Simple MOSFET Timer Circuit

Ever hear of a MOSFET?

It is a tiny part that is becoming popular in electronics because it works better than older parts for some things.

This article shows you how to build a cool timer circuit using a MOSFET.

Because MOSFETs are so good at their job, this timer can delay things for a long time and be super accurate.

What is MOSFET Timer Circuit:

A MOSFET timer circuit typically involves the use of a Metal Oxide Semiconductor Field Effect Transistor as a switching element in combination with other components to create a timer or delay circuit.

MOSFETs are semiconductor devices that can be used as electronic switches, making them suitable for various timing applications.

Circuit Working:

Parts List:

Pressing and releasing the push button triggers the circuits operation.

The 100uF capacitor accumulates the 12V supply serving as the energy reservoir to power the MOSFET.

The stored voltage in the capacitor activates the MOSFET initiating its conducting state.

The MOSFET, in turn energizes the relay leading to the circuits operational phase.

Subsequent to activation, the 100uF capacitor undergoes a controlled discharge through the 10M resistor.

As the voltage across the capacitor drops below 5V, signifying the depletion of stored energy the MOSFET begins transitioning to its non conducting state.

The MOSFET, now in the process of turning off ceases to power the relay leading to its deactivation.

The duration for which the MOSFET and relay remain active is determined by the combination of the 100 uF capacitor and the 10M resistor.

Advantages of MOSFET Implementation:

The MOSFETs distinctive advantage lies in its virtually infinite gate impedance.

This characteristic prevents the 100uF capacitor from discharging through the MOSFET gate ensuring precise control over the timing mechanism.

The high gate impedance enables the use of an exceptionally large resistor even up to 50M.

This flexibility results in the MOSFET timer circuit accommodating very long delay intervals, making it ideal for applications requiring extended timing precision.

Formulas:

Below mentioned formula is utilized to get the fundamental resistor capacitor RC circuits time constant (τ).

Although it is commonly called the delay time, it is more realistic to think of it as a distinctive time constant that affects the capacitors charging and discharging behavior.

T = R * C

here,

• T (τ): Represents the time constant of the circuit in seconds (s).
• R: Represents the resistance of the resistor in ohms Ω, here R = 10MΩ (10 million ohms).
• C: Represents the capacitance of the capacitor in farads F, here C = 100µF (microfarads).

Calculation:

• Convert microfarads to Farads: 100µF = 0.000 1F
• Apply the formula: T = R * C = 10MΩ × 0.000 1F = 0.1 seconds

Therefore, the time constant of your RC circuit with a 10MΩ resistor and 100µF capacitor is approximately 0.1 seconds.

How formula works:

The duration required for the voltage across the capacitor Vc to reach around 63.2% of the applied voltage source Vs during charging or to drop to 36.8% of Vs while discharging is indicated by the time constant (τ).

It does not indicate how long it takes for the supply voltage or the voltage to reach 0V.

Note:

This formula assumes perfect components and is only an approximation.

Leakage current may occur in real capacitors, while minor tolerance differences may occur in resistors.

For simple RC charging and discharging applications, this method works well.

Different calculations may be required for more complicated circuits based on the particular application.

Knowledge the behavior of RC circuits in a variety of applications, such as timers, filters and signal coupling, requires a knowledge of the time constant.

How to Build:

Building the simple MOSFET based timer circuit requires careful attention to detail and adherence to the provided specifications.

• Collect all the required components listed above.
• Identify the pins of the N channel enhancement type MOSFET.
• It typically has three pins: Gate, Source and Drain.
• Place the MOSFET on the breadboard and connect it as follows:
• Connect the MOSFETs gate pin between 10M resistor and a bush button.
• Connect the MOSFETs source pin to the ground.
• Connect the MOSFETs drain pin to one end of the 12V relay.
• Connect one leg of the capacitor to the MOSFETs gate pin and to ground.
• Connect one end of the 10M resistor to the MOSFETs gate pin and other to ground.
• Connect one leg of relay to 12V positive source.
• Connect diode 1N4007 between MOSFETs drain pin and 12V positive supply.

Test the Circuit

• Press and release the push button to initiate the circuit.
• Observe the relay activation and deactivation based on the timing determined by the capacitor and resistor values.

Circuit Operation:

• Pressing the push button charges the capacitor.
• The MOSFET is activated, powering the relay.
• The capacitor discharges through the resistor determining the timing.
• When the voltage across the capacitor drops below 5V the MOSFET deactivates turning off the relay.

Conclusion:

In conclusion, the simple MOSFET based timer circuit described herein exemplifies a sophisticated timing solution with unique advantages.

By leveraging the characteristics of MOSFETs and carefully selecting component values, this circuit offers precision and flexibility in timing applications making it a valuable asset in electronic projects requiring extended delay intervals.

References

Delayed start with RC and MOSFET