This simulator models an NPN transistor used as a low-side switch for an LED load: a control source drives current through a base resistor, the transistor's base current (scaled by beta) determines whether it sits in cutoff, forward-active operation or saturation, and a separately powered supply drives current through the LED and its series resistor into the transistor's collector-emitter path. Adjust load supply, control voltage, base and load resistors, beta, drive mode (steady DC or slow PWM) and PWM timing, and inject base or collector-emitter faults to see how the switch responds.
• A real-time 3D cutaway workbench of the control source/pulse generator, the base resistor and input terminals, a magnified NPN package showing die and bond wires, the separate load supply, the LED and current-limiting resistor, the emitter return/current shunt, and an operating-region recorder, with home view, focus-selected-part, toggleable full enclosure, exploded view, auto-rotate and expand controls, tappable numbered components with callouts matching the diagram reference, and a labels toggle. • Nine live controls: load supply voltage, high control voltage, base resistor, LED series resistor, forward-active current gain (beta), a control-mode selector (steady DC or slow PWM demonstration), PWM repetition rate, PWM duty cycle, and a fault selector (healthy, open base path, open collector-emitter path, shorted collector-emitter path). • Play/pause, single-step (0.1 s) and larger-step (1 s) time controls, a playback-speed selector from real time to 1 minute per second, plus dedicated "Set input HIGH" and "Set input LOW" actions in addition to start/stop. • A Reset laboratory action, a live "what is happening" sequence narrative with operating-region status tokens and a readings table. • Nine live metrics: instantaneous control input, base current, collector/LED current, collector-emitter voltage, transistor power dissipation, series-resistor power dissipation, LED electrical power, actual forced beta (IC/IB, zero when IB is zero), and relative LED brightness. • A Curves & measurements tab with two charts (control vs. collector-emitter voltage; base current vs. collector current), the complete model equation set, and snapshot measurements. • An Experiments tab with four guided scenarios (saturated switch, underdriven base, PWM operation, shorted device), a model-verification bench of independent automated checks, and a timestamped event log with a copyable trial report. • A Learn & assess tab with guided lessons (drive the base, supply the load, reach saturation, diagnose the failed path), a knowledge-check quiz with reset, and a written scope/reference statement.
Base current follows IB = max(0, (Vcontrol − 0.7)/RB) — control voltage above the approximate 0.7 V junction drop drives current through the base resistor, while lower or zero control voltage leaves the transistor in cutoff with no base current. Collector current is then IC = min(β·IB, max(0, (Vs − 2 V − 0.2 V)/RL)): the transistor tries to deliver beta times the base current, but the load circuit's own supply voltage, LED drop and series resistance impose a hard ceiling. When beta times base current would exceed that load-limited ceiling, the device is in saturation and VCE approaches about 0.2 V; below that point it is in forward-active operation, where beta times base current is the limiting factor instead.
The underdriven-base experiment keeps the transistor forward-active (not saturated) by combining a large base resistor with low beta, producing reduced LED current and a higher collector-emitter voltage than the saturated case. The shorted collector-emitter fault is the sharpest diagnostic: with control voltage forced to zero, the load path still conducts and lights the LED because current now flows through the short rather than being controlled by the transistor at all — demonstrating that load current comes from the load supply, not the base circuit.
This is a piecewise NPN switch and LED model with fixed junction drops, a generic (non-manufacturer) beta and ideal PWM transitions. It does not include charge storage time, junction capacitance, reverse operation, self-heating, or device rating certification, and the illustrative package geometry should not be used for real-world pin identification. Component changes are treated as new design experiments at the retained state, not a model of physically swapping energized parts; use Reset laboratory to start a fresh trial.
The load circuit does — specifically the load supply voltage, the LED's forward drop, and the series resistor. Once beta times base current would exceed what the load path can supply, the transistor saturates (VCE ≈ 0.2 V) and further increases in base drive no longer raise the LED current.
From the separate load supply, not from the base control input. The base circuit only controls whether and how much current the transistor allows to flow; the transistor does not generate or supply the load's power itself.
A collector-emitter short bypasses the transistor's control entirely — current from the load supply flows straight through the short and the LED path regardless of base drive, which is why the LED stays lit even though the control input and base current are both zero.
It uses fixed junction voltage drops, a generic (non-datasheet) current gain, and ideal PWM transitions. It excludes charge storage/turn-off delay time, junction capacitance, reverse-mode operation, self-heating effects, and any device safe-operating-area or rating certification — use real datasheet values for actual part selection.