SIMPLE DELAY TIMER CIRCUIT SERIES
For shorter delays, the circuit works fine but for longer delays, a 12V relay may be unstable and oscillations of the armature can be found.įor longer delays, it is advised to use a 6V relay with a 100Ω resistor in series with the coil must be used. The delay of the relay can be controlled by variable resistor and 1000♟ capacitor. The 100♟ capacitor that is connected to the base of the transistor, is used to keep a stable base bias of the transistor so that there are no relay clicking. As a result, the contacts of the relay switch. Hence, the relay coil gets energized when the transistor is switched on. The relay coil is connected to collector of the transistor. Once the charge across the 1000♟ capacitor reaches 3.3V, Zener diode starts conducting.Īs the Zener is connected to the base of the transistor, it triggers the transistor and it is turned on. When the power to the circuit is turned on, the 1000♟ capacitor charges through the 100KΩ variable resistor.
The circuit is based on the RC time delay and Zener controlled switch. The working is very simple and is explained below. Time Delay Relay can provide a short delay after power is turned on and before switching on the device. The aim of this project is to demonstrate the functioning of a time delay relay. This prevents the disastrous effects of voltage spikes or input surge currents. Hence, it is safe to provide a time delay before giving the power to device. Input surge current at power on or power resuming after failure can cause severe damage to SMPS systems in electronic devices. Such power systems are vulnerable to spikes in mains power supply. Modern day electronic devices use SMPS based power supply systems. To show the switching operation of the relay, an LED is connected to Normally Open contact of the relay and the Com contact is connected to supply. An LED along with a current limiting resistor is connected from the collector of the transistor. One end of the relay coil is connected to the Collector terminal of the transistor and the other end of the coil is connected to supply.Ī diode is placed between the coil terminals. Negative terminals of both the capacitors and the Emitter terminal of the transistor are connected to ground. Anode of Zener diode is also connected to the base of transistor TIP122. The wiper terminal of the variable resistor is also connected to the cathode of the Zener diode.Īnode of the Zener diode is connected to positive terminal of 100♟ capacitor.
The wiper of the variable resistor is connected to the positive terminal of a 1000♟ capacitor. A 1KΩ resistor, a 100KΩ variable resistor and another 1KΩ resistor are connected in series between the supply and ground.
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