Relay is an electrical control device commonly used in automation control circuits, playing a role in automatic regulation, safety protection, conversion circuits, etc. It is actually an “automatic switch” that uses small current to control high current operation.
1. Determine relay types based on different input signals
There is no problem selecting electromagnetic, temperature, time, or photoelectric relays based on whether the input signals are electrical, temperature, time, or optical signals. The selection of voltage and current relays is specifically explained here. If the relay coil supplied by the entire machine has a constant current, a current relay should be selected, and if it has a constant voltage value, a voltage relay should be selected.
2. Selection of input parameters
The input quantity closely related to the user is the coil working voltage (or current), while the pull-in voltage (or current) is a parameter that the relay manufacturer controls, judges, and evaluates the sensitivity of the relay. For users, it is just a working limit parameter value. The control Factor of safety is the working voltage (current)/pull in voltage (current). If the relay is used at the pull in value, it is unreliable and unsafe. When the ambient temperature rises or the relay is under vibration and shock conditions, the relay will not work reliably.
When designing the entire machine, the no-load voltage cannot be used as the basis for the working voltage of the relay, but the actual voltage should be calculated by connecting the coil to the load, especially when the internal resistance of the power supply is high. When using a transistor as a switching element to control the coil on/off, the transistor must be in the on/off state. For relays with operating voltages below 6VDC, the saturation voltage drop of the transistor should also be deducted. Of course, the higher the working value, the better. Exceeding the rated working value too high will increase the impact wear of the armature, increase the number of contact bounce, and shorten the electrical life.
3. Select according to the usage environment
The environmental conditions for use mainly refer to temperature (maximum and minimum), humidity (generally the maximum relative humidity at 40 ℃), Low-pressure area (not considered below 1000 meters), vibration and shock. In addition, there are requirements for packaging methods, installation methods, external dimensions, and insulation. Due to different materials and structures, the environmental mechanical conditions that the relay can withstand vary. If used beyond the environmental mechanical conditions specified in the product standard, it may damage the relay. Therefore, the relay can be selected according to the environmental mechanical conditions of the entire machine or higher level conditions.
It is best not to use AC powered relays around devices that are sensitive to electromagnetic or radio frequency interference. When selecting DC relays, products with coil transient suppression circuits should be selected. For areas where solid-state devices or circuits are used to provide excitation and are sensitive to peak signals, products with transient suppression circuits should also be selected.
4. Select the type and capacity of relay contacts based on load conditions
It is important to determine parameters based on the size of the load capacity and the nature of the load (resistance, inductance, capacitance, lamp load, and motor load). It is incorrect to believe that a small switching load on a contact is necessarily more reliable than a large switching load. Generally speaking, relay switching is negative
