How to Choose an Optocoupler: CTR, Speed and Isolation

1. What an optocoupler does

An optocoupler passes a signal as light across an isolation barrier: an LED on one side, a photodetector on the other. No electrical connection means no ground loop and no shock path — which is why they sit between mains and low-voltage control, or between noisy power stages and sensitive logic.

2. CTR is the number to watch

Current transfer ratio (CTR) is the output current divided by the input LED current, typically 50–600%. CTR falls with age and with LED drive current, so design with margin: assume ≤ 50% of the datasheet CTR at end of life, and keep the LED current inside the guaranteed test window.

• Choose the CTR grade that leaves ≥ 2× design margin
• CTR drifts with temperature and over 10+ years
• For digital transfer, a photo-IC or high-speed coupler avoids CTR dependency

3. Isolation and creepage

Spec	Typical values	What it means
Isolation voltage	2500–5000 VRMS	1-minute withstand test
Creepage / clearance	≥ 5 mm / 8 mm for mains	physical gap on the PCB
CTR	50–600%	gain across the barrier
Speed	phototransistor kHz; photo-IC 10–50 Mbps	max data rate

Isolation voltage is a test rating, not a working voltage. For mains-referenced designs follow the creepage and clearance rules of your safety standard (IEC 62368 / 60950), and keep the isolation slot clean and unbroken.

4. Which type for which job

• Phototransistor — general digital/analog isolation, up to ~100 kHz
• Photo-IC / logic output — clean digital levels, 1–50 Mbps
• Photovoltaic / gate-drive — driving MOSFET/IGBT gates
• Linear / analog — feedback in isolated supplies, needs matched CTR

5. How to choose

• Set the isolation working voltage and required creepage first
• Then pick the speed class; only then choose the CTR grade
• Budget ≥ 2× CTR margin at end of life
• Keep an ESD/TVS part on the connector side of the barrier

Related products

• Optocoupler Series
• Transistor (SOT-23) Series
• TVS/ESD Protection
• SMD LED Series