1. Selection according to the use environment
The environmental conditions of use mainly refer to temperature (maximum and minimum), humidity (generally the maximum relative humidity at 40°C), low air pressure (not to be considered when the altitude is below 1000 meters), vibration and shock. In addition, there are still requirements for packaging methods, installation methods, dimensions and insulation. Due to different materials and structures, the relays are subjected to different environmental mechanical conditions. If used under the environmental mechanical conditions specified in the product standard, the relay may be damaged. It can be selected according to the environmental mechanical conditions of the whole machine or the higher-level conditions.
It is best not to use AC-excited relays around devices that are sensitive to electromagnetic interference or radio frequency interference. When choosing DC relays, products with coil transient suppression circuits should be used. Those who use solid-state devices or circuits to provide excitation and are more sensitive to spikes should also choose products with transient suppression circuits.
2. Determine the relay type according to the input signal
According to the input signal is electricity, temperature, time, light signal to determine the selection of electromagnetic, temperature, time, photoelectric relay, this is no problem. The selection of voltage and current relays is specially explained here. If the whole machine provides a constant current to the relay coil, a current relay should be used, and a voltage relay should be used for a constant voltage value.
3. Selection of input parameters
The input quantity closely related to the user is the working voltage (or current) of the coil, and the pull-in voltage (or current) is the parameter used by the relay manufacturer to control the sensitivity of the relay and to judge and evaluate it. For the user, it is only a working limit parameter value. The control safety factor is the operating voltage (current) / pull-in voltage (current). If the relay is used under the pull-in value, it is unreliable and unsafe. The increase in ambient temperature or vibration or shock conditions will cause the relay to The work is unreliable. When designing the whole machine, the no-load voltage cannot be used as the basis for the working voltage of the relay. Instead, the coil should be connected as a load to calculate the actual voltage, especially when the internal resistance of the power supply is large. When using a triode as a switching element to control the on-off of the coil, the triode must be in the on-off state. For relays with a working voltage below 6VDC, the saturation voltage drop of the triode should also be deducted. Of course, it is not that the higher the working value is, the better. Exceeding the rated working value will increase the impact wear of the armature, increase the number of contact bounces, and shorten the electrical life. The general working value is 1.5 times the pull-in value, which is lower than the working value. The error is generally ±10%.
4. Choose the type and capacity of relay contacts according to the load situation
Long-term practice at home and abroad has proved that about 70% of faults occur on the contacts, which shows that the correct selection and use of relay contacts is very important.
The contact combination form and the number of contact groups should be determined according to the actual situation of the controlled circuit. The moving-close contact pair in the moving-close contact group and the change-over contact group, due to less contact bounces when connected and large compensation after contact ablation, its load capacity and contact reliability are higher than that of the moving-break contact The pair of moving-breaking contacts in the group and the change-over contact group should be high, and the whole line of the whole machine can be adjusted appropriately by adjusting the position of the contact, so as to use the moving-closing contact as much as possible.
It is very important to determine the parameters according to the size of the load and the nature of the load (resistive, inductive, capacitive, lamp load and motor load). It is incorrect to think that the contact switching load is smaller and more reliable than the switching load. Generally speaking, when the relay switching load is at the rated voltage, the current is better than 100mA and less than 75% of the rated current. Current less than 100mA will increase contact carbon deposits and decrease reliability, so 100mA is called the test current, which is the assessment content of the process conditions and level of the relay manufacturer by professional standards at home and abroad. Since general relays do not have low-level switching capability, the user needs to indicate when ordering relays that are used to switch loads below 50mV and 50μA. If necessary, please ask the relay manufacturer to assist in the selection.
The rated load and life of the contact of the relay refer to the number of resistive actions under the rated voltage and current. When the rated voltage is exceeded, it can be selected by referring to the contact load curve. When the nature of the load changes, its contact load capacity will change.
