Simple Time Delay Relay Circuit

Can you imagine a tiny timer for your electronics.

A time delay relay circuit is like a stopwatch for power.

It can wait a set amount of time before turning something on or off like a light or a motor.

This delay can be fixed, like waiting 5 seconds every time, or adjustable, like setting a timer knob.

The main reason to use one is to control exactly when things happen in a system.

Circuit Working:

Parts List:

Safeguard your equipment using this compact Simple Time Delay Relay Circuit.

The switched mode power supply SMPS in modern electronics is susceptible to mains line spikes, so this circuit introduces a one minute delay before powering the device preventing damage from inrush current and power on spikes.

When power is initially applied or restored after an outage inrush current can harm the SMPS in electronic devices.

The power on spike that occurs when power returns is caused by high magnetic flux in the distribution transformer of the mains network.

Providing a brief delay can prevent such damage.

The described time delay relay circuit serves this purpose supplying power to the device only after a delay of one to two minutes following power activation.

The circuit operates as a zener controlled switch.

Capacitor C1 charges through resistors R1 and VR.

When the voltage across C1 exceeds 3V the zener diode conducts triggering transistor T1.

The relay connected to T1s collector activates providing power through the relays common and normally open contacts.

The relay remains engaged as long as the mains voltage remains stable.

Capacitor C2 maintains a steady base bias for T1 preventing relay chatter.

Diode D1 protects against back electromotive force when T1 switches off and a red LED indicates when the relay is active.

The delay time is determined by the value of C1.

Formulas:

Formula for time delay relay circuit is mentioned below:

Time Constant (τ):

An essential factor in calculating delay time, this is the product of capacitance C and resistance R.

It shows how long it takes for a capacitor to fill up or empty to a particular voltage.

A simple formula to estimate the time delay is as follows:

Delay Time = k × τ

where,

• Delay Time: This is the amount of time, expressed in seconds (s), that the relay needs to activate when power is applied.
  k: This constant is dependent on the transistors operating point and circuit layout.
• Depending on the particular circuit design, it might vary somewhat from the average of 0.693 for simple designs.
  τ (tau): The time constant may be computed as follows:

τ = R × C

where,

• R: This is the timing resistors resistance, measured in ohms.
• C: This is the timing capacitors capacitance, measured in farads.

Key Points:

This is only an estimate, and other variables like transistor characteristics and component tolerances may have an impact on the actual delay time.

The circuit is assumed to use a single RC timing network in the formula.

Complex circuits may have more parts or stages, necessitating the use of distinct methods to determine the delay.

Applying the Formula and its works:

• In the circuit diagram, locate the timing resistor Rt and capacitor Ct.
• To find the time constant (τ), multiply the values of Rt and Ct.
• To find the predicted delay time, multiply τ by the constant k, which is typically around 0.693.
• Recall that this is a condensed method.
• Advanced analytical methods or circuit modeling tools may be required for a more accurate computation.

How to Build:

To build a Simple Time Delay Relay Circuit you need to follow the below mentioned assembling steps:

Prepare the Components:

• Gather all the required components listed above.

Connect the Components:

• Follow these connections:
• Connect the anode of the zener diode to the base of the NPN transistor T1.
• Connect the cathode of the zener diode to the junction of R1 and VR.
• Connect the collector of T1 to one terminal of the relay coil.
• Connect the other terminal of the relay coil to the positive supply voltage 12V.
• Connect the emitter of T1 to the negative supply voltage ground.
• Connect one terminal of the normally open NO contact of the relay to the positive supply voltage.
• Connect the other terminal of the NO contact to the load electronic device.
• Connect the common terminal of the relay to the negative supply voltage.
• Connect capacitor C1 between the zener cathode and the negative supply ground.
• Connect capacitor C2 between the negative supply voltage and the base of T1.
• Connect resistor R1 in series between VR upper arm and the positive supply voltage.
• Connect resistor R2 between the lower arm of the preset and ground.
• Connect diode D1 across the relay coil, with the cathode connected to the positive supply voltage and the anode connected to the collector of T1.

Testing and Adjustment:

• Power up the circuit and adjust VR until the relay activates after the desired delay time one to two minutes.
• You can use a multimeter to measure the delay accurately.

Finalize the Circuit:

• Once you have tested and adjusted the circuit you can finalize the connections on a prototyping board or PCB for a more permanent setup.

Connect the Load:

• Connect your electronic device load to the relays NO contact.
• The device will now receive power only after the specified delay protecting it from inrush current and power on spikes.

Note:

• Always exercise caution when working with electrical circuits.
• If you are unsure seek assistance from someone with experience in electronics.

Conclusion:

To conclude, the 12V time delay relay circuit offers a simple and effective solution to protect electronic devices with SMPS based power supplies.

By introducing a delay of one to two minutes before applying power the circuit helps prevent damage from inrush current and power on spikes, improving the overall reliability and lifespan of the equipment.

References:

RC time delay circuit to trigger relay

Time delay off relay design