In project practice, PLC output ports are frequently damaged due to improper use. PLCs are often the core of equipment control, comprising both hardware and software, and are expensive. Once internal damage occurs, the repair and replacement cycle is long. Even if you're wealthy and can afford to replace it, you might not have the control program; even if you do have the program, you might not be able to configure and download it to the system. In short, it's very troublesome and frustrating for the customer.
First, it should be noted that the output points of transistors using MOSFETs as control elements have an output current of only 0.5A to 0.7A per point and lack comprehensive overload and overvoltage protection. They can only directly drive small resistive DC loads, such as input control signals for frequency converters and servo units, or drive status indicator lights. If you need to drive inductive loads or high-power loads, be sure to use solid-state relays or DC intermediate relays for isolation, and then decide whether to use contactors based on the power rating.
1. When the transistor output point is used for position control and outputs high-speed pulses, since most servo units or stepper drivers are mainly used to connect to machine tool CNC systems, their position command input ports are differential drives designed for 5V systems, while the transistor output H level of the PLC is close to 24VDC. At this time, a suitable current-limiting resistor needs to be configured to interconnect. Otherwise, the servo input port or PLC output port will be damaged. As shown in Figure 1, the 2.7K resistor is the current-limiting resistor.
2. Both AC and DC solid-state relays have built-in comprehensive overvoltage protection circuits to absorb surge voltage and back electromotive force, and can achieve DC-to-AC isolation conversion. When controlling low-power loads such as frequently operating hydraulic solenoid valves, electromagnets, pneumatic valves, and single-phase motors, solid-state relays are generally used to protect and isolate the PLC, convert power, and convert DC to AC power. See Figures 2 and 3.
3. For high-power AC loads, an AC contactor can be controlled by a DC relay. It is important to note that the relay should be a model with a freewheeling diode, and preferably also have an operation indicator light; the AC contactor should be a model with a resistor-capacitor protection module to improve system reliability and anti-interference performance, as shown in Figure 4.
Alternatively, this can be achieved by using an AC solid-state relay to control an AC contactor coil. In this case, a contactor without a resistor-capacitor module can be selected because the AC solid-state relay contains a similar circuit, as shown in Figure 5.
The PLC relay-type output point is the NO dry contact of its internal miniature relay output. It can control both AC and DC loads, but its driving capability is very poor, with each point outputting only 0.5A~2A of current. In industrial settings, the contact DC power supply is DC24V, and the AC power supply is AC220V. It is important to note that the contact AC power supply cannot be AC380V, as 380V is very unsafe for miniature relays. 1. For resistive DC small loads and servo unit control signals, the PLC output contacts can be used for direct control. However, a conduit-type fuse must be installed close to the power supply to prevent damage to the PLC output point in case of short circuits due to broken wire insulation or component short circuits. As shown in Figures 1 and 2, FU1 and FU2 are conduit-type fuses.
2. For small AC loads, direct drive using PLC contacts is generally not recommended. This is because industrial AC loads typically operate on AC 220V or AC 380V. While the load itself doesn't draw a large current, the current can be substantial during a short circuit. Direct control using PLC outputs would cause the contacts to melt and stick together instantly, or the entire miniature relay to burn out. To protect the PLC outputs, DC relays are used for isolation, and the relay's NO contact is used to control the small AC load. Even if the DC relay burns out, replacement is convenient, minimally costly, and fast, as shown in Figure 3. To improve overall system reliability, a single-phase circuit breaker should be installed on the AC power supply side to provide short-circuit and overload protection, as shown by QF1 in Figure 3.
3. For high-power loads, AC contactors are generally used for control. In this case, the PLC output contacts can directly control the contactor coil. It is important to note that the contactor should be a model with an RC snubber module, and a fuse should be installed on the AC220 control power supply side, as shown in Figure 4 (FU1). To improve the overall system reliability, the NC contacts of a thermal protector, a phase loss protector, and an emergency stop button should be connected in series in the main contactor coil circuit. This ensures timely power cut-off and shutdown in case of a main circuit fault, as shown in Figure 4 (FR1, OP1, and SB1).
