Differences Between Three Types of PLC Outputs1. Relay Output:The advantage is that different common points can carry different AC and DC loads, and the voltage can also be different, with a load current of up to 2A per point; however, relay output is not suitable for high-frequency loads, which is determined by the relay’s lifespan. Its lifespan decreases with the increase in load current, generally ranging from hundreds of thousands to over a million times, and some company products can reach over 10 million times, with a response time of 10ms.2. Transistor Output:The biggest advantage is its suitability for high-frequency operations, with a short response time, generally around 0.2ms. However, it can only carry loads of DC 5-30V, with a maximum output load current of 0.5A per point, but no more than 0.8A for every 4 points.3. Thyristor Output:The load capacity is 0.2A per point and can only carry AC loads, suitable for high-frequency operations, with a response time of 1ms.Therefore, when your system output frequency is below 6 times per minute, relay output should be the first choice due to its simple circuit design, strong anti-interference, and load-carrying capacity. When the frequency is below 10 times/min, either relay output can be used; or PLC output can drive Darlington transistors (5-10A), which can significantly reduce the load.2. The common output types are relay type and transistor type, so what are the differences between the two?1. Different load voltage and current types:Load type: Transistors can only carry DC loads, while relays can carry both AC and DC loads.Current: Transistor current is 0.2A-0.3A, while relays can handle 2A.Voltage: Transistors can connect to DC 24V (generally max around DC 30V), while relays can connect to DC 24V or AC 220V.2. Different load capacities:The load-carrying capacity of transistors is less than that of relays. When using transistors, sometimes additional components are needed to drive large loads (such as relays, solid-state relays, etc.).3. Different overload capacities:Generally, when there are situations with large inrush currents (such as light bulbs, inductive loads, etc.), transistors have a smaller overload capacity and need to be derated more.4. Different response speeds:The relay output principle is that the CPU drives the relay coil, causing the contacts to engage, allowing the external power supply to drive the external load through the closed contacts, with an open-circuit leakage current of zero and a slow response time (about 10ms).The transistor output principle is that the CPU uses optocouplers to turn the transistor on and off to control external DC loads, with a fast response time (around 0.2ms or even less).Transistors are mainly used for positioning control and need to output pulses. Relays cannot output pulses, hence cannot be used for positioning control.If relays are used to control positioning servos or steppers, a positioning module is needed, which is not economical. However, using a transistor output can control servos, etc.5. Different lifespans:Under rated working conditions, relays have a limited number of operations, while transistors only age without a limit on the number of uses. Relays are mechanical components, so they have an operational lifespan, while transistors are electronic components, only aging without a usage limit. The number of switching times per minute for relays is also limited, while transistors do not have this limitation.6. Different working voltages:Relays provide dry contacts that can be used for both AC and DC, with a wide voltage range, but in some cases, the signal may not be “clean” enough (with spikes).Transistor (including thyristor) outputs are active switches, requiring the load to match or adapt to the corresponding power supply. For inductive loads, a diode may also need to be considered.3. What are the advantages and disadvantages of these three types of outputs?1. Relay:Advantages: Can drive both AC and DC loads; large rated load current;Disadvantages: Cannot operate at very high frequencies, and relays have a lifespan of generally 1 million times.2. Transistor:Advantages: Operating frequency can reach several hundred kHz, no contacts, thus no mechanical lifespan;Disadvantages: Can only connect to DC loads (generally below DC 30V), with relatively small current.3. Thyristor:Advantages: High operating frequency, long lifespan;Disadvantages: Can only connect to AC loads, with relatively small rated load current.
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