The time relay is the relay playing a role of controlling action time in circuit, it is to use the electromagnetic principle or mechanical acting principle to delay (0.4 ~ 180 seconds) to replace a coil in DC circuit. Hereinafter only the air damping- type time relay is introduced (at present, it is possibly not widely used, the new type commonly used is the time relay for large scale integrated circuit, it is using the principle of resistance capacity to realize delay action, but the air damping-type time delay has not been completely eliminated, so it is necessary to understand its working principle, hereinafter the working principles of air damping time delay is introduced). 

Time-Delay Relays of JS7-A Series:  It is using the air in air sac and using the principle of orifice throttling to obtain delay action. According to the characteristics of the contact time delay, it can be divided into two kinds of energizing delay action (such as Model JS7-2A ) and deenergizing delay reset type (such as JS7-4A). 

Air type time relay:  

JS7- A series time relay is composed of four parts of electromagnetic system, working contact, air chamber and transmission mechanism. Of which the electromagnetic system is composed of coil, and armature and iron core, and the counterforce spring and spring piece. Working contact: It is composed of two pairs of instantaneous contact (a pair of instantaneous closing, and the other pair of instantaneous disconnecting) and two pairs of delay contact. Air chamber: Inside the air chamber there is a rubber membrane moving along with the increasing and decreasing of air. The adjusting screw on the top of the air chamber can adjust the duration of the time delay. Transmission mechanism: It is composed of pushing plate, piston rod, and lever and pyramid-type spring. 

Working principle of the air type time relay: 

JS7 - A time relay has two kinds of energizing （electricity） delay and deenergizing(power outage) delay. The right diagram is the energizing-type time delay relay. The main functions are that the after the coil is energized, the contact will act after a period of time delay; But when the coil is deenergized, the contact will be reset immediately. 

So, the working principles are as follows: When the coil is energized, the armature overcomes the resistance of the counter-force spring and close the fixed iron core immediately, the piston rod moves downwards under the function of pagoda, so that the rubber membrane connecting with the piston rod  moves upwards too, but due to the limit of the velocity of air intake from the air inlet, under the rubber membrane there is a thin air space, which forms pressure difference with the air on the rubber membrane, thus producing damping effect on the movement of the piston. The air enters to the air sac from the air inlet. Over a period of time, the piston can finish its whole stroke and press the micro- switch, making the normally closed contact disconnect delay and the normally open contacts close delay. The duration of the delay time is determined by the throttling degree of the orifice, the faster the air intake is , the shorter the time delays. Twisted throttle screw can adjust the size of the air inlet, so as to achieve the purpose of adjusting the delay time length. The delay range is two kinds of 0.4 ~ 60 s and 0.4 ~ 180 s. After the microswitch is closed（attracted) by the armature, it will immediately act through the pushing plate, to make the normally closed contact instantaneously disconnect and the normally open contact instantaneously close. 

When the coil is deenergized, under the function of spring, the armature pushes the piston to the most bottom through the piston rod, at this moment, the air inside the air chamber under the rubber membrane passes through the non-refeed valve  locally formed by rubber membrane, the weak spring and piston and quickly exhausts from the aperture of the air chamber at top of the rubber membrane, so that the normally closed contact of microswitch instantaneously closes, the normally open contacts  instantaneously  disconnects and the contacts also reset immediately. 

To sum up: For the relay of energizing delay, the coil is energized, the contact will act in delay; the coil is deenergized, the contact will instantaneously reset. The principle of action of the deenergizing delay type: The coil is energized, the contact will act instantaneously; the coil is deenergized, the contact will reset in delay.