The rule of thumb is that when a diode is fully on, it looks like a short circuit (ideally). So (to make this easy), say I have 100 V source and a diode which breaks down at 10V. If my power supply/source impedance is 1 Ohm, will the current through my diode be:
- I = (Vsupply - Vdiode)/Rsupply = (100 - 10)/1 = 90/1 = 90 Amps, or
- I = Vsupply/Rsupply = 100/1 = 100 Amps?