The principles of selecting relays

In order to correctly select relays you must know the characteristics to ensure relays they meet with the practical requirements. The principles of selecting relays can be seen in the table below.

Step 1: Contact

Item		Considered Points	Influence Factors
Contact	Contact load	AC, DC, size and types (inductive or resistive)	Ambient temperature AC load - operation & the load (Synchronous or not) Does the contact material match the load?
	Contact arrangement	NO or NC or switching (how many pairs per contact)
	Electrical endurance	The frequency and expected operation times
	Contact material	Which material?
	Contact resistance	How much and the testing conditions?

Step 2: Coil

Item		Considered Points	Influence Factors
Coil	Rated voltage	How much, direction, AC/DC?	Ambient temperature Power fluctuation]Voltage drop driver by semi-conductor
	Coil resistance	How much? Input power consumption?
	Operate Voltage	How much? Influence of the power wave?
	Release voltage	How much? Influence of the power fluctuation?
	Max. allowable voltage	How much? How long?
	Coil temperature rise	How much? Insulation level?

Step 3: Performance

Item		Considered Points	Influence Factors
Performance	Enclosure type	Unenclosed type, dust protected, flux proofed or plastic sealed	Ambient temperature Safety requirements
	Dielectric strength	How much? Where?
	Insulation resistance	How much? Where?
	Vibration resistance	How much? Functional or destructive?
	Shock resistance	How much? Functional or strength?

Step 4: Practical Environment

Item		Considered Points	Influence Factors
Practical Environment	Ambient temperature	High or low? How long?	Insulation level Method of encapsulation Life
Practical Environment	Atmosphere	Humidity? Harmful gases?	Insulation level Method of encapsulation Life

Step 5: Outline and Mounting

Item

Considered Points

Influence Factors

Outline & Mounting

Outline

Size an dimensions

Required mounting size
Mounting method

Welding mode

Manual , wave , reflow solder?

Is cleaning needed or not?

Step 6: Others

Item		Considered Points	Influence Factors
Others	Safety approval	UL, VDE, TUV, CQC etc	Zone customers requirements
Others	Special requirements & conditions	Customer requirements	Zone customers requirements

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