(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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