mA VCE = 5 V GB rank GR rank BL rank Collector to emitter Saturation voltage VCE (sat) IF = 5 mA, IC = 1 mA min 1.0 55 typ. 1.15 ⎯ max 1.3 ⎯ Unit V V
7 ⎯ ⎯ 50 100 100 200 ⎯
⎯ 10 2 ⎯ ⎯ ⎯ ⎯ 0.1
⎯ 100 50 600 600
V nA µA
ICEO
% 300 600 0.4 V
146 155
