Glossary · Electrical engineering and control panels
Transistor output
Also known as: Semiconductor output, Solid-state output
German: Transistorausgang
In industrial control, a transistor output is a digital output channel in which a transistor switches a DC load, typically at 24 V DC, allowing fast, wear-free switching of small loads such as solenoid valves, indicators and relays.
- Control panels
In one sentence
A transistor output switches DC loads with a transistor, fast and wear-free, typically at 24 V DC.
Example
The PLC's transistor outputs drive the valve terminal's solenoids at 24 V DC with 0.5 A per channel.
How it applies
- Engineering: Transistor outputs are sourcing (PNP, switching +24 V) or sinking (NPN, switching 0 V); the load wiring must match. Check current per channel and per group, and protection against short circuits and inductive kickback.
- Maintenance: A shorted transistor can leave an output permanently on. Channel diagnostics report some but not all faults.
- Documentation: State output type (sourcing or sinking), voltage and current rating in the I/O documentation. Service manuals should explain that a transistor output in the off state may still show a small leakage current, which can keep sensitive loads such as LEDs glowing.
Transistor output vs. relay output
A Relay output can switch AC and different voltages with isolation. A transistor output is limited to DC but switches much faster and without wear.