The timing of the circuit can be changed by reducing or increasing the RC values of R7 and E2. Once the charge has been discharged (which will take around 150 - 210 secs for the values shown), transistor Q2 will be able to turn ON. When the mains supply cuts off, the charge at capacitor E2 will cause the base of transistor Q2 to be reverse bias and cannot turn ON until almost all the charges have been discharged through resistor R7. During the ON stage of the SCR, the capacitor E2 is charged to its maximum value. The use of C1 and R6 across the SCR acts as a snubber circuit to reduce the switching noise generated by the SCR when it turns OFF/ON. Once these two transitors are ON, the SCR will turn ON as well. No heatsink is required for the SCR.Īt power on, there is no charge at E2, hence the transistor Q2 will be forward bias and turn ON when Q3 turn ON. Relays used should have coil ratings below 1A in order not to overheat the SCR. Relay outputs provide isolation between the supply circuit and the outpu. The thyristor used can be either 2P6M or MCR106-8 or equivalent parts available in the market. Zelio timer relay enables simple automation cycles to be set up using wired logic. More accurate timing can be achieved by using low tolerance resistor and capacitor. Once the mains power supply cuts off, the relay will only be able to turn ON after a period of 150 - 210 secs depending on the tolerance of the RC circuit represented by resistor R7(5.1 Mohm) and electrolytic capacitor E2(47uF). The schematic below shows the circuit diagram of the ON delay timer. All parts are potted to prevent users from touching the parts. Once tested working, the components should be potted using epoxy with only the terminals exposed. Circuit should be tested using printed circuit board and an isolating variable transformer where the voltage is slowly ramped up from zero. It is not recommended to use breadboard to test the circuit. Some parts may "burst" if there are some short circuit in the circuit. It works with two switches, one for rest and the. This project should be handled by experienced electronics designer as its part are powered on directly from the mains supply.Īs all parts is "live", one may get electric shock if care is not taken when testing the circuit. The 555 delay IC circuit is a popular, versatile, and simple timer manufacturers use to design other circuits. If a microcontroller based solution is used, extra circuitry such as backup battery or supercapacitor need to be incorporated in order to retain the memory of the MCU and to ensure that the clock still runs even after the supply has cuts off. The use of RC circuitry as a timer circuit is reliable and is not susceptible to "memory loss" as in the case of microcontroller. Time-delay, or time-release relays, allow necessary actions to happen at specific times in an electrical apparatus because they, in essence, act as a timer. The use of microcontroller based devices are not reliable in that if the power supply cuts off and came back again in a short period of time, it will reset and "forgotten" its previous state.
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