Isolation Monitor, switched-divider

by VladM

22 parts

A 400V pack floats between P and N, with chassis as the reference. Riso+ and Riso− (2MΩ each) are its healthy isolation; "fault", a 1MΩ pot used as a rheostat (500kΩ at 50%), is a leak from P to chassis. The monitor connects a 1MΩ measuring chain (Rm 990kΩ + Rt 10kΩ, the tap) from P to chassis while CLK is high (S1) and from N to chassis while it is low (S2) — one second each, about 0.8s on screen at ×0.25. The formula. With U1 = V(P → chassis) in the first half (tapA × 100), U2 = V(chassis → N) in the second (−tapB × 100), R0 = 1MΩ and V = 400V: R_iso+ = R0·(V − U1 − U2)/U2 R_iso− = R0·(V − U1 − U2)/U1 Worked value, the fault at 50%: tapA 0.500V → U1 = 50V; tapB −2.500V → U2 = 250V. R_iso+ = 1 MΩ × 100/250 = 400 kΩ (2 MΩ ∥ 500 kΩ); R_iso− = 1 MΩ × 100/50 = 2 MΩ. The lower, 400kΩ = 1,000 Ω/V, passes a 500 Ω/V limit (200kΩ).