What this dual-channel optocoupler does for your isolation barrier
The HCPL2531SDM is a dual-channel optocoupler from onsemi that provides 5000Vrms of galvanic isolation between a DC input and a transistor output — enough to separate a 24V industrial field bus from a 3.3V logic side without ground loops. Each channel can sink up to 8mA at the output, with a typical turn-on time of 250ns and turn-off of 260ns, which is fast enough to pass 1 MHz digital signals or isolated PWM gate drives.
Sizing the load resistor for the CTR spread
The current transfer ratio (CTR) is specified between 19% and 50% at a forward current of 16mA — that means at the minimum CTR, the output transistor can only pull 3mA (19% of 16mA). If you need a logic-high output, the pull-up resistor must be sized so that 3mA is enough to drop the output voltage below the logic threshold; at the maximum CTR the same resistor will saturate the output faster, so the timing stays consistent. Compared to the through-hole HCPL2530M which offers 2500Vrms isolation and slower 450ns/500ns switching, the HCPL2531SDM doubles the isolation voltage and nearly halves the propagation delay for the same dual-channel function.