Optocouplers

An optocoupler, also called an optoisolator, is a device that transfers electrical signals between two isolated circuits using light. It is essential in applications where galvanic isolation must be maintained between a high-voltage power circuit and a low-voltage control circuit, protecting sensitive microcontrollers and logic circuits from dangerous voltage spikes.

Core Concept Explanation

An optocoupler packages an infrared LED and a light-sensitive device, typically a phototransistor, inside a single IC package. When a current flows through the LED on the input side, it emits infrared light. This light crosses a small transparent gap and falls on the base region of the phototransistor on the output side, turning it on. The two sides share no electrical connection whatsoever, which is the defining feature.

The key parameter describing input-output transfer efficiency is the Current Transfer Ratio (CTR), defined as the ratio of the output collector current to the input LED forward current. A CTR of 100% means 1 mA of LED current produces 1 mA of collector current. Practical devices range from 20% to over 300% depending on type and operating conditions.

The isolation voltage specifies the maximum voltage that can exist between the input and output terminals without breakdown. Common values range from 1500 V to 5000 V RMS. This is tested between the two sides of the package and is the primary safety specification in industrial designs.

Optocouplers are available in several output configurations. The basic type uses a phototransistor output. Higher-speed variants use a photodiode with a separate transistor amplifier. Triac output optocouplers are used directly in AC power control. Logic-output types integrate a Schmitt trigger for clean digital switching.

Mathematical Expression

The current transfer ratio defines the relationship between input drive and output current. It is expressed as a percentage and is the most critical parameter when designing the input drive resistor. The output collector current is:

IC = CTR x IF, where IC is the phototransistor collector current and IF is the LED forward current. To ensure the output transistor is fully saturated, the designer must ensure IC is large enough to pull the output low through the load resistor. The input resistor is calculated as R = (VCC_in - VF) / IF, where VF is the LED forward voltage, typically 1.2 V for infrared LEDs.

Practical Understanding

In a typical microcontroller to relay driver interface, the MCU GPIO pin cannot directly drive a relay coil due to current limitations and voltage differences. An optocoupler is inserted between the MCU and the transistor driving the relay. This ensures that any inductive spike or voltage transient from the relay side cannot reach and damage the MCU.

The bandwidth or switching speed of an optocoupler is limited by the capacitance of the phototransistor junction and the transit time of carriers. Standard optocouplers like the 4N35 operate up to about 10 kHz to 50 kHz. High-speed types like the 6N137 use a photodiode and logic output, reaching 10 Mbps and above, suitable for digital communication isolation.

CTR degrades with age and temperature. Designs must include CTR margin to ensure reliable operation over the device lifetime. Using the LED at lower than maximum forward current also extends its lifespan and reduces CTR degradation rate.

Given:
VCC_input = 5 V, LED forward voltage VF = 1.2 V, desired IF = 10 mA
CTR = 100% (minimum), load resistor RL = 1 kΩ, VCC_output = 12 V

Why this formula applies:
We need input resistor R1 to set correct LED current, then verify output saturation.

Formula:
R1 = (VCC_input - VF) / IF
IC = CTR x IF
Vout_low = VCC_output - IC x RL

Substitution:
R1 = (5 - 1.2) / 0.010 = 3.8 / 0.010
IC = 1.00 x 10 mA = 10 mA
Vout_low = 12 - (0.010 x 1000)

Calculation:
R1 = 380 Ω (use 390 Ω standard value)
IC = 10 mA
Vout_low = 12 - 10 = 2 V (transistor not yet saturated, increase CTR margin or reduce RL)

Final Answer:
R1 = 390 Ω, IC = 10 mA, output voltage drop = 10 V across RL confirming transistor is conducting.

Exam Tip: CTR is always expressed as a percentage. If CTR = 100% and IF = 5 mA, then IC = 5 mA exactly. GATE often asks to find R1 given VCC, VF, and required IC using CTR. Do not confuse CTR with voltage gain.

• Current flows through input LED, producing infrared light proportional to IF.
• Light crosses the transparent isolation gap with no electrical conduction path.
• Phototransistor receives photons at its base-collector junction, generating base current.
• Collector current IC flows through output load, controlled by CTR and IF.
• Two grounds can be at completely different potentials without damage.

Quick Revision

• Optocoupler transfers signal using light, providing galvanic isolation between input and output circuits.
• CTR = (IC / IF) x 100%, key parameter for output drive strength calculation.
• Input resistor: R1 = (VCC - VF) / IF, with VF approximately 1.2 V for IR LEDs.
• Isolation voltage rating (1500 V to 5000 V RMS) defines the maximum voltage between input and output sides.
• Standard optocouplers (4N35) operate below 50 kHz; high-speed types (6N137) reach 10 Mbps.
• CTR degrades over time; design with margin above minimum CTR for reliable long-term operation.
• GATE trap: CTR is a current ratio, not a voltage gain. It does not depend on VCC of the output side.