An NPN transistor needs a resistor in the base, the emitter on ground and the load between the supply and the collector.
What the message means
The base current is what turns the transistor on. Too small a current leaves it half on; no resistor can damage the pin or the transistor.
Fix it in this order
- Put the load between the positive supply and the collector, and the emitter to ground.
- Join the grounds of the load supply and the microcontroller.
- Use a base resistor of about 1 kohm as a start for a small load, and calculate it from the load current if you need more.
- Check the transistor current rating is above the load current, with margin.
- Check the pin order of your transistor from its datasheet, since packages differ.
- Put a diode across a relay coil or motor.
If you see this, do this
| You see | Do this |
|---|---|
| Load never turns on | Pin order, base resistor or ground. |
| Load dim, transistor hot | Base current too small or load too large. |
Related
MOSFET not switching, relay active low.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.