APPLICATION GUIDE
Overview
The electronic control system of small automation equipment (such as packaging machines, labeling machines, simple assembly tooling) usually does not require large PLCs or complex networks. The core is a compact combination of "PLC/relay logic + HMI human-machine interface + 24V power supply + sensors + actuators".
A common question from buyers is "how small a PLC should such a small device be equipped with, and how to calculate the power supply?"
Selection Criteria
PLC selection: first check the actual number of I/O points required, and reserve a certain expansion headroom; for independent small equipment, a compact CPU is usually sufficient. In situations involving networking or data collection, it is necessary to confirm whether the communications interface is sufficient.
HMI selection: The screen size and interaction method are determined according to the complexity of the operation. A text display can be used for simple parameter display. If multi-screen switching or graphical operation is required, touch screen HMI is preferred.
24V power supply power calculation: PLC power consumption + sum of sensor power consumption + HMI power consumption. It is recommended to leave a 20–30% headroom and consider the surge current when the solenoid valve/contactor coil is started.
Sensor selection: The output type (NPN/PNP) of the proximity switch and photoelectric switch needs to match the wiring method of the PLC input module.
Relay: When the PLC output needs to drive loads with larger currents or different voltage classes such as contactor coils and solenoid valves, it is usually necessary to install an intermediate relay for isolation.
Common Pitfalls
PLC selection only depends on the current number of I/O points and ignores the reserved expansion headroom. When adding functions later, the whole machine can only be replaced.
The 24V power supply is selected based on the nominal power consumption, and the surge current during energization of the solenoid valve/contactor is not considered, resulting in occasional undervoltage during operation.
The sensor NPN/PNP output type does not match the PLC input module, causing the signal to not be read correctly.
Frequently Asked Questions
Is it necessary to use PLC for small equipment?
When there are few points and fixed logic, relays and timers can also be implemented; but when multiple work steps are involved and parameters need to be modified frequently, PLC is more flexible and easier to maintain.
How to choose between HMI and text display?
The operation is simple, just use a text display to display numbers or status; if a graphical interface or multi-screen switching is required, it is recommended to use a touch screen HMI.
Can multiple devices share one 24V power supply?
Yes, but it is necessary to calculate the total power consumption and consider the impact of a single point of failure on the entire line. It is recommended that key equipment be powered independently.
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