LESSON 01 / 10
Mini Course 02 • Interactive prototype
Bipolar Junction
Transistors (BJTs)
Make a small base current control a larger load current. Explore NPN and PNP switching, identify saturation, measure circuit voltages, and diagnose an automotive electronic driver.
10 guided lessonsLive circuit simulationDiagnostic labKnowledge checkYOUR PROGRESS
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Interactive electronics lab
Control the load with a transistor
This simplified, educational DC model assumes a silicon BJT, a resistive load, and a base resistor. It approximates on-state VBE = 0.7 V and saturated VCE = 0.2 V; real transistors vary with device, temperature, and current.
NPN low-side switch
NPN: load connects toward positive supply; the transistor completes its path to ground. The drive is through RB. Diagram is conceptual rather than a wiring diagram.
Base drive is on. Change a control to see whether the transistor saturates.
Workbench controls
Base current IB—
Load current IC—
On-state VCE / VEC—
Transistor state—
Transfer curve: available base drive vs. load current
For a given load, collector current increases with base drive until the load limits it. A BJT switch is designed with sufficient base drive to reach saturation.Automotive connection
An ECU can use a transistor driver to switch a relay coil or another load. An NPN low-side driver controls the ground side; a PNP high-side driver controls supply to a load.
Real-world caution: The simulator shows a resistive load. Relay coils and solenoids are inductive: opening their circuit can cause a damaging voltage spike. Use manufacturer-specified suppression and never experiment on a live vehicle module without an approved procedure.
Try switching the drive off, then reduce base drive until the BJT leaves saturation.The Challenger • Automotive case study
Cooling fan relay driver: no operation
A conceptual 12 V relay-coil circuit is controlled on its ground side by an ECU's NPN transistor. The ECU is commanding the fan relay ON, but the relay does not energize. Diagnose from the voltage observations; this is an educational model, not a vehicle-specific procedure.
Virtual technician's meter
DIGITAL MULTIMETER
0.00 V
Black probe is on chassis ground for single-ended measurements. For coil voltage, probes are across the coil. All readings are approximate and with relay commanded ON.
Select a probe point to gather evidence.
Reason from the evidence
For each case, use all four meter positions and choose the most consistent explanation.
Cases solved: 0 / 5
Interpretation note: An open relay coil and an open transistor driver can both leave the driver terminal near battery voltage via circuit paths, depending on the actual vehicle design. Here the open-coil case uses the simplified measurement behavior shown by the virtual circuit. In practice, verify with the OEM wiring diagram, proper load testing, and circuit isolation before condemning a module.
Final knowledge check
Prove your understanding
Answer eight questions. A score of 6/8 or higher completes the knowledge check; you can retry as often as you need.
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Finish all lessons, complete the workbench and diagnostic cases, and pass the final knowledge check.