This difference may be enough to cause problems in high frequency circuits or where accurate timing is required. 14.)įigure 7 demonstrates that for a 4.7 kΩ load on a 5 V supply we could expect a 10 µs fall time (PHL) (photo-transistor turning on) but a 30 µs rise time (PLH) (photo-transistor turning off). (Test configuration is a low side switch. Propagation delay is different for high to low (PHL) and low to high (PLH). While Table 5 gives the impression that a fixed time delay of 2 µs is given, Figures 7 and 8 show that the situation is more complex. Switching characteristics Table 5. The switching characteristics lead one to believe that a fixed delay of 2 µs is given. In most applications we design the circuit to drive the output transistor into saturation. If more current is required then further amplification is required – e.g., the Darlington arrangement shown at the top of this page. Table 4 and Figure 6 show us that for this device when the LED is run at 10 mA we can expect to get only 2 to 5 mA maximum out of the transistor. Note datasheet has CTR curves for other temperatures also.
Current transfer ration (CTR) as a function of LED current. The forward voltage of an LED is temperature dependent and will decrease with increasing temperature. From this we can see that only the IR LED could result in the 1.3 to 1.5 V forward voltage at 50 mA. Typical IV curves for various colours of LEDs. Other parameters such as capacitance may be important at high frequencies. (3) At 10 V there will be up to 50 nA leakage current between the collector and emitter when the LED is dark (unpowered). (2) The maximum voltage we can switch with this device is 30 V. From this we can infer that it is an infra-red type – see Figure 5 – but this is stated in the description on page 1. (1) The forward voltage of the LED is only 1.3 V. Note that in most cases the PCB pad radius will reduce this distance significantly.
(3) The clearance distance is the shortest distance through the air between the any input and output. See Vishay’s Design Guidelines for Optocoupler Safety Agency Compliance for details on this. Creepage distance is measured along the outside of the package while clearance is the shortest distance through the air. This is the shortest distance along the surface of the package between any input and output pin. (1) 5,000 V isolation between the input and output. Isolation, creepage and clearance Table 2. e.g., If there is 3 V across the output transistor then the maximum output current will be given by \(I_ = 50 \, \mathrm mA\).
(5) Watch the output transistor power dissipation: if the transistor is not turned hard-on there will be a voltage drop across it and the power dissipation may rise. (4) The transistor output is capable of 100 mA continuous. There will be a recovery time associated with this. (3) Surprisingly, perhaps, the LED peak current is 3 A but note well that this is for ≤ 10 µs. (2) The maximum continuous forward current is 60 mA. (1) The maximum reverse voltage the LED can take is 5 V. Essential reading.įrom the Absolute Maximum Ratings section we can see: Careful reading of the datasheets and experience will be your guide. As always, in most applications certain parameters become more significant than others. The information below is published in the Vishay 4N25 datasheet. Note the different ground symbols on the 5 V and 12 V circuits indicating isolated grounds. The 12 V logic level will then switch from low to high. When Q1 is switched on U1 LED will illuminate and the collector-emitter resistance will fall to a low value. Switching a 12 V circuit with a 5 V control signal.įigure 3 shows a simple application where a 5 V logic signal in one circuit is controlling a 12 V signal in another. This one makes the transistor’s base available on pin 6 but this is not always the case – particularly on high-density packages. DIL opto-isolator package and pinout.įigure 2 shows a typical pinout for a 6-pin DIL single opto-isolator package. Usually opto-isolators transfer digital (on-off) signals, but some techniques allow them to be used with analog signals. In operation the photons from the illuminated LED that hit the base-collector junction in the transistor generate electrons and these are injected into the base, turning on the transistor. The most common type of opto-isolator consists of an LED and a photo-transistor combined in an opaque package. Opto-isolator schematic symbol showing the LED and photo-transistor elements. A 5 V logic signal, for example, might switch a mains circuit using an opto-isolator without risk of the mains feeding into to the low-voltage logic. 1 kV isolation is common for mains powered circuits. An opto-isolator is a component that transfers electrical signals between two isolated circuits by using light. This allows transmission of electrical signals between systems of different voltages with complete electrical isolation up to the rated isolation voltage.
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