NPN · low-side switch
SaturationReleased · switch OFF
● Base current IB● Collector current IC● Flyback through D1
Live readings
LED on| Node voltages | Junction / device | Currents |
|---|---|---|
| VCC | VBE | IB |
| VBB | VCE | IC |
| VB | VBC | IE |
| VC | — | Coil |
| VE | — | D1 |
| GND0.00 V | — | — |
Nodes are relative to GND. VBE = VB − VE; VCE = VC − VE; VBC = VB − VC. Currents follow the reference arrows; IE = IB + IC.
Transistor power PQ
Collector current against VBB
Drive voltage · left axis · Collector current · right axis
Each coloured trace holds IB constant. The white point is the circuit operating point; the dashed line is the DC load line. Curves use the same idealised fixed-drop model as the circuit: a vertical saturation segment at VCE(sat), then IC = βIB in the active region. Real curves have a rounded knee and an Early-effect slope; these curves are not measured device data.
Drag the white cursor, or use the slider, to pause at any point. Arrow keys move the cursor; Home and End select the cycle limits.
Workbench
Cut-off, active operation and saturation
Cut-off
No forward base current flows in the model. The transistor interrupts the collector path and the load is off. NPN switches turn off when the VBB is LOW. PNP switches turn off when the VBB reaches the emitter supply.
Active operation
The base current limits the collector current. Increasing forward base-emitter bias or reducing RB raises the load current. An active transistor has not reached the low-voltage ON state expected of a saturated switch.
Saturation
The load and supply limit the collector current. Additional base current gives no further load-current increase in this model. Saturation provides the low collector-emitter voltage used for the ON state of a transistor switch.
The animation shows conventional current direction. Marker speed aids visibility and does not represent electron speed. The model assumes constant LED and junction voltage drops and an adjustable active-region gain β. The gain control represents β in active operation, rather than the forced saturation gain βsat specified in the tutorial. Real devices have varying gain, leakage, switching delay and temperature dependence. The sine drive varies from 0 V to VCC with a DC offset. The load turns off when the forward junction voltage falls below the assumed junction drop; during the remaining interval, load current may vary before saturation. In cut-off, the ideal LED model leaves the collector node voltage unspecified; the display reports “not fixed”.
A switch for the Crypto-Box
Comparator timer and relay explorer: replace VBB with an RC comparator timer and use the relay contacts to switch the box power rail.
A transistor can switch an LED indicator, a relay coil, a buzzer or a solenoid latch, provided the transistor and supply meet the load ratings. Select the relay load to see the coil energise and the normally open contacts close. The dashed mechanical link shows that the K1 coil operates the normally open K1 contacts. The contacts could enable a separate low-voltage circuit when the correct code is accepted.
The relay schematic includes D1 across the coil. D1 is reverse-biased during normal operation and provides a path for coil current after Q1 switches off. The relay ON model uses steady-state coil resistance and a chosen pickup-current threshold. At switch-off, the pink loop shows coil current circulating through D1 while Q1 is OFF. The decay uses an illustrative inductance of 0.5 H and a 0.7 V diode drop. A 1.8-second slow-motion replay displays the calculated decay; real elapsed time is shown beside the diode status. Turn-on transients, mechanical delay and contact hysteresis are omitted. The relay settings describe a teaching model, rather than a specific stocked relay.
A servo needs power, ground and a position-control signal. Switching servo power alone does not command the lid to open. Use the transistor-switch model to study load switching, then design the servo control separately.
References: Relay manufacturer: transistor drive and coil protection · Arduino: servo control
Build and test in the lab
Use page 4 of the tutorial to build an NPN switch from the components available in your kit or the Skills Lab store. Check the actual transistor pinout and component ratings before wiring. Measure the base and collector currents, then compare the LED behaviour with the explorer. Complete the three tutorial questions and compare notes within your group; the student pack contains no final answers or solutions.
References: Build Electronic Circuits: transistor-switch guide · onsemi 2N3904 datasheet · onsemi 2N3906 datasheet