7.2 Choosing the relay type according to input signals

   There is no question to choose electromagnetic relay, temperature relay, time relay or photoelectrical relay according to the different input signals of electricity, temperature, time or light. Here, we specially indicate the choosing of voltage relay and current relay. You should choose current relay if the equipment could provide the coil constant current and choose voltage relay if the machine could provide the coil constant voltage.

7.3 Choosing the input parameter

   The input parameter that is tightly connected to the user is the coil operating voltage (or current). The pickup voltage (or current) is the parameter to control the sensitivity, to judge and examine the relay for the relay manufacturer. To the users, the pickup voltage (or current) is just a lower limit of the operating parameter. The control safety coefficient is operating voltage (current) / pickup voltage (current). It is unreliable and unsafe to use the relay under the pickup values. The ambient temperature rise, vibration and shock would lead the relay to operate unreliably. When designing the equipment, it is not right to use the unload voltage as the criterion of the relay operating voltage. The right method is to calculate the actual voltage regarding the coil as the load, especially when the inner resistance of the power source is high. When using transistor as the controller to switch on or off the coil, the transistor must be on the on-off state. For the relay that the operating voltage is less than 6VDC, it is required to deduct the saturation voltage drop of the transistor. Of course, it doesn’t mean that the operating value can be increased unlimitedly. If the operating value exceeds the rated value too much, the shock abrasion of armature and contact bounce times would be increased; the electrical life would be shortened. Generally, the operating value is 1.5 times of the pickup value; the error of the operating value is in the range of ±10%.

7.4 Choosing the contact type and capacity according to the load

   It is verified by practice that about 70% of the faults are because of the contact, which indicates that correctly choosing and using the relay contact is very important.
The contact form and amount of sets should be determined by the actual conditions of the controlled circuit. The common contact arrangements are showed in table 6. Because of less amount of bounce times when the circuit is switched on and larger amount of compensation after ablation, the load capacity and reliability of the make-contacts and make-contact in changeover contacts is higher than that of the break-contacts and break-contact in changeover contacts. So the circuit can adjust the contact position to choose make contacts as long as possible.

It is very important to determine the parameters according to the load capacity and characteristics (resistive, inductive, capacitive, lamp load and motor load). It is wrong to consider that small contact-switching load is certainly more reliable than large contact-switching load. Generally, it is best that when the switching load is under the rated voltage, the current exceeds 100mA and less than 75% of the rated current. If the current is less than 100mA, the contact carbon collection will increase and thus the reliability will decrease. So 100mA is regarded as the experiment current, which is the examined content to the world relay manufacturers to estimate the relay manufacturing technics level and conditions according to the professional criterion. Because most relays don’t have the low level switching capability, when ordering the relay with switching load less than 50mV、50μA, the user must give special note or ask the manufacturer to assist to choose the right type of relay if necessary.
The rated load life of relay is the operating times of relay under rated voltage, rated current and resistive load. When the voltage exceeds the rated voltage, the relay can be chosen according to the contact load curve. When the characteristics of the load changes, the contact switching capability will change, and the user can change the load current according to table 8.

(Table 8)