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Simple Time Delay Relay Circuit

Can anyone think small timer for their electric things?

Time delay relay circuit work like stopwatch for power.

It waits for some time before turning ON or OFF something like light or motor.

This wait time can be same always like 5 second or can change using timer knob.

Main reason to use it is to control when thing start or stop in system.

Circuit Working:

Parts List:

CategoryComponentQuantity
Resistors1k 1/4 watt2
Preset 50k1
CapacitorsElectrolytic 1000µF 25V1
Electrolytic 100µF 25V1
SemiconductorsTransistor BC5471
Zener diode 3V 1/2 W1
Diode 1N40071
Red LED 5mm 20mA1
Relay 12V1

Protect the device using this small simple time delay relay circuit.

Modern electronics use SMPS power supply which can get damage from sudden power spike.

This circuit wait for one minute before turning ON the device.

So it stop big current rush and spike at power ON.

When power comes back after the cut big current can hurt the SMPS.

This power spike happen because of strong magnetic force in power transformer.

If we wait little time before to turn ON we can stop this damage.

This time delay circuit does that which gives power to device after 1 or 2 minutes of delay when power comes back.

This circuit work like switch controlled by Zener diode.

Capacitor C1 slowly get charge through resistor R1 and VR.

When C1 voltage goes above 3V then Zener diode starts work and turn ON the transistor T1.

Relay is connected to T1 but when T1 turns ON relay also turns ON.

Then it gives power to device using relay contacts.

Relay stays on as long as power is stable.

Capacitor C2 help T1 to stay ON properly and stop relay from clicking ON and OFF.

Diode D1 stops back current when T1 turn OFF.

Red LED shows when relay is working.

Delay time depends on size of C1.

Formulas:

Formula for Time Delay Relay Circuit:

Time Constant (τ):

This is main thing to find delay time.

It is just resistance (R) times capacitor (C).

It tells how long capacitor take to charge or discharge for the needed voltage.

Simple formula to imagine delay time:

Delay Time = k × τ

where,

One can find it like in the below formula:

τ = R × C

where,

Important Points:

This is just rough idea but actual delay can change because of parts difference and how transistor work.

This formula works for simple circuit with one resistor and one capacitor for RC network.

Complex circuits may need different method to find delay.

How to Use Formula:

To find timing resistor Rt and capacitor Ct in the circuit:

How to Build:

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

Get the Parts:

Make the Connections:

Test and Set:

Fix the Circuit:

Connect the Device:

Be Safe:

Conclusion:

This Simple Time Delay Relay Circuit is easy and useful way to protect electronic devices that use SMPS power.

It waits for 1 to 2 minutes before giving power so it stops damage from big current rush and power spikes.

This make the device work better and live longer.

References:

RC time delay circuit to trigger relay

Time delay off relay design

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