Each cell follows the single-diode model, solved here for voltage V at a given current I via Newton–Raphson:
I = I_L − I₀(e^(V/nV_T) − 1) − V/R_sh
I_L ∝ irradiance G (fully shaded ⇒ I_L ≈ 0)
Cells in a string are wired in series, so the same current I flows through all of them — the string can only pass as much current as its worst cell allows. When a shaded cell's I_L drops below the string current, that cell can no longer supply I by generating power: instead it is driven into reverse bias (negative V) to force the deficit through its own shunt resistance R_sh. Since P = I·V, a reverse-biased cell doesn't generate — it dissipates power as heat, which is exactly how field "hotspots" that crack cell glass form.
A bypass diode wired antiparallel across a substring turns on once that substring's voltage drops below about −0.5 V and diverts the string current around it instead of through the shaded cells. This caps the reverse voltage (and the heat) each cell must absorb, at the cost of losing that substring's power contribution entirely.
- Shading buttons — set which of the 18 cells (I_L → 0) are shaded.
- Bypass diodes — toggle the three per-row protection diodes on/off.
- Irradiance — sets I_L for every unshaded cell.
- String current — the operating point (set by the external load); push it past a shaded cell's I_L to force reverse bias.
Approximation: reverse-region avalanche breakdown is modeled as a soft drop in R_sh below about −14 V rather than a full breakdown curve — enough to show the physical mechanism without needing manufacturer breakdown data.