Interactive simulation
Inductive kickback simulation
Switch a coil off without a diode and watch the spike that kills transistors.
Watch the instant the transistor opens: the coil's current cannot stop, so the collector node flies to whatever voltage keeps it flowing — into avalanche with no diode (drawn at 300 V here; the real ceiling is your transistor's avalanche rating), or a harmless V + 0.7 through the flyback diode. Adding a series resistor kills the current faster but pushes the node back up by i × Rs — the trade every relay driver makes. The plot's window is always eight coil time constants (8 × L/R), so one release fills it whatever L and R you pick: raising L stretches the same shape instead of flattening it.
What you can change
- Coil inductance
- Coil resistance
- Supply
- Snubber R
