24V Input Optocoupler Selection

Question

Problem: 24V Input Optocoupler Selection. Solve this hands-on electronics task on EWskills.

Accepted Answer

1. Component Purpose

Field-side 24V digital inputs carry noise from industrial wiring. Connecting these directly to a 3.3V MCU input would expose it to ground potential differences, common-mode noise, and transient spikes.

An optocoupler receives the 24V signal on its input LED (through a series resistor: R = (24V − 1.2V) / 10mA ≈ 2.2kΩ) and transfers the logic state to the MCU side through an isolated phototransistor output.

Required: Transistor Output · 1-Ch · ≥3000Vrms isolation · CTR ≥50% · Vout ≥30V · DIP-4 · Through Hole

2. How to Find It on DigiKey

Go to DigiKey.com → Isolators → Optoisolators → Transistor, Photovoltaic Output

Apply these filters:

Voltage - Isolation → 3000Vrms or higher
Output Type → Transistor
Number of Channels → 1
Current Transfer Ratio (Min) → 50% or higher
Voltage - Output (Max) → 30V or higher
Package / Case → 4-DIP
Part Status → Active

Tip: The series resistor on the 24V input LED is critical. Calculate: R = (Vin − Vf) / If = (24 − 1.2) / 0.01 = 2.28kΩ. Use standard 2.2kΩ.

3. Key Specifications & What They Mean

Specification	Required	Why
Output Type	Transistor	Simple ON/OFF signal isolation.
Channels	1	Single digital input.
Isolation	≥3000Vrms	Protects MCU from field-side noise.
CTR (Min)	≥50%	Reliable signal at ~10mA LED drive.
Vout Max	≥30V	Margin for output-side voltage.
Package	4-DIP	Through hole for prototyping.
Temp Range	−40°C to +85°C	Standard industrial range.
RoHS	Yes	Compliance required.

4. Selecting a Safe, Production-Ready Part

The filtered list will show matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?

Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide

5. Filtered DigiKey Link

➡ View Matching Parts — Transistor, 1-Ch, ≥3000Vrms, CTR≥50%, DIP-4

64. 24V Input Optocoupler Selection