Know-How
Completed
Relay or transistor output: which one and how to check it
Short answer: use a relay output for AC loads, for loads with their own supply or when you need isolation. Use a transistor output for DC loads that switch often or need PWM. Most output faults come from a missing output supply, a load above the rating or an inductive load without protection.
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This works for any PLC or I/O module with digital outputs. At the end you will find the page for your PLC range.
The idea in one minute
A relay output is a contact. It closes a circuit that can carry AC or DC and is isolated from the PLC. It is slow, a few milliseconds, and wears out with the number of operations.
A transistor output switches DC electronically. It is fast, supports PWM and does not wear out, but it works only with DC, in one polarity, and usually needs a supply on its common.
Which one to use
| Need | Relay | Transistor |
|---|---|---|
| AC load | Yes | No |
| DC load | Yes | Yes, within its current rating |
| Fast switching or PWM | No | Yes |
| Many operations a day | Wears out | Does not wear out |
| Isolation from the PLC | The contact is isolated | Depends on the model |
Five checks when an output does not work
| Check | What to do | Good result |
|---|---|---|
| 1. Output LED | Activate the output from the program | LED on. If not, check the program or the output configuration |
| 2. Output supply | On transistor outputs, measure the supply on the output common | Supply present and within range |
| 3. Voltage at the load | Measure across the load with the output on | Close to the supply voltage |
| 4. Load current | Compare the load current with the output rating | Below the rating of each output and of the group |
| 5. Inductive loads | Check the protection on coils and valves | Diode on DC loads, RC snubber or varistor on AC loads |
What the readings tell you
- LED on, no voltage at the output: no supply on the output common (transistor), or the load circuit is open (relay).
- The output works and then stops: overload or short circuit protection. Measure the load current.
- Relay output that stays closed: contact welded by a high inrush current. Use an interface relay or a contactor.
- Transistor output with an AC load that does nothing: transistor outputs switch DC only.
If the output has never worked
- On a relay output, power off the load circuit and measure continuity between the contacts with the output on and off.
- On a transistor output, check the polarity of the load and the supply on the common.
- Check the switches that select the output mode, where your PLC range has them.
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