Learn Automation with MINATA #34: PLC Outputs, Relay and Transistor — Choose Before You Energise the Load
PLC Outputs, Relay and Transistor: Choose Before You Energise the Load
A PLC output is not a universal switch. Relay outputs and transistor outputs each have their own strengths in load type, switching speed and contact life. Choose the wrong one, or feed a load beyond what the module can carry, and you damage the output and lose hours of production with it.
Relay output against transistor output
A relay output switches through a mechanical contact. It is flexible with AC or DC within the catalogue limits, isolates more naturally, and suits commands that do not switch quickly. In exchange, the contact wears, the number of operations is limited, and it is unsuitable for high-speed pulses.
A transistor output switches DC through a semiconductor: fast and long-lived when the load is right. It works only with DC, following the polarity and configuration of the module, and is not a drop-in replacement for an AC load. Inductive loads — valve coils, relays, contactors — need the suppression the manufacturer specifies in order to protect the transistor.
Read the load before choosing the output
Before you run a wire from the PLC out to a load, answer four questions:
- Is the load DC or AC, and what is its rated voltage?
- What is its holding current, and what is its inrush or pulse current?
- Is it inductive: a valve coil, a contactor coil, a motor, a relay?
- Does it need fast or precise switching, or only occasional ON and OFF?
A small 24VDC valve may suit a transistor output once the current and the suppression have been checked. A large contactor coil, a multi-tier tower lamp, a siren or an AC load usually belongs behind a suitable interposing relay or contactor. "The PLC controls the load" does not mean "the PLC supplies the load directly".
What an interposing relay is for
An interposing relay puts a stage between the PLC and the field device. It can change the contact type, split one command into several branches, supply the right kind of AC or DC contact, and let you replace a cheap relay instead of an entire PLC module. But another relay is also another contact, another coil supply and another point of failure — use one when there is a clear technical reason.
For an inductive DC load, a flyback diode or an equivalent protective device must be fitted the right way round and to the recommendation. For an AC coil, use an RC snubber, a varistor or the equivalent from the datasheet. Do not apply one suppression component to every kind of load.
Relay and transistor outputs side by side
| Criterion | Relay output | Transistor output |
|---|
| Load type | AC and DC within limits | DC only, correct polarity |
| Switching speed | Milliseconds, limited operations | Fast, suits pulses and PWM |
| Service life | Mechanical contact wear | High with the right load |
| Current per point | Usually higher | Usually lower — check the datasheet |
| Leakage when OFF | Practically zero | A small leakage current exists |
| Isolation | The contact isolates naturally | Watch the shared polarity |
Current limits per point and per common group
An easy way to destroy an output is to look only at the current of one point and forget the total current of the common group. A module may allow 0.5A per point while the group of eight is limited to 2A in total; switching several loads at once exceeds the limit even though each point is still within its rating. Build a load table listing the holding and inrush current of each output, add them up per common group, and compare with the module catalogue before the layout is fixed.
Choosing suppression by load type
| Load type | Usual suppression |
|---|
| DC coil (valve, relay) | Flyback diode across the coil, correct polarity |
| AC coil (contactor) | RC snubber, or varistor/MOV |
| Inductive load on a transistor output | Exactly as the module datasheet recommends |
The leakage current of a transistor when OFF is worth attention too: it can be enough to hold a miniature relay or a sensitive input in a false ON state. When a transistor output drives a small relay or another PLC input, compare that leakage against the release current of the receiving device.
A worked engineering situation
A PLC controls a 24VDC solenoid valve and a motor contactor. The transistor output can drive the small valve once current and suppression have been checked. The contactor command goes through an interposing relay with a suitable contact and protection; the contactor's own auxiliary feedback returns to a PLC input so the HMI shows the real state, not merely the command that was sent.
This is an illustrative arrangement. Fuse sizing, total group current and the suppression configuration must follow the module catalogue, the loads and the approved electrical drawings.
PLC output checklist
- [ ] Every load has a defined voltage, holding current and AC/DC type.
- [ ] The total current per output group stays within the module and supply capability.
- [ ] Inductive loads have the right type of suppression.
- [ ] Power loads and AC loads go through suitable switchgear.
- [ ] Real status feedback exists wherever an operating fault must be detected.
- [ ] Load power wiring and signal wiring are separated and labelled.
A good output is one chosen from the load, not from habit. Post #35 continues with timers and counters — small blocks that create sequence faults very easily when they are used without state.
Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation
Previous — #33: PLC inputs, PNP and NPN: https://minatavn.com/en/blog/automation-33-plc-inputs-pnp-npn
Next — #35: PLC timers and counters: https://minatavn.com/en/blog/automation-35-plc-timers-counters
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