Opto-Isolator

How it works

An opto-isolator is the simplest galvanic isolator in production: two semiconductor devices in one moulded package, separated by a transparent dielectric and looking at each other across it. On the input side, an infrared LED — typically gallium-arsenide, emitting around 940 nm — converts forward current into photons. On the output side, a silicon photodetector absorbs those photons and produces a collector current, a switched MOSFET channel, a gated SCR, or a tiny photodiode current that drives an internal amplifier. The two halves of the package share nothing but a transparent epoxy or silicone gel; there is no copper trace, no bond wire, no resistive path between input ground and output ground.

That asymmetry is the entire point. The input LED can sit on logic ground at 5 V, while the output transistor can ride on a floating rail that is thousands of volts above (or below) that logic ground. Photons cross the gap at the speed of light regardless of the potential difference. The barrier between them is rated by its dielectric strength — the voltage at which the gel arcs over and lets current jump — typically 2.5 kV, 5 kV, or 7.5 kV for a one-minute hi-pot test, with reinforced-isolation parts going to 10 kV.

Current transfer ratio

The fundamental DC transfer specification is the current transfer ratio:

CTR = I_C / I_F   (typically 20 – 100% for phototransistor parts)

Drive 10 mA into the LED and a CTR-50% part will sink about 5 mA at the collector — assuming the collector load lets the transistor stay in its active region. The number is a property of the LED-detector geometry, the LED's external quantum efficiency, and the phototransistor's internal current gain. It is also a moving target: LEDs slowly degrade with hours-at-temperature-at-current, so the same part that ships at CTR = 100% may drop to 50% after a few thousand hours of continuous operation. Designs target the worst-case end-of-life CTR with margin, not the typical fresh value on the datasheet.

For higher CTR, the output stage can be a Darlington pair (4N32, 6N138) which multiplies gain by a factor of 100-1000 — giving CTRs above 500% — at the cost of speed. Higher-current LEDs and matched detector geometry can also bump CTR; the Avago/Broadcom HCPL series tightens part-to-part variation by laser-trimming the LED and binning detectors.

Speed and bandwidth

Speed is the single dimension along which opto-isolator families differ the most. Three regimes dominate the market.

Family	Output	Typical part	t_PHL / t_PLH	Max data rate
Slow phototransistor	BJT	4N25, PC817, CNY17	3 – 10 μs	~10 kHz
Darlington	BJT Darlington	4N32, 6N138	30 – 50 μs	~2 kHz
High-speed digital	Photodiode + logic gate	6N137, HCPL-2601, HCPL-2611	50 – 100 ns	10 Mbit/s
Gate driver	Bipolar push-pull, 2-4 A peak	HCPL-3120, ACPL-W346, FOD3120	200 – 500 ns	Switching, not data
Linear	Matched photodiode + sigma-delta	HCPL-7800, ACPL-C87B	1 – 3 μs	~100 kHz analog BW
SCR / triac driver	Photo-SCR or photo-triac	MOC3041, MOC3081	ms-scale	Mains-frequency switching
Photo-MOSFET (SSR)	Two photo-MOSFETs back-to-back	AQY212, TLP222	0.1 – 1 ms	Solid-state relay

The 4N25's speed limit comes from base-charge storage in its phototransistor. The base is its photoactive region — there is no base lead to bleed off stored charge — so when the LED switches off, the carriers in the base must recombine on their natural lifetime, which is microseconds. The 6N137 sidesteps this by using a discrete photodiode for detection and feeding its photocurrent into a high-speed logic-gate amplifier with active pull-down. Result: 75 ns propagation delay and 10 Mbit/s clean.

Worked example: driving a 4N25 from a microcontroller

A 3.3 V GPIO needs to switch a 24 V relay coil via a 4N25 phototransistor optocoupler. The relay coil draws 60 mA at 24 V. Design the LED drive resistor and check that the optocoupler can sink the coil current.

LED forward drop:  V_F  ≈ 1.2 V  @ I_F = 10 mA
GPIO high level:   V_OH = 3.0 V  (worst case)
LED drive resistor:
   R_LED = (V_OH − V_F) / I_F = (3.0 − 1.2) / 0.010 = 180 Ω

Output requirement:
   I_C(needed) = 60 mA

End-of-life CTR (4N25, worst case): ~20%
   I_F required = I_C / CTR_min = 60 / 0.2 = 300 mA

That is FAR beyond the 4N25's I_F(max) of 50 mA.
The 4N25 alone cannot drive 60 mA.

Two fixes are common. First, use the optocoupler output to switch a small MOSFET that handles the coil current — the 4N25 only needs to source a few microamps of gate charge, and the actual coil current flows through the MOSFET on the isolated side. Second, replace the 4N25 with a Darlington-output part (4N32, CTR > 500%) and accept the slower turn-off. For relays this is fine; for PWM it is not.

Where they show up

• MOSFET / IGBT gate driver isolation. Half-bridges and three-phase inverters need the high-side switch's gate driver to float with the source rail. An HCPL-3120 or ACPL-W346 delivers a 2 A gate pulse referenced to the floating MOSFET source from a controller on logic ground.
• AC mains zero-cross detection. A current-limited LED across the rectified mains pulses off at every zero crossing. The output gives the microcontroller a 100 Hz / 120 Hz timing signal for phase-cut dimmers, triac firing, and inverter synchronisation.
• Switch-mode power supply feedback. The output voltage on the isolated secondary side feeds a TLV431 shunt regulator that pulls current through the LED of a PC817; the primary side sees the photodetector current and adjusts the duty cycle. Decades of every wall-wart, ATX PSU, and laptop brick.
• Industrial 4-20 mA loops. A factory PLC's analog inputs need to reject hundreds of volts of common-mode noise picked up by long cable runs across a plant. An optocoupler on every loop breaks the ground path and rejects everything that is not differential.
• USB and RS-232 isolation. Dedicated isolated USB transceivers (ADuM3160, USB1T20) and isolated UART parts sit between a host PC and a sensor that cannot share grounds — common in test instruments, automotive ECUs, and medical sensors.
• Medical patient-leakage isolation. IEC 60601-1 caps body-contact leakage at 10 μA (CF parts). Every signal line that crosses the patient boundary is opto- or digital-isolated to force leakage through the multi-kV barrier instead of the cable shield.
• Solid-state relays. A photo-MOSFET output (AQY212) or photo-SCR (MOC3041) replaces a mechanical relay's contacts with a fully isolated semiconductor switch — zero contact bounce, silent, no wear.

Picking a part

The first cut is the output type:

• Slow logic / control signals. 4N25, PC817, CNY17 — cheap, ubiquitous, 10 kHz max.
• Fast digital data (UART, SPI clock, PWM). 6N137, HCPL-2611 — 10 Mbit/s with the photodiode-plus-amplifier topology.
• Gate drive into a power FET. HCPL-3120, ACPL-W346, FOD3120 — push-pull output, 2-4 A peak source/sink, designed for V_iso ≥ 5 kV and high CMTI (common-mode transient immunity).
• Analog feedback. HCPL-7800 family, ACPL-C87B — matched photodiodes with feedback linearisation; 8-12 bit equivalent linearity.
• AC line switching. MOC3041 (zero-cross triac driver), MOC3081 (random-phase triac driver) — the canonical interface from a 5 V MCU to a 230 VAC load.
• Solid-state relay. Photo-MOSFET parts (AQY212, TLP222) for sub-amp signal switching with no contact bounce.

The second cut is the isolation rating: 2.5 kV is sufficient for "functional" isolation inside a low-voltage product; 5 kV is the standard for "basic" safety isolation against mains; 7.5 to 10 kV reinforced is required when the part is the sole safety barrier between a user-touched surface and AC mains.

Versus digital isolators

Since roughly 2010, capacitive (Silicon Labs Si84xx) and giant-magnetoresistive (Analog Devices ADuM, NVE IsoLoop) digital isolators have eaten significant share from optocouplers in the high-speed and high-reliability slots. Their advantages are concrete:

Metric	Optocoupler (6N137)	Digital isolator (Si8410)
Propagation delay	75 ns	10 ns
Data rate	10 Mbit/s	150 Mbit/s
Channel-to-channel skew	~50 ns	< 2 ns
Service life (50% CTR drop)	10⁴ – 10⁵ hours	Effectively unlimited
Common-mode transient immunity	10 – 25 kV/μs	50 – 100 kV/μs
Quiescent supply	~5 mA per channel	1 – 3 mA per channel
Cost at 1 ku	$0.30 – $1.50	$1 – $3

Optocouplers still hold the field where: (a) the part needs to bridge analog feedback with photo-linearity, (b) the application is low-speed and cost-sensitive (PC817 is pennies), (c) the output happens to be a SCR or photo-MOSFET that digital isolators do not natively provide. Most newly-designed isolated UART, SPI, and CAN links are now digital isolators; new SMPS feedback loops increasingly use digital isolators plus a digital controller; new gate-driver designs split — high-speed silicon-carbide drives push to digital, mainstream IGBT drives still ship with optocoupled gate drivers because the existing reference designs work.

Common pitfalls

• Designing to typical CTR instead of end-of-life worst case. A part that works on the bench at CTR = 100% may drop to CTR = 30% after 5000 hours, leaving the design starving for output current. Always size for the minimum CTR over temperature and lifetime.
• Forgetting the LED drive resistor. The LED is a forward-biased diode; without a series resistor it is a short circuit and burns up at first power-on. Compute R = (V_drive − V_F) / I_F.
• Treating the 4N25 as a digital part. Its 10 μs turn-off is a bandwidth ceiling, and its CTR drift makes its on-resistance unpredictable. Above a few kHz, use the 6N137 family.
• Ignoring CMTI in gate-driver applications. When a high-side MOSFET switches the bus, dV/dt across the optocoupler can hit 50 kV/μs. A part with CMTI of 10 kV/μs will glitch — your gate fires when it shouldn't, the bridge cross-conducts, the inverter dies. Always pick a gate-driver optocoupler with CMTI well above the worst-case dV/dt of your switching node.
• Trusting isolation voltage as a working voltage. A 5 kV-rated part is rated for a one-minute hi-pot test. The maximum continuous working voltage across the barrier is set by creepage, clearance, and pollution-degree rules (IEC 60664-1) — often a few hundred volts on the same part. Always read the working-voltage spec, not just the isolation rating.
• Using a phototransistor optocoupler for analog signals. CTR is nonlinear in I_F, drifts with temperature, and ages with time. A plain optocoupler in an analog feedback path bakes those nonlinearities into your control loop. Use a linear optocoupler (HCPL-7800) or a digital isolator + ADC.