1. Introduction
With the advancement of technology, PLCs are now widely used. Today, we'll introduce the output types of PLCs: relays and transistors. Their operating parameters differ significantly, and it's crucial to distinguish between them before use to avoid misuse and product damage. This article briefly introduces the characteristics of relay and transistor outputs and precautions for their use.
2. Working principle of relay and transistor output
A relay is an electronic control device with a control system (also known as an input circuit) and a controlled system (also known as an output circuit). It is commonly used in automatic control circuits and essentially acts as an "automatic switch" that uses a smaller current to control a larger current. An electromagnetic relay generally consists of an iron core, coil, armature, and contact springs (as shown in Figure 1). When a certain voltage is applied across the coil, a current flows through it, generating an electromagnetic effect. The armature, attracted by the electromagnetic force, overcomes the tension of the return spring and is drawn towards the iron core, causing the moving contact to engage with the stationary contact (normally open contact). When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the spring's reaction force, causing the moving contact to engage with the original stationary contact (normally closed contact). This engagement and release cycle achieves the purpose of connecting and disconnecting the circuit. From the working principle of a relay, it can be seen that it is an electromechanical component that uses mechanical action to achieve the opening and closing of contacts; it is a contact element.
Figure 1. Structure diagram of an electromagnetic relay
A transistor is an electronic component that controls the conduction between its collector and emitter by using base current. It is a contactless component.
3. The main differences between relay and transistor outputs
(1) Relay output
The advantages are that different AC or DC loads can be connected between different common points, and the voltage can also be different, with a load current of up to 2A/point; however, the relay output method is not suitable for high-frequency operating loads, which is determined by the lifespan of the relay. Its lifespan decreases with the increase of the load current, generally ranging from hundreds of thousands to millions of times, with some companies' products reaching more than 10 million times, and a response time of 10ms.
(2) Transistor output
Its biggest advantage is that it is suitable for high-frequency operation and has a short response time, usually around 0.2ms. However, it can only carry DC 5-30V loads, and the maximum output load current is 0.5A/point, but it must not exceed 0.8A for every 4 points.
When your system output frequency is less than 6 times per minute, relay output should be the first choice because of its simple circuit design, strong anti-interference ability, and strong load-carrying capacity. When the frequency is less than 10 times/min, you can use relay output or use PLC output to drive Darlington transistor (5-10A) and then drive the load, which can greatly reduce the load.
relay
Advantages: Can drive both AC and DC loads; large rated load current;
Disadvantages: The operating frequency cannot be too high, and relays have a limited lifespan, typically 1 million cycles.
transistor
Advantages: The operating frequency can reach several hundred kHz, and there are no contacts, so there is no such thing as mechanical lifespan;
Disadvantages: It can only be connected to DC loads (generally below DC 30V), and the current is relatively small;
Bidirectional thyristor output: Can only be connected to AC loads, has a relatively high operating frequency and long lifespan, but the rated current of the load is also relatively small.
Transistors are primarily used for positioning control, requiring the crystal's output to emit pulses. Relays, on the other hand, cannot emit pulses and therefore cannot be used for positioning control. Using a relay to control a positioning servo or stepper motor would require an additional positioning module, which is economically inefficient. A single transistor output can control servos, etc. That's the gist of it. Based on production process requirements, the start/stop of various indicator lights and inverters/digital DC speed controllers should use transistor outputs, as they are suitable for high-frequency operations and have short response times. If the PLC system's output frequency is less than 6 times per minute, relay outputs should be preferred. This method simplifies the output circuit design and provides strong anti-interference and load-carrying capacity.
1. Different load voltage and current types
Load type: Transistors can only drive DC loads, while relays can drive both AC and DC loads.
Current: Transistor current 0.2A-0.3A, relay current 2A.
Voltage: Transistors can be connected to DC 24V (generally the maximum is around DC 30V), while relays can be connected to DC 24V or AC 220V.
2. Different load capacities
Transistors have a lower load-carrying capacity than relays. When using transistors, sometimes other components (such as relays, solid-state relays, etc.) need to be added to drive large loads.
3. The overload capacity of a transistor is less than that of a relay.
Generally speaking, when there is a large inrush current (such as in light bulbs, inductive loads, etc.), the transistor has a small overload capacity and needs to be dated more.
4. Transistors have a faster response time than relays.
The principle of relay output type is that the CPU drives the relay coil to close the contacts, so that the external power supply drives the external load through the closed contacts. Its open circuit leakage current is zero, and the response time is slow (about 10ms).
The principle of transistor output is that the CPU controls the external DC load by switching the transistor on and off via optocoupler. The response time is fast (about 0.2ms or even less). Transistor output is generally used for high-speed output, such as servo/stepper motors, and for outputs with high operating frequencies, such as temperature PID control, mainly used in stepper motor control, as well as servo control and solenoid valve control (where valves operate at high frequencies).
5. Under rated operating conditions, relays have a limited lifespan based on the number of operations, while transistors only age and have no limit on the number of uses.
Relays are mechanical components and therefore have a limited lifespan, while transistors are electronic components that only age and have no limit on the number of uses. Relays also have a limited number of switching cycles per minute, while transistors do not.
6. Different service life
Relays, being mechanical components, have a limited lifespan due to the number of operations they can operate on, and this lifespan is related to the load capacity, as detailed in Table 2. The table shows that as the load capacity increases, the contact lifespan decreases almost exponentially. Transistors, being electronic components, only age and have no limited lifespan.
7. Different prices
Transistor outputs are slightly more expensive.
4. Selection Principles for Relay and Transistor Outputs
Relay-type outputs have high drive current, slow response, and limited mechanical life, making them suitable for driving intermediate relays, contactor coils, indicator lights, and other applications with low operating frequencies. Transistor-type outputs have low drive current, high frequency, and long life, making them suitable for applications requiring high frequency and long life, such as servo controllers and solid-state relays. In high-frequency applications, if driving a large load simultaneously is required, additional devices (such as intermediate relays or solid-state relays) can be added for driving.
4.1 Impact of driving inductive loads
Figure 2 shows the instantaneous high voltage generated when driving an inductive load.
When relays control inductive loads such as contactors, a momentary voltage spike occurs between the two contacts of the relay due to the non-sudden change in current caused by inductance, according to U=L*(dI/dt). This voltage spike exceeds the derating of the relay's contact withstand voltage. For electromagnetic relays, the withstand voltage between the contacts is 1000V (1min). If the voltage between the contacts operates at around 1000V for an extended period, it can easily cause metal migration and oxidation of the contacts, leading to increased contact resistance, poor contact, and contact adhesion. The faster the operating frequency, the more severe the problem. As shown in Figure 2, this instantaneous high voltage lasts less than 1ms and has an amplitude exceeding 1KV. The same problem exists when the transistor output is an inductive load; this instantaneous high voltage can damage the transistor.
Therefore, when driving an inductive load, a surge protection circuit should be connected across the load terminals. When driving an inductive load in a DC circuit (such as a relay coil), the user circuit needs to connect a freewheeling diode in parallel (note the diode polarity); if driving an inductive load in an AC circuit, the user circuit needs to connect an RC surge absorption circuit in parallel to protect the PLC output contacts. The protection circuit for the PLC output contacts is shown in Figure 3.
4.2 Precautions during use
Currently, customers frequently experiencing relay problems in the market share a common characteristic: the faulty output point operates at a relatively high frequency, and the driven loads are inductive loads such as relays, solenoid valves, or contactors, lacking any absorption protection circuitry. Therefore, it is recommended to pay attention to the following points when selecting and using PLC output types:
(1) Pay close attention to the load capacity. The output port must comply with the maximum allowable current limit (as shown in Table 1) to ensure that the heat generation of the output port is limited to the allowable range. The service life of the relay is related to the load capacity. When the load capacity increases, the contact life will be greatly reduced (as shown in Table 2), so special attention should be paid to this.
(2) Pay close attention to the nature of the load. As analyzed in Section 4, inductive loads generate instantaneous high voltage at the moment of switching. Therefore, although the load capacity may not appear large on the surface, it is actually very large, which will greatly shorten the life of the relay. Therefore, when driving an inductive load, an absorption protection circuit should be connected across the load terminals. It is especially important to add a protection circuit when the operating frequency is relatively high. From the customer's experience, the improvement effect after adding an absorption protection circuit is very obvious.
Based on the characteristics of capacitors, if a capacitive load is directly driven, a surge current will be generated at the moment of conduction. Therefore, in principle, capacitive loads should not be connected to the output port. If necessary, it should be ensured that the surge current is less than the maximum current specified in the specifications (see Table 1).
(3) Pay close attention to the operating frequency. When the operating frequency is high, it is recommended to choose the transistor output type. If a large current needs to be driven at the same time, the transistor output driving intermediate relay mode can be used. When controlling stepper motors/servo systems, or using high-speed output/PWM waves, or in situations where the operating frequency is high, only the transistor type can be selected. PLCs do not require the output types of expansion modules and main modules to be consistent. Therefore, when the number of system points is large and the functions are different, you can consider expanding the main module with relay output to transistor output or expanding the main module with transistor output to relay output to achieve the best matching.
5. Conclusion
This article briefly compares the working principles and characteristics of relay and transistor output types in PLCs, and proposes points to note during selection and use. It has been proven that proper selection and system design based on load characteristics, capacity, and operating frequency significantly reduces the failure rate of output ports, resulting in high customer satisfaction.
