Select the contact type and contact load of electromagnetic relay: the same type of relay usually has a variety of contact ways to choose (electromagnetic relays are: single contact, double contact, multiple contact points and normally open contact, normally closed contact, etc.), should choose the appropriate application circuit contact type. 2, choose the rated working current of the electromagnetic relay coil: the DC electromagnetic relay driven by transistors or integrated circuits, the rated working current of the coil (usually 2 times the pull-in current) in the output current range of the driving circuit. 3, choose the appropriate volume of the electromagnetic relay: the size of the relay volume is usually related to the size of the relay contact load, the choice of more or less product of the relay, but also according to the request of the application circuit. 4, the choice of electromagnetic relay coil power voltage: the choice of electromagnetic relay, first should choose the relay coil power voltage is AC or DC. The rated operating voltage of the relay should generally be less than or equal to the operating voltage of its control circuit.