1, according to the use of environment selection
The environmental conditions used mainly refer to temperature (maximum and minimum), humidity (generally referred to as maximum relative humidity at 40 ° C), low air pressure (may not be considered for use below 1000 m), vibration and shock. In addition, there are requirements for packaging methods, mounting methods, external dimensions and insulation. Due to different materials and structures, the environmental mechanical conditions of the relay are different. If it is used under the environmental mechanical conditions specified by the product standard, the relay may be damaged, and it may be selected according to the environmental mechanical conditions of the whole machine or a higher level.
It is best not to use AC-energized relays around devices that are sensitive to electromagnetic interference or radio frequency interference. For DC relays, use a product with a coil transient suppression circuit. Where solid-state devices or circuits provide excitation and are sensitive to spikes, products with transient suppression circuits are also selected.
2. Determine the type of relay according to the input signal.
According to the input signal is electricity, temperature, time, light signal to determine the choice of electromagnetic, temperature, time, photoelectric relay, this is no problem. Here, the selection of voltage and current relays is specifically described. If the whole machine supplies a constant current to the relay coil, the current relay should be selected. For a constant voltage value, the voltage relay is selected.
3. Selection of input parameters
The input quantity closely related to the user is the coil operating voltage (or current), and the pull-in voltage (or current) is the parameter that the relay manufacturer controls the sensitivity of the relay and judges and evaluates it. For the user, it is just a working limit parameter value. The control safety factor is the working voltage (current) / pull-in voltage (current). If the relay is used under the pull-in value, it is unreliable and unsafe. If the ambient temperature rises or is under vibration or shock conditions, the relay will be made. The work is not reliable. When designing the whole machine, the no-load voltage can not be used as the basis of the working voltage of the relay. Instead, the coil should be connected as the load to calculate the actual voltage, especially when the internal resistance of the power supply is large. When the triode is used as the switching element to control the on/off of the coil, the triode must be in the on-off state. For relays operating below 6 VDC, the triode saturation voltage drop should also be deducted. Of course, the higher the working value is, the higher the better. If the rated working value is too high, the impact wear of the armature will increase, the number of contact bounces will be increased, and the electrical life will be shortened. The general working value is 1.5 times of the suction value. The error is generally ±10%.
4. Select the type and capacity of the relay contacts according to the load.
Long-term practice at home and abroad has proven that about 70% of faults occur on contacts, which is important to properly select and use relay contacts.
The contact combination form and the number of contact groups should be determined according to the actual situation of the controlled loop. The moving contact group in the moving contact group and the switching contact group has a lower load returning degree and a larger compensation amount after the contact is ablated, and the load capacity and contact reliability are lower than the dynamic breaking contact. The set of break contacts in the group and the switch contact group should be high, and the whole machine line can be adjusted by adjusting the position of the contacts as much as possible.
It is important to determine the parameters based on load capacity and load characteristics (resistance, inductive, capacitive, lamp load, and motor load). It is considered that the contact switching load is small and reliable than the switching load. Generally speaking, the relay switching load is rated at a rated voltage, and the current is greater than 100 mA, which is preferably less than 75% of the rated current. If the current is less than 100 mA, the carbon deposit in the contact will increase and the reliability will decrease. Therefore, 100 mA is called the test current, which is the assessment content of the professional conditions of the relay manufacturer and the level of the process. Since the general relay does not have the low-level switching capability, the relay is used to switch the load of 50mV and 50μA or less. The user should indicate that the relay manufacturer should assist in the selection if necessary.
The contact rated load and life of the relay refer to the number of times the load is resistive at rated voltage and current. When the rated voltage is exceeded, the contact load curve can be selected. When the nature of the load changes, its contact load capacity will change.
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