Capacitors, Inductors and Conversion
What an Optocoupler Does—and Does Not Do
Learn input/output isolation, transfer ratio and output logic.
You need to size or verify a real system using “What an Optocoupler Does—and Does Not Do”, with units, duty and limits that match the application.
Follow a signal across an optical isolation barrier, translate current-transfer ratio into guaranteed output current, and design the pull-up and logic thresholds without assuming that one isolation-voltage number makes a system safe.
- Select or check first
- Isolation transfers information, not energy for the output circuit. The output side still needs its own supply or bias path. A phototransistor optocoupler does not reproduce input voltage, provide a precision current gain, interrupt unlimited fault energy or automatically satisfy a product safety standard.
- Controlling assumption
- Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
- Top risk
- “5 kV isolation” does not authorize a 5 kV working circuit. A short-duration isolation test voltage is not necessarily the permitted continuous working voltage. Creepage, clearance, repetitive peak voltage, insulation class and the applicable standard must all agree.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “What an Optocoupler Does—and Does Not Do”
Electrical-engineering basics
Electrical behaviour becomes easier to reason about when every voltage has a reference and every current has a complete return path.
- Voltage
- Voltage is potential difference between two points. A voltage statement is incomplete unless both points or its reference are known.
- Current
- Current is charge flow through a path. It does not disappear after a component; in a complete circuit it returns to the source.
- Resistance and impedance
- Resistance opposes steady current. Impedance extends that idea to changing signals and can depend on frequency through capacitance and inductance.
- Power and heat
- Electrical power is the rate of energy transfer. Some becomes useful output and some becomes heat that must stay within component and enclosure limits.
- Rating versus operating point
- A rating is a limit or qualified condition, not a target. The operating point must remain inside all relevant voltage, current, power, temperature and timing limits.
How to use this article: Draw the source, load, reference and return path before calculating. Treat exposed or stored electrical energy as hazardous until isolation is proved.
On this page
A common phototransistor optocoupler contains an input LED and a light-sensitive output transistor in one package. Input current produces light; the detector responds on the other side of an insulation barrier. The two circuits can use different reference potentials because the intended signal does not need a metallic conductor across that barrier.
Diagram description
A conceptual package cutaway places an illuminated input die opposite a detector die. Arrows show optical transfer and text states that conductors do not cross the barrier.
First set a safe LED current
Input series-resistor screen
R_IN = (V_DRIVE − V_F)/I_F
Use worst-case drive voltage, data-sheet LED forward-voltage limits, controller output capability and resistor dissipation. The absolute-maximum LED current is not a normal design target. Reverse voltage can also be tightly limited for an optocoupler LED, so AC or bipolar inputs may need an antiparallel diode or an AC-input device.
- R_IN
- input current-limiting resistance, Ω
- V_DRIVE
- actual voltage applied to the resistor-plus-LED path, V
- V_F
- LED forward voltage at the relevant current and temperature, V
- I_F
- LED forward current, A
For a 5.0 V drive, an assumed 1.2 V LED drop and I_F = 5 mA, the arithmetic gives 760 Ω. A designer then selects a standard value and rechecks current at every V_F and drive-voltage corner.
CTR relates input LED current to output collector current
Diagram description
A four-pin optocoupler and two current circles are connected by a light-transfer arrow. The equation CTR equals collector current divided by LED current times 100 percent appears above them.
Current-transfer ratio
CTR = (I_C/I_F) × 100%; I_C ≈ (CTR/100%)I_F
The second form predicts current only in the data-sheet operating region, usually before deep output saturation. For robust design, use the guaranteed minimum CTR at the relevant I_F, V_CE and temperature, then apply any documented aging allowance. Use maximum CTR where excessive output current or storage time matters.
- CTR
- current-transfer ratio, percent
- I_C
- phototransistor collector current, A
- I_F
- input LED forward current, A
If guaranteed minimum CTR is 40% at I_F = 5 mA under the chosen conditions, the guaranteed active-region collector-current basis is 0.40 × 5 mA = 2 mA—not a typical-value prediction.
| Influence | Design consequence |
|---|---|
| CTR bin and production spread | Use the exact ordering suffix and guaranteed minimum/maximum |
| LED current | CTR can change substantially away from its specified test current |
| Collector-emitter voltage | The output operating region and available collector current change |
| Temperature | LED efficiency and phototransistor gain shift |
| LED aging | Light output can decline, reducing CTR over life |
| Output saturation | Simple active-region CTR arithmetic no longer predicts switching time or voltage accurately |
The output circuit turns photocurrent into a logic voltage
Diagram description
Two circuit states show the same collector pull-up and grounded emitter. With the LED off, the output node is high; illumination turns on the phototransistor and the node becomes low.
Pull-up current and low-level requirement
I_PULLUP = (V_CC − V_OL)/R_P; require I_C(sink) ≥ I_PULLUP + I_LOAD
Choose R_P large enough that the worst-case optocoupler can pull the node below the receiver's V_IL maximum, yet small enough to charge node capacitance and overcome leakage for a valid high. Include receiver input current and any external loads.
- V_CC
- output-side pull-up supply, V
- V_OL
- target low output voltage, V
- R_P
- collector pull-up resistance, Ω
- I_LOAD
- other current drawn from the output node, A
With V_CC = 5 V, R_P = 4.7 kΩ and V_OL = 0.4 V, the pull-up demands about 0.98 mA. A 2 mA worst-case collector-current budget has margin for that resistor, but receiver current, temperature and CTR aging still need inclusion.
Rise-time intuition
τ_node ≈ R_P C_node
A larger pull-up reduces sink-current demand but normally makes the rising edge slower. Deep phototransistor saturation can add storage delay on turn-off. Data-sheet switching tests use specific resistors, voltages and input pulses; copy neither the time nor the resistor without matching those conditions.
- τ_node
- first-order node time constant, s
- C_node
- phototransistor, receiver, wiring and PCB capacitance, F
A 10 kΩ pull-up driving 100 pF has an RC time constant of about 1 μs before phototransistor storage and receiver threshold effects are included.
| Input LED | Phototransistor | Output risks to check |
|---|---|---|
| Off | Leakage only | Leakage × pull-up resistance, receiver leakage and noise must still leave a valid high |
| On | Sinking current | Minimum CTR and available LED current must produce a valid low |
| Changing | Charging, discharging or saturated | Propagation delay, edge rate, storage time and pulse-width distortion |
| Failed open | Usually off | System may interpret a permanent high; decide whether that is safe |
| Barrier transient | Parasitic displacement current | Common-mode transient immunity and filtering may matter in fast power circuits |
A component rating is only one part of an isolation design
| Question | What to verify |
|---|---|
| What voltage is present? | Working, repetitive peak, transient and test voltages are different specifications |
| What insulation is required? | Functional, basic, supplementary and reinforced insulation serve different purposes |
| What environment? | Pollution degree, material group, altitude, overvoltage category and contamination affect spacing |
| What geometry? | Internal insulation, external creepage, clearance, PCB slots and copper spacing all contribute |
| Which approvals? | Use the exact package option, safety certificates and end-product standard |
| What can fail? | Input resistor, LED, package, output transistor, PCB contamination and adjacent copper need fault analysis |
Design a digital optocoupler channel from both sides
| Step | Action |
|---|---|
| 1 | Define isolation purpose, working voltage, transient environment and applicable standard |
| 2 | Select the exact optocoupler package and approval option |
| 3 | Choose LED current from guaranteed CTR, life, temperature and driver limits |
| 4 | Calculate input resistance at voltage and V_F extremes |
| 5 | Choose pull-up from V_IL, V_IH, minimum CTR, leakage and speed requirements |
| 6 | Check dissipation, switching waveforms and common-mode transients |
| 7 | Verify creepage and clearance across the complete PCB and enclosure |
| 8 | Test logic corners and safe response to open/short faults |
Input-current and pull-up calculations use the resistor methods in Combining Resistors and Building Voltage Dividers, while the distinction between a pulled-up idle state and an active sink is developed in Pull-Ups, Current Shunts and Failed Components.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for What an Optocoupler Does—and Does Not Do?
Isolation transfers information, not energy for the output circuit. The output side still needs its own supply or bias path. A phototransistor optocoupler does not reproduce input voltage, provide a precision current gain, interrupt unlimited fault energy or automatically satisfy a product safety standard.
Which assumption can change the conclusion?
Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
What can go wrong if I copy the example directly?
“5 kV isolation” does not authorize a 5 kV working circuit. A short-duration isolation test voltage is not necessarily the permitted continuous working voltage. Creepage, clearance, repetitive peak voltage, insulation class and the applicable standard must all agree.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- VO617C — Low Input Current Optocoupler, Phototransistor OutputVishay Semiconductors · 2026-10-13Manufacturer data sheet reviewed for CTR bins and test conditions, switching characteristics, maximum ratings, isolation parameters and package creepage/clearance.
- Phototransistor Optocouplers Selection GuideVishay Intertechnology · 2026-10-13Manufacturer selection guide reviewed for package options, CTR definition, creepage and phototransistor optocoupler applications.
- VO617C Product InformationVishay Intertechnology · 2026-10-13Manufacturer product record reviewed for current documents, approvals and product-level isolation claims.
- RC Circuits — University Physics Volume 2OpenStax, Rice University · 2026-10-13University physics reference reviewed for the first-order pull-up-capacitance time-constant model and its limitations.