About PLC output type selection and precautions in use
1 . introduction A relay is an electronic control device. It has a control system (also called an input circuit) and a controlled system (also called an output circuit). It is usually used in an automatic control circuit. It actually uses a smaller current to control a larger one. A "automatic switch" of current. Electromagnetic relays generally consist of iron cores, coils, armature, and contact reeds (as shown in Figure 1). As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil to generate an electromagnetic effect, and the armature will absorb the pulling force of the return spring to the core under the action of the electromagnetic force to attract the armature. The movable contact and the stationary contact (normally open contact) are attracted. When the coil is de-energized, the electromagnetic suction force also disappears, and the armature returns to its original position at the reaction force of the spring, causing the movable contact to pull in contact with the original stationary contact (normally closed contact). This sucks and releases so as to achieve the purpose of turning on and off in the circuit. From the working principle of the relay, it can be seen that it is an electromechanical element. It is a contact element that achieves the on-off of contacts through mechanical actions. Due to the different working principle of the relay and the transistor, there is a big difference in the operating parameters of the two. The Emerson EC series PLC related data is used as an example for comparison and description (see the main specifications of the output port in Table 1). Relay output port Transistor output port External power supply 250Vac, 30Vdc or less 5-24Vdc Circuit insulation Relay mechanical insulation Optocoupler insulation Action instructions Relay output contact closure indicator is lit The indicator lights up when the optocoupler is driven Leakage current at open circuit / Less than 0.1mA/30Vdc Minimum load 2mA/5Vdc 5mA (5-24Vdc) Maximum output current Resistive load 2A/1 points; Y0, Y1: 0.3A/1 point others: 0.3A/1 point Inductive load 220Vac, 80VA Y0, Y1: 7.2W/24Vdc Light load 220Vac, 100W Y0, Y1: 0.9W/24Vdc Response time ON-OFF Up to 20ms Y0, Y1: 10us OFF-ON Up to 20ms Y0, Y1 highest output frequency / 100kHz per channel Output common Y0-COM0; Y1-COM1; After Y2 Up to every 8 ports use 1 common port, each common port is isolated from each other Fuse protection no Table 1 Output port specifications Action frequency condition Contact life 220VAC, 15VA 1 second ON/1 second OFF 3.2 million times 220VAC, 30VA 1 second ON/1 second OFF 1.2 million times 220VAC, 60VA 1 second ON/1 second OFF 300 thousand times Table 2 Relay life Relays control the opening and closing moments of inductive loads such as contactors. Since the inductor has a non-disruptive current characteristic, an instantaneous spike voltage will be generated between the two contacts of the relay according to U=L*(dI/dt). The voltage amplitude exceeds the derating of the contact resistance of the relay; the electromagnetic relay used in the relay, the withstand voltage between the contacts is 1000V (1min), if the voltage between the contacts works long-term around 1000V, It is easy to cause the migration and oxidation of the contact metal, resulting in the phenomenon of large contact resistance, poor contact, and contact adhesion. And the faster the operating frequency, the more serious the phenomenon. The transient high pressure is shown in Figure 2 below. The duration is within 1ms and the amplitude is above 1KV. This problem also exists when the transistor output is an inductive load. This transient high voltage can cause transistor damage. A common feature of the customer's site where relay problems often occur on the market today is that the faulty output point has a faster operating frequency, and the driven loads are inductive loads such as relays, solenoid valves or contactors, and do not absorb the protection circuit. Therefore, it is recommended that the following points should be noted when selecting and using PLC output types: According to the characteristics of the capacitor, if the capacitive load is directly driven, an inrush current will be generated at the moment of conduction. Therefore, in principle, the output port should not be connected to a capacitive load. If necessary, ensure that the surge surge current is smaller than the specification (see table 1) The maximum current in the description. 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PLC output types are relay and transistor types. The operating parameters of the two are quite different. Before use, they need to be differentiated to avoid misuse and cause product damage. This article briefly introduced the characteristics of the relay and transistor output and the precautions in use. 
Figure 1 electromagnetic relay structure
A transistor is an electronic component that controls the collector and emitter conduction through a base current. It is a contactless element.
(1) for driving a load can be connected to a different type of relay <br> 220V AC or 24V DC load requires no polarity; type transistor connected only 24V DC load, polarity requirements.
The load current of the relay can reach 2A, and the load current of the transistor is 0.2-0.3A. At the same time with the type of load, see Table 1 for details.
8A/4 point group public end;
8A/8 point common
0.8A/4 points
1.2A/6 points
1.6A/8 point
Increases the total current by 0.1A for each additional point above 8 o'clock
Others: 12W/24Vdc
Others: 1.5W/24Vdc
Others: 0.5ms
(2) the response time of the relay <br> different response times slower (about 10ms-20ms), the transistor is relatively fast response time of about 0.2ms-0.5ms, Y0, Y1 even up to 10 us.
(3) Since the service life of the relay is different <br> mechanical element is subjected life limits the frequency of operation, and is related to the load capacity as shown in Table 2, it can be seen from the table, with the increase in the load capacity of the contact life is almost Decrease by level. Transistors are only aging electronic components, no life-limits.
4 . Relay and transistor output selection principle <br>Relay-type output drive current, slow response, mechanical life, suitable for driving intermediate relays, contactor coils, lights and other operating frequency is not high occasions. Transistor output drive current is small, high frequency, long life, suitable for controlling servo controllers, solid state relays and other applications requiring high frequency and long life. In high frequency applications, if you need to drive a large load at the same time, you can add other devices (such as relays, solid state relays, etc.) to drive. 
Figure 2 Instantaneous high voltage generated when driving an inductive load
Therefore, when the inductive load is driven, the absorption protection circuit should be connected to both ends of the load. When driving inductive loads (such as relay coils) of the DC circuit, the user circuit must be connected in parallel with a freewheeling diode (requiring attention to the polarity of the diode); if the inductive load of the AC circuit is driven, the user circuit needs to connect the RC surge absorbing circuit in parallel to protect PLC output contacts. PLC output contact protection circuit shown in Figure 3. 
Figure 3 PLC output contact protection circuit
Emerson EC20 Series Programmable Controller User Manual