Simple Power ON Delay Circuit

A simple power on delay circuit using IC 555 establishes a delay between power supply and circuit activation.

When a trigger is received, the IC 555 runs in monostable mode, which causes it to produce a single output pulse for a predetermined amount of time.

The timing network that establishes the delay time is made up of the resistor R1 and capacitor C2.

The timers output remains low (circuit off) while the capacitor charges.

Applying power causes the capacitor to begin charging through R1.

The 555s output flips high, turning on the circuit when the capacitor voltage reaches two thirds of the supply voltage.

After that, the capacitor discharges via a route inside the timer clearing the circuit and delaying the start of the subsequent power cycle.

Circuit Working:

Parts List:

The above circuit works as a power on delay circuit with a relay output, utilizing a 555 IC and a bridge rectifier.

Power Up and Rectification:

AC to DC Conversion:

AC voltage is supplied by the transformer when the circuit is plugged in.

This AC voltage is rectified into DC voltage by the bridge rectifier, which is made up of 4 diodes 1N4007.

The remaining components of the circuit are powered by this rectified DC voltage.

Delay Timer with 555 IC:

Monostable Mode:

The monostable mode is selected for the 555 IC.

In this mode, when a trigger is received, it generates a single high pulse for a set amount of time.

The power on event serves as the trigger in this circuit.

Timing Network and Delay:

A timing network made up of a resistor R1 and a capacitor C2 regulates the delay time.

Capacitor C2 begins charging through resistor R1 as soon as power is applied.

As C2 charges, the voltage across it progressively increases.

During this charging period, the relay stays inactive (off) due to the timers output pin 3 remaining low 0V.

Triggering the Delay:

The timers output pin 3 flips to high voltage (around the supply voltage level) when the voltage on C2 reaches about two thirds of the supply voltage (as calculated by internal resistors in the 555 IC).

This starts the subsequent phase.

Relay Activation and Device Turn On:

Relay Control:

The relay coil is activated by the high voltage on pin 3 of the timer.

Relay contacts switch as a result finishing the circuit for the connected device.

The device then turns on.

Electrolytic Capacitor C1:

By connecting across the relay coil, capacitor C1 creates a snubber circuit.

In order to shield the circuit from voltage spikes, it assists in absorbing the voltage spike that happens when the relay coil disengages.

Reset and Waiting for Next Cycle:

Resetting the Timer:

Capacitor C2 internally discharges through the 555 IC discharge pin 7 after the predefined delay specified by R1 and C2.

The timer is reset as a result, and pin 3s output returns to low voltage 0V.

The linked gadget shuts off when the relay coil de energizes.

Waiting for Next Power Cycle:

Until you apply power again, the circuit remains in this state (relay off, timer reset).

After that, the cycle is repeated delaying the activation of the linked device each time the power is turned on.

Optional LED for Visualization:

To see the delay attach an LED and a current limiting resistor to the timers output pin 3.

When the delay is over, the LED will turn on after being off throughout that time.

Formula:

The delay time T produced by the RC network and the 555 IC is determined using the following crucial formula in this circuit:

T = 1.1 * R1 * C2

where:

• T is the delay time in seconds
• R1 is the value of the timing resistor R1 in the circuit in ohms
• C1 is the value of the timing capacitor C2 in the circuit in farads

With this formula, you may find the circuits delay duration depending on the resistor and capacitor values that you select.

You may change the delay time by substituting various values for R1 or C2, then recalculating T using the formula.

How to Build:

To build a Simple Power ON Delay Circuit follow the below mentioned steps:

• Gather all the components mentioned in the circuit diagram above:
• Connect pin 1 of IC1 555 to ground.
• Connect pin 2 to pin 6 of IC1 555, connect capacitor C2 and resistor R1 in series to ground.
• Connect pin 3 of IC 555 to base of transistor Q1 through resistor R2.
• Connect pin 4 and pin 8 of IC 555 to positive supply.
• Connect collector of transistor Q1 to positive supply through diode D1, connect transistor Q1 emitter to ground.
• Connect capacitor C1 from positive supply to ground.
• Connect 12V relays one coil terminal to positive supply and other coil terminal between collector of Q1 transistor and diode D1.
• Connect bridge rectifier connections a shown in circuit diagram.

Conclusion:

To conclude, In this simple power on delay circuit when you connect in your gadget, the circuit produces a little pause (delay) before turning it on.

The wait time is set by a chip IC 555 using a resistor and capacitor.

These are the components that you may modify to alter the delay.

References:

A simple delay ON circuit