MCU Interfaces

PC817 Optocoupler Galvanically Isolated Logic Interface

Sharp PC817 4-pin phototransistor optocoupler circuit providing 5000 Vrms galvanic isolation between field digital inputs and sensitive microcontroller logic with antiparallel reverse protection and passive collector pull-up.

Galvanic Isolation Voltage: 5000 Vrms (1 min dielectric withstand)

Components: 4
Formulas: 4
Gotchas: 4

Open in Web CAD

Verified source

Galvanic Isolation Voltage: 5000 Vrms (1 min dielectric withstand)
Components: 4
Formulas: 4
Gotchas: 4

Click any component in the schematic above to inspect its electrical ratings, function, and connections.

Circuit overview

The PC817 optocoupler isolated logic interface provides robust, unidirectional galvanic isolation between noisy or high-voltage external field circuits (such as 24 V industrial automation PLCs, long sensor lines, and motor encoders) and sensitive 3.3 V or 5.0 V microcontroller logic systems.

Inside the compact 4-pin DIP-4 package, electrical isolation is achieved by transmitting signals optically across a transparent internal silicone dielectric barrier capable of withstanding 5000 Vrms. An input gallium arsenide (GaAs) infrared emitting diode (IRED) with a forward drop of VF ≈ 1.2 V emits light proportional to the forward current IF set by input series resistor R1. Photons traverse the isolation gap and illuminate the base region of a high-voltage NPN silicon phototransistor (VCEO = 80 V).

When a logic HIGH voltage is applied at the input port P_IN, forward current IF flows through R1 and the IRED to the field ground GND1. The emitted photons induce base photocurrent, driving the phototransistor into deep collector-emitter saturation (VCE(sat) ≤ 0.1 V to 0.2 V). This vigorously pulls the isolated output port P_OUT down to GND_ISO against collector pull-up resistor R2. When the input voltage drops below the IRED conduction threshold (VF ≈ 1.0 V), the diode turns off, base photocurrent ceases, and R2 pulls P_OUT cleanly up to VCC_ISO (+5.0 V). The circuit thus acts as an inverting digital isolator with sub-microsecond edge integrity.

To ensure extreme field durability, antiparallel high-speed diode D1 (1N4148) is connected directly across the IRED terminals. Because the internal GaAs emitter diode possesses a low reverse breakdown rating of VR = 6.0 V, inductive kickback, cable discharge, or reverse polarity wiring faults could otherwise cause destructive avalanche breakdown. Diode D1 safely clamps reverse voltage transients to approximately -0.7 V, preserving optocoupler health.

By physically and galvanically segregating the input return ground (GND1) from the isolated system ground (GND_ISO), this interface completely halts ground loops, rejects common-mode noise spikes (high dV/dt), and prevents hazardous ground voltage differentials from damaging digital host controllers.

Key Technical Specifications

Galvanic Isolation Voltage	5000 Vrms (1 min dielectric withstand)
Operating Logic Level	+5.0 V DC (scalable from 3.3 V to 24 V)
Nominal Isolated Supply Rail	+5.0 V DC (compatible with 3.3 V to 36 V)
Input Forward Current (IF)	9.74 mA nominal (at 5.0 V input)
Output Collector Current (IC)	1.04 mA in saturation
Current Transfer Ratio (CTR)	80% to 160% (Rank A at IF = 5 mA)
Collector-Emitter Breakdown (VCEO)	80 V DC minimum
Saturation Voltage (VCE(sat))	≤ 0.2 V max (0.1 V typical)
Response Switching Time (tr / tf)	4 μs / 3 μs typical (at RL = 100 Ω)

Theory of Operation & Working Principle

Circuit Analysis

The PC817 galvanically isolated logic interface operates through five sequential electrical and optical phases:

1. **Input Signal Assertion & IRED Forward Biasing:** When the external controller or field sensor asserts a logic HIGH level (nominal +5.0 V) at input port P_IN, current flows through series metal-film current-limiting resistor R1 (390 Ω). Once the input voltage overcomes the GaAs diode forward knee (VF ≈ 1.2 V), forward current IF = (VIN - VF) / R1 ≈ (5.0 V - 1.2 V) / 390 Ω ≈ 9.74 mA establishes stable forward conduction.

2. **Optical Photon Transmission Across Galvanic Barrier:** The forward current excites radiative recombination in the GaAs active region, producing infrared photons at a peak wavelength of approximately 940 nm. These photons traverse the transparent internal optical silicone barrier, maintaining complete electrical isolation with a continuous withstand rating of 5000 Vrms and typical coupling capacitance of only 0.6 pF.

3. **Phototransistor Conduction & Collector Saturation:** Photons penetrating into the light-sensitive base of the NPN phototransistor generate electron-hole pairs, establishing base photocurrent Iph. With a minimum Current Transfer Ratio (CTR) of 80% to 160% (Rank A), the transistor provides ample collector sink current IC = IF × CTR ≈ 9.74 mA × 0.8 ≈ 7.8 mA of available sinking capability, vastly exceeding the ~1.04 mA demanded by pull-up resistor R2. This forces the phototransistor into hard saturation, driving collector voltage VCE down to VCE(sat) ≤ 0.1 V.

4. **Output Pull-Up & Inverting Logic Restoration:** When the input signal transitions to logic LOW (0 V), forward current collapses to zero. Emitted photon flux ceases, and photocarriers in the base region recombine over the storage and fall time (ts + tf ≈ 3 μs to 5 μs). As the phototransistor turns off into high-impedance cutoff (collector dark current ICEO ≤ 100 nA), collector pull-up resistor R2 (4.7 kΩ) restores output port P_OUT up to the isolated supply rail VCC_ISO (+5.0 V).

Design Equations & Calculations

Input Forward Current

I_F = V_IN − V_FR₁

Calculates the forward drive current flowing through the internal GaAs infrared emitting diode from a logic HIGH input voltage, accounting for the 1.2 V diode forward voltage drop.

Symbol	Parameter	Typical value
I_F	IRED forward current	9.74mA
V_IN	Input logic HIGH voltage	5.0V
V_F	GaAs LED forward voltage drop	1.2V
R₁	Input current-limiting resistor	390Ω

Required Current Transfer Ratio and Saturation Current

CTR_req = I_CI_F × 100% = V_{CC_ISO} − V_CE(sat)R₂ · I_F × 100%

Determines the minimum Current Transfer Ratio required to sink the pull-up load current and pull the output down to VCE(sat), ensuring valid logic LOW detection.

Symbol	Parameter	Typical value
CTR_req	Required Current Transfer Ratio	10.7%
I_C	Collector pull-up sink current	1.04mA
V_{CC_ISO}	Isolated supply voltage	5.0V
V_CE(sat)	Collector saturation voltage	0.1V
R₂	Collector pull-up resistor	4.7kΩ
I_F	IRED forward drive current	9.74mA

Overdrive Saturation Margin Factor

K_OD = CTR_minCTR_req

Quantifies the safety factor by which the optocoupler's guaranteed minimum CTR exceeds the required saturation CTR, ensuring reliable operation over 10-year LED aging and elevated temperature.

Symbol	Parameter	Typical value
K_OD	Overdrive safety margin factor	7.5dimensionless
CTR_min	Datasheet minimum CTR (Rank A)	80%
CTR_req	Minimum CTR needed for saturation	10.7%

Estimated Switching Cut-Off Frequency

f_c ≈ 12π · R₂ · C_L

Estimates the -3 dB switching frequency corner imposed by the collector pull-up resistor and total output nodal capacitance (phototransistor junction capacitance plus wiring capacitance).

Symbol	Parameter	Typical value
f_c	Switching cut-off frequency	33.8kHz
R₂	Collector pull-up resistance	4.7kΩ
C_L	Total output load capacitance	100pF

Bill of Materials (BOM)

Every part needed to build this circuit. Select a reference designator to inspect it on the schematic.

Component	Value / rating	Package	Role in circuit	Reference
PC817X1NSZ0F Sharp Corporation PC817X1NSZ0F 4-pin through-hole DIP package.	PC817	DIP-4	Phototransistor Output Optocoupler (5000 Vrms isolation, Rank A CTR 80%..160%)	PC817X1NSZ0F pinout & datasheet
MFR-25FRF52-390R Yageo 390 Ω 1% 250 mW metal film axial resistor.	390 Ω	Axial 0207	Input IRED current-limiting resistor (IF ≈ 9.7 mA at 5.0 V IN)	—
1N4148 onsemi 1N4148 100 V 200 mA 4 ns fast recovery diode.	1N4148	DO-35	Antiparallel transient reverse protection clamp diode	—
MFR-25FRF52-4K7 Yageo 4.7 kΩ 1% 250 mW metal film axial resistor.	4.7 kΩ	Axial 0207	Isolated collector pull-up load resistor	—

Practical Applications & Target Domains

Galvanically isolated digital logic inputs for STM32, Arduino, ESP32, and Raspberry Pi GPIO

24 V DC industrial PLC digital I/O interface modules and line receivers

Ground loop breaking in distributed sensor networks and motor control environments

Isolated contact closure, limit switch, and optical interrupter sensing

Mains AC zero-cross detection modules (downstream of bridge rectifier)

Critical Design Traps & Engineering Gotchas

Traps found in real builds, with the fix for each.

Critical

Bridging Galvanic Isolation Grounds on PCB Layout

Accidentally tying the input signal return ground (GND1) to the isolated microcontroller system ground (GND_ISO) on the PCB ground plane or power supply defeats the entire 5000 Vrms isolation barrier, allowing ground loops and high-voltage industrial surges to reach sensitive digital logic.

Mitigation

Maintain strict physical segregation on the PCB layout with a minimum 6.3 mm (or 8.0 mm for reinforced isolation) keepout creepage gap beneath the DIP-4 package, keeping all copper planes, vias, and traces separated between input and output domains.

Warning

Long-Term CTR Degradation and Temperature Derating

GaAs infrared emitter diodes suffer from gradual luminous flux degradation (typically 10% to 30% reduction over 5 to 10 years of continuous operation) and reduced efficiency at high operating temperatures (up to -0.5%/°C), causing marginal circuits designed with 1:1 CTR to drop out of saturation.

Mitigation

Incorporate an overdrive safety margin factor of K_OD ≥ 2.0 to 3.0 when calculating R1 and R2, ensuring the minimum initial collector sinking capability exceeds the pull-up current by at least 200% across the operating temperature range.

Warning

Slow Turn-Off Delay with High Pull-Up Resistor Values

Selecting an excessively high collector pull-up resistor (such as R2 = 47 kΩ or 100 kΩ) to save power dramatically increases turn-off rise time (tf > 20 μs to 50 μs) due to phototransistor collector-base junction capacitance (Cob ≈ 10 pF) and stray PCB capacitance, corrupting fast serial data or PWM pulses.

Mitigation

Keep R2 between 2.2 kΩ and 4.7 kΩ for digital switching up to 20 kHz to 50 kHz. For higher baud rates (>100 kbps), use a high-speed digital isolator or add a buffer stage (e.g., 2N3904 or Schmitt trigger inverter).

Critical

IRED Reverse Voltage Breakdown Without Antiparallel Diode

Connecting long field cables without an antiparallel clamp diode exposes the PC817 input LED to inductive spikes, ESD, or reverse cabling faults. Because GaAs LEDs have a fragile reverse breakdown rating of VR = 6.0 V, even brief negative transients cause irreversible degradation or short circuits.

Mitigation

Always populate fast recovery clamp diode D1 (1N4148) in antiparallel across PC817 pins 1 and 2, which conducts whenever input polarity reverses and clamps transient negative voltages to -0.7 V.

Primary sources

https://global.sharp/products/device/lineup/data/pdf/datasheet/pc817xxnsz1b_e.pdf

Help keep this page accurate

Built, simulated or reviewed this circuit? Confirm it, or report a part, value or connection that is wrong.

Report an error in the schematic Confirm this schematic

Opens a public form on GitHub with this page already filled in; a free GitHub account is needed. How reports and confirmations work

Theme

Height

HTML snippet

<iframe src="https://circuitbase.tech/embed/schematic/pc817-optocoupler-isolated-logic-interface?theme=paper" width="100%" height="500" frameborder="0" loading="lazy" style="border: 1px solid #e5e7eb; border-radius: 6px; max-width: 100%;" title="PC817 Optocoupler Galvanically Isolated Logic Interface — CircuitBase Interactive Schematic"></iframe>

PC817 Optocoupler Galvanically Isolated Logic Interface

Circuit overview

Key Technical Specifications

Theory of Operation & Working Principle

Circuit Analysis