The 4N25 and 4N35 are widely used phototransistor optocouplers for transferring signals between electrically isolated circuits. Although they share the same basic operating principle and similar pin configurations, their electrical characteristics can vary by manufacturer, device grade, and test conditions. This article compares the 4N25 and 4N35 in terms of CTR, electrical ratings, switching behavior, real-world performance, and circuit requirements to help you choose the right device or determine whether they can be safely substituted.
4N25 vs 4N35 Differences
The 4N25 and 4N35 use the same basic LED-to-phototransistor isolation architecture, so the most useful comparison is between the electrical characteristics that can affect circuit performance and interchangeability.
| Parameter | 4N25 | 4N35 |
|---|---|---|
| CTR | Manufacturer-, grade-, and test-condition-dependent; verify the guaranteed minimum CTR in the specific datasheet | Manufacturer-, grade-, and test-condition-dependent; some product families or grades may specify a higher minimum CTR, so verify the specific datasheet |
| Output-current capability | Depends on LED current and guaranteed CTR | Depends on LED current and guaranteed CTR |
| Switching performance | Depends on load, LED drive, saturation, and device specifications | Depends on load, LED drive, saturation, and device specifications |
| Electrical interchangeability | Must be verified from the specific datasheet | Must be verified from the specific datasheet |
| Main selection consideration | Guaranteed CTR, electrical ratings, and switching requirements | Guaranteed CTR, electrical ratings, and switching requirements |
The practical distinction is how effectively LED current produces phototransistor collector current. However, CTR varies by manufacturer, grade, and test conditions, so the 4N35 does not always guarantee higher CTR than the 4N25. Selection should be based on the specific datasheet and circuit requirements.
What Are the 4N25 and 4N35?
The 4N25 and 4N35 are single-channel phototransistor optocouplers containing an infrared LED on the input side and an NPN phototransistor on the output side. The two elements are optically coupled while remaining electrically isolated.
When current drives the LED, its infrared light activates the phototransistor, allowing a signal to cross the isolation barrier without a direct conductive connection. Their basic operating principle is essentially the same; practical differences depend primarily on the electrical characteristics specified by the particular manufacturer and device grade.
4N25 vs 4N35 Pinout and Internal Circuit
A common 6-pin configuration for these optocouplers is:
| Pin | Function |
|---|---|
| 1 | Anode |
| 2 | Cathode |
| 3 | No connection |
| 4 | Emitter |
| 5 | Collector |
| 6 | Base |
Pins 1 and 2 connect to the internal LED, while pins 4 and 5 provide the phototransistor emitter and collector connections.
Pin 6 exposes the phototransistor base. In many simple circuits, the base is left open. In other designs, a resistor may be connected between the base and emitter to influence leakage, switching behavior, and sensitivity.
Common DIP versions use the same basic pin arrangement, making physical substitution possible in many circuits. However, pin compatibility does not guarantee electrical interchangeability. Before substitution, verify the relevant electrical ratings and guaranteed CTR in the specific manufacturer's datasheet.
4N25 vs 4N35 Electrical Characteristics
Electrical specifications determine whether the 4N25 or 4N35 can meet the voltage, current, isolation, and switching requirements of a circuit. The most important characteristics include current transfer ratio, input LED characteristics, output-side ratings, isolation ratings, and switching speed.
Current Transfer Ratio (CTR)
Current transfer ratio, or CTR, indicates how effectively input LED current produces collector current on the phototransistor output.
The basic formula is:
CTR = (IC / IF) × 100%
Where:
• IC = phototransistor collector current
• IF = LED forward current
CTR is not a fixed transistor gain. It varies with LED forward current, collector-emitter voltage, temperature, individual device variation, and LED aging. It is also manufacturer- and device-grade-dependent.
For reliable circuit design, use the guaranteed CTR specified in the applicable datasheet under conditions relevant to the application, rather than relying on a typical value or assuming that one part number always has higher CTR than the other. Section 6 demonstrates how different hypothetical CTR values affect collector-current capability in a practical microcontroller interface.
Input LED Characteristics
The input side of the 4N25 and 4N35 behaves like an infrared LED and requires proper current limiting. Important specifications include forward voltage, forward current, peak forward current, reverse-voltage rating, and power dissipation.
The LED current-limiting resistor can be estimated using:
RLED = (VIN − VF) / IF
Where:
• RLED = LED current-limiting resistance
• VIN = input or drive voltage
• VF = LED forward voltage
• IF = desired LED forward current
The selected resistor should provide the required LED current without exceeding the ratings of the optocoupler or driving circuit. Use the forward-voltage range and allowable current specified in the applicable manufacturer's datasheet.
Output-Side Ratings
The phototransistor output must match the voltage and current requirements of the controlled circuit. Important specifications include collector-emitter voltage, collector current, collector-emitter saturation voltage, collector leakage current, and output power dissipation.
The required collector current should also be compared with the current available from the guaranteed CTR at the selected LED current. A device may have a sufficiently high maximum collector-current rating but still be unable to provide the required current if its guaranteed CTR is too low. Saturation voltage and leakage current are also important in switching applications because they affect the output LOW level and OFF-state behavior.
Isolation Ratings
Both devices provide galvanic isolation between the input LED and output phototransistor. However, the specified isolation test voltage should not automatically be treated as the allowable continuous working voltage. For safety-critical designs, working voltage, creepage, clearance, insulation requirements, PCB layout, and applicable safety standards must also be considered.
Switching Speed
The 4N25 and 4N35 are general-purpose phototransistor optocouplers, rather than high-speed digital isolation devices. Their actual switching performance depends on LED drive current, collector load resistance, output capacitance, collector current, phototransistor saturation, and base-terminal configuration.
Driving the phototransistor deeply into saturation can increase stored charge and slow its turn-off response. For low-frequency control and status signals, this may not be significant. When faster digital communication or tighter timing is required, a high-speed optocoupler or digital isolator may be more appropriate.
How the 4N25 and 4N35 Perform in Real Circuits
Microcontroller, Logic, and Low-Current Signal Isolation
The 4N25 and 4N35 can isolate microcontroller GPIO and other low-current control signals.
A typical path is:
MCU GPIO → Current-Limiting Resistor → Optocoupler LED → Isolation Barrier → Phototransistor → Logic Input
The phototransistor is commonly used with a pull-up resistor and must sink enough current to produce a valid logic LOW. The key considerations are LED current, guaranteed CTR, pull-up resistance, and logic thresholds. Section 6 demonstrates this relationship numerically.
Power-Supply Feedback and Control
Phototransistor optocouplers are commonly used to transfer feedback or control information across the isolation barrier in power supplies. In an isolated feedback loop, changes in optocoupler collector current can influence the feedback signal seen by the primary-side controller.
CTR variation can therefore affect feedback-loop gain, control range, regulation behavior, and available operating margin. The design may need to account for minimum and maximum CTR, temperature effects, LED aging, and device-to-device variation rather than relying on a single typical value. In precision or tightly regulated power supplies, the optocoupler characteristics should be evaluated as part of the complete feedback-loop design.
AC and Mains Detection
The 4N25 and 4N35 may also be used to detect the presence or state of an AC signal. The optocoupler provides isolation, but it does not by itself make a mains-connected circuit safe. Appropriate current limiting, protection, insulation spacing, component voltage ratings, and other applicable safety measures remain necessary.
Practical Example: 4N25 vs 4N35 With a Microcontroller
Consider a microcontroller that must send a logic signal to an electrically isolated circuit.
The basic arrangement is:
MCU GPIO → LED Resistor → 4N25/4N35 → Pull-Up Resistor → Logic Input
Assume:
• GPIO voltage = 5 V
• LED forward voltage = 1.2 V
• LED current = 10 mA
The LED resistor is:
RLED = (5 V − 1.2 V) / 0.01 A
RLED = 3.8 V / 0.01 A
RLED = 380 Ω
Now consider the output using two hypothetical CTR cases. The 20% and 100% values below are used only to demonstrate how CTR affects potential collector current. They should not be interpreted as universal CTR specifications for the 4N25 and 4N35.
For a hypothetical CTR of 20%:
IC = 10 mA × 0.20
IC = 2 mA
For a hypothetical CTR of 100%:
IC = 10 mA × 1.00
IC = 10 mA
These calculations indicate the collector current associated with the assumed CTR values under the relevant conditions. They do not by themselves guarantee a particular saturated output LOW voltage in an actual switching circuit.
For a 5 V output with a 4.7 kΩ pull-up:
I = V / R
I = 5 V / 4700 Ω
I ≈ 1.06 mA
Under this simplified calculation, both hypothetical CTR cases indicate sufficient collector-current capability for the approximately 1.06 mA pull-up current. In an actual circuit, the required output LOW voltage, collector-emitter voltage, saturation behavior, temperature variation, logic-input threshold, and appropriate design margin must also be verified.
Reducing the pull-up resistance to 1 kΩ increases the approximate current requirement:
I = 5 V / 1000 Ω
I = 5 mA
If the guaranteed collector current under the relevant operating conditions is only about 2 mA, it would not provide sufficient margin for a pull-up path requiring approximately 5 mA. A device capable of substantially more collector current under the same conditions would provide greater design margin.
4N25 vs 4N35: Which Should You Choose?
The correct device depends on the electrical requirements rather than the part number alone.
| Requirement | Selection Consideration |
|---|---|
| Limited LED drive | Ensure sufficient output-current margin at the available LED current |
| MCU or logic interface | Check output LOW/HIGH margins, pull-up requirements, and switching time |
| Power-supply feedback | Evaluate CTR range, temperature behavior, aging, and impact on the control loop |
| Faster signal switching | Evaluate rise/fall times, saturation, and circuit loading |
| Existing circuit replacement | Match electrical ratings, package, pinout, isolation, and switching requirements |
| High-speed communication | Consider a high-speed optocoupler or digital isolator instead |
Common Problems When Using the 4N25 or 4N35
| Problem | Likely Causes | What to Check |
|---|---|---|
| Output Does Not Switch Fully | Insufficient LED current, inadequate CTR, incorrect LED resistor, excessive output load, unsuitable pull-up resistance, or incorrect pin connections | Verify LED drive, required collector current, pull-up resistance, load, and pin connections against datasheet limits. |
| Output Switching Is Too Slow | Deep phototransistor saturation, collector resistance, capacitance, LED drive level, or base-terminal configuration | Reduce unnecessary saturation and evaluate the collector resistor, capacitance, LED drive, and base connection. Use a faster optocoupler if necessary. |
| Circuit Works with One Part but Not the Other | Insufficient electrical margin or different device characteristics | Design around guaranteed electrical limits rather than the performance of an individual sample. |
| Circuit Becomes Unreliable at Temperature Extremes | CTR, leakage current, LED characteristics, and switching behavior vary with temperature | Verify operation across the required temperature range and provide sufficient design margin. |
Conclusion
The 4N25 and 4N35 are functionally similar optocouplers, but they are not automatically interchangeable. Choose between them based on LED drive, output-current capability, electrical and isolation ratings, and switching requirements. For applications requiring substantially faster or more predictable timing, consider a high-speed optocoupler or digital isolator.
Frequently Asked Questions [FAQ]
Q1. Can the 4N25 and 4N35 be used as direct replacements for each other?
In some circuits, yes, because common 4N25 and 4N35 DIP versions share a similar pinout and phototransistor architecture. However, pin compatibility does not guarantee electrical interchangeability. Before substitution, compare the specific manufacturers' datasheets for guaranteed CTR, voltage and current ratings, isolation requirements, switching characteristics, package details, and test conditions.
Q2. Why should minimum guaranteed CTR be used instead of typical CTR when designing a circuit?
Typical CTR represents expected performance rather than a guaranteed limit. Using the guaranteed minimum CTR helps ensure sufficient collector-current capability across permitted device variation and operating conditions, which is particularly important for logic interfaces and power-supply feedback.
Q3. How do LED current and pull-up resistance affect 4N25 or 4N35 switching performance?
LED current influences the collector current available through CTR, while the pull-up resistor determines how much current the phototransistor must sink to pull the output LOW. A smaller pull-up resistance requires more collector current. If the available collector current is insufficient, the output may not reach the required logic LOW level. Excessive saturation can also increase turn-off time, so both current margin and switching speed should be considered.
Q4. Why can a circuit work with one 4N25 or 4N35 but fail with another device of the same part number?
Individual devices can have different CTR and other electrical characteristics while still meeting the same datasheet specifications. If a circuit works only with higher-performing samples, it may have insufficient design margin. The circuit should therefore tolerate the full guaranteed operating range of the selected device.
Q5. When should you choose a high-speed optocoupler or digital isolator instead of a 4N25 or 4N35?
Consider an alternative when the application requires higher data rates, shorter or more predictable propagation delay, tighter timing, or faster switching than a general-purpose phototransistor optocoupler can reliably provide. High-speed optocouplers such as the 6N137 are better suited to many digital signals, while digital isolators can provide high-speed logic isolation without relying on LED-to-phototransistor CTR.
