A MOSFET is controlled mainly by gate-to-source voltage, but real designs must also account for gate charge, resistance, transient voltage, and heat. Motor solenoid and relay currents cannot stop instantly.
The main ideas are: Motor solenoid and relay currents cannot stop instantly. Flyback paths limit voltage. Clamp choice changes release speed. Mosfet voltage rating needs transient margin.
Do not ask only whether the gate voltage exceeds threshold. Ask what RDS(on) is specified at the actual VGS you will provide, how much current flows, what transient voltage exists, how quickly the gate is charged, and whether the thermal path can remove the resulting heat.
The safest design usually has an intentional answer to three questions: what happens at startup, what happens during a fault, and where stored energy goes at turn-off.
Worked thinking: A simple diode across a solenoid gives gentle current decay while a TVS or zener clamp can allow faster release at higher controlled voltage. The example is meant to demonstrate the method, not to replace the ratings and test conditions of the actual part you use.
Design view – motor solenoid and relay currents cannot stop instantly. Ask which component or connection establishes this behavior and which rating could be exceeded if the load changes. This turns a descriptive fact into a selection rule.
Troubleshooting view – flyback paths limit voltage. Imagine the reading if this condition were missing, too weak, too high, reversed, or intermittent. A useful test is one that separates those possibilities instead of merely producing another number.
Design view – clamp choice changes release speed. Ask which component or connection establishes this behavior and which rating could be exceeded if the load changes. This turns a descriptive fact into a selection rule.
Troubleshooting view – MOSFET voltage rating needs transient margin. Imagine the reading if this condition were missing, too weak, too high, reversed, or intermittent. A useful test is one that separates those possibilities instead of merely producing another number.
A switch should be evaluated in both steady state and transition state. ON-state voltage drop creates heat, OFF-state voltage stress can damage the device, and the brief transition between states can be the most stressful period during fast or repetitive switching.
Finish by verifying the real load. A driver that works with an LED may still fail with a relay, motor, solenoid, heater, or long cable because startup current, inductance, wiring resistance, and electrical noise change the conditions dramatically.
Worked Example
A simple diode across a solenoid gives gentle current decay while a TVS or zener clamp can allow faster release at higher controlled voltage.
Hands-On Mini-Lab
- Identify gate drain and source from the datasheet.
- Verify the gate has a pull resistor that defines OFF.
- Measure VGS rather than gate-to-ground voltage alone.
- Measure load current and voltage drop across the MOSFET.
- Estimate conduction loss and observe temperature after several minutes.
Common Beginner Mistakes
- Using VGS(th) as the fully-on voltage.
- Leaving the gate floating.
- Choosing only by headline drain-current rating.
- Ignoring switching loss during PWM.
Fault-Finding Lens
- Verify the actual supply voltage at the load and switching device while the load is commanded ON.
- Measure the control quantity at the device itself: base current for a BJT, VGS for a MOSFET, or coil voltage for a relay.
- Check the load independently when possible so a failed load is not mistaken for a failed driver.
- Inspect protection parts, grounds, connectors, and wiring for open circuits, shorts, reversed polarity, or excessive voltage drop.
- After the repair, repeat the original operating condition and verify current, voltage drop, temperature, and turn-off behavior.
Check Your Understanding
- Explain mosfets with inductive loads in your own words.
- Which voltage or current would you measure first to verify this stage?
- What is one failure mode that could make the switch run hot?
- What protection component or design margin is most relevant here?
- How would the circuit behave during controller startup or reset?
Key Points
- Motor solenoid and relay currents cannot stop instantly.
- Flyback paths limit voltage.
- Clamp choice changes release speed.
- Mosfet voltage rating needs transient margin.