Article Overview
An optocoupler's voltage is defined by the forward voltage of its input LED, the reverse voltage limit of the LED, and the maximum isolation voltage between input and output circuits.
Input Side Voltage
The input of an optocoupler consists of a light-emitting diode (LED). The forward voltage of this LED typically ranges from 1.2V to 1.4V for standard infrared LEDs, depending on the specific device and current applied . The reverse voltage of the LED is usually low, often around 5V, and exceeding this can cause a reverse current that may damage the LED or reduce its light-emitting efficiency . The LED's voltage drop is proportional to the current flowing through it, and datasheets specify the maximum forward current to prevent overheating or destruction.
Output Side Voltage
The output side of an optocoupler is usually a phototransistor, photodarlington, or photo-SCR/triac. The voltage across the output device depends on the collector-emitter voltage (V_CE) for phototransistors or the blocking voltage for photo-SCRs and triacs. Typical phototransistor optocouplers can handle collector-emitter voltages from 20V up to 80V, while specialized devices may support higher voltages . The output voltage is controlled by the external circuit and the current transfer ratio (CTR) of the optocoupler.
Isolation Voltage
A key voltage specification is the isolation voltage, which defines the maximum voltage that can be applied between the input and output without breakdown. Standard optocouplers provide isolation up to 5kV RMS, ensuring that high voltages on the input side do not affect the output side . This voltage is tested for a short duration (e.g., 1 minute) and guarantees insulation under normal operating conditions .
Summary
- Forward voltage (LED input): ~1.2–1.4V
- Reverse voltage (LED input): ~5V maximum
- Output voltage (phototransistor V_CE): 20–80V typical
- Isolation voltage: up to 5kV RMS Understanding these voltage limits is essential for designing circuits that safely use optocouplers to transfer signals between high-voltage and low-voltage sections while maintaining electrical isolation .
Optocoupler
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