Signal across an isolation barrier

by ourcircuits

6 parts

A 1 kHz logic pulse crosses an isolation barrier. The input side drives the coupler's LED — a real junction sitting at a measured 2.3025 V forward while 9.991 mA flows through its 270 Ω resistor — and the output side lives on its own 12 V supply whose common is biased 48.000 V above the input ground, so the two halves are nowhere near the same potential and the signal crosses anyway: the collector sits at 52.507 V in absolute terms and swings 4.507–12.000 V against its own common. Inside the part there is no conductance whatsoever between the LED pins and the transistor pins; the only coupling is a behavioural current source, I_C = CTR × I_LED, measured at 4.9954 mA against 9.9908 mA. The simulator needs one common reference node, so that 48 V bias source stands in for the galvanic gap a real barrier would simply leave open — and where a real coupler's CTR drifts with current, temperature and age, this one is a fixed number.