CHAPTER 7 : TIMING RELAYS

One kind of industrial control system is timing relays. Timing relays used to start of event must be delayed until another event has occurred.

Timing relays are conventional relays that equipped with an additional hardware mechanism or circuity to delay the opening or or closing of loads contacts.

There are two kind of timing relays :

1. Pneumatic (air) timing relay uses mechanical linkage and air bellows system to achieve its timing cycle. In pneumatic relay consists of two timed and two instantaneous contacts. when the coil is energized the timed contacts are prevented from opening or closing. When the coil is deenergized the contacts return immediately to their normal state.
   
2. Solid State Timing relay uses electronic circuitry to achieving its time cycle. Some of these timers use a resistor/capacitor (RC) time constant to obtain the time base, and others are quartz clocks as the time base. An RC oscillator networks generates  a highly stable and accurate pulse that it used to provide the different time delay increments and switch a contact output.
   

Time-delay relays can be classified into two basic :

• ON delay relay is often referred to as DOE (Delay On Energized) when power is connected to the coil of ON delay timer , the contacts delay changing position for some period of time.
• OFF delay relay is often referred to as DODE (Delay On Deenergize), the operation is opposite of the operation of the ON delay timer, when voltage is applied to the coil of the OFF delay timer, the contacts will change position immediately. when the coil is deenergized there is a time delay before the contacts change to their normal position.

Reference :

Petruzella, D. Frank (1996). Industrial Electronics. New York : McGraw-Hill