Simple MOSFET Timer Circuit

Have anybody heard about MOSFET before?

This small part is now getting popular in electronics because it does some jobs better than old parts.

In this Simple MOSFET Timer Circuit we will show how to use MOSFET for making one cool timer circuit.

Because MOSFET work very good and this timer can give good accuracy and make delay for long time also.

What is MOSFET Timer Circuit:

When making delay or timer circuit MOSFET timer mostly work like switch with other components.

MOSFET is a semiconductor part which is used like electronic switch.

That is why it is good for many types of timing jobs.

Circuit Working:

Parts List:

This circuit starts working when push button is pressed and then released.

MOSFET get power from 100uF capacitor which take charge from 12V supply.

This stored charge in capacitor turn ON the MOSFET so it start conducting.

Then MOSFET turn ON the relay and circuit begin to work.

After that capacitor slowly start to discharge through 10M resistor.

When voltage in capacitor goes below 5V it means energy is almost finished.

Then MOSFET begin to turn OFF and does not conduct anymore.

Because of this MOSFET stop giving power and relay also turn OFF.

The 100uF capacitor and 10M resistor together decide how long MOSFET and relay stay ON.

Why use MOSFET:

MOSFET have special thing very big gate resistance and no current flow at gate.

Because of this timing is more stable and the 100uF capacitor cannot lose charge through gate.

Even 50M resistor can be used because gate does not take current.

This is why MOSFET timer can make very long and accurate delay which is good for high precision timing jobs

Formulas:

RC circuit have basic formula to find time constant (τ) and this is not exact delay time but special time number that show how fast capacitor charge or discharge.

Formula:

T = R × C

where:

• T (τ) is time constant in seconds (s)
• R is resistor value in ohms Ω and in this circuit it is R = 10MΩ (10 million ohms)
• C is capacitor value in farads F and here C = 100µF

Calculation:

First change microfarads to farads: 100µF = 0.0001F

Now use formula:

T = 10MΩ × 0.0001F = 0.1 seconds

So the time constant for 10MΩ resistor and 100µF capacitor is around 0.1 sec.

How this formula work:

Time constant means how long it takes for capacitor voltage (Vc) to:

Goes up to 63.2% of battery voltage (Vs) when charging

Goes down to 36.8% of Vs when discharging

This does not mean voltage becomes 0 it just show speed of charging or discharging.

Important Note:

This formula gives only close idea and in actual circuit:

Capacitors may leak small current

Resistors may not be 100% exact value

Still for simple circuits like RC timers or signal filters this method works good.

In big or advanced circuits more calculation are needed.

But knowing time constant can help to understand how RC circuit behaves in many electronics like timer, filter, delay and signal jobs.

How to Build:

To build a Simple MOSFET Timer Circuit follow the following steps:

Get All Components Ready:

• Take all parts we need from above list.

Check MOSFET Pins:

• MOSFET N-channel enhancement type have 3 pins: Gate (G), Source (S), Drain (D)
• Place MOSFET on Breadboard
• Gate pin goes between 10M resistor and push button
• Source pin connect to GND
• Drain pin connect to one end of 12V relay coil

More Connections:

• Positive side of capacitor 100µF connect to gate pin of MOSFET
• Negative side connects to ground
• 10M resistor also connect to gate pin on one side and other side to ground
• One coil pin of relay connects to +12V supply
• Other coil pin connects to drain pin of MOSFET
• Put diode 1N4007 between drain pin of MOSFET and +12V supply and cathode to +12V and anode to drain

Test the Circuit:

• Press and leave the push button
• Watch how relay turn ON and OFF
• Timing depend on capacitor and resistor values

How Circuit Works:

• Push button is pressed → Capacitor gets charged
• Gate of MOSFET receives voltage → MOSFET turns ON
• MOSFET allows current → Relay gets power and activates
• Capacitor now slowly discharges through resistor 10MΩ
• When voltage goes below 5V then MOSFET turns OFF
• Relay deactivates and circuit stop

Conclusion:

This easy and Simple MOSFET Timer Circuit is a good example of smart timing idea with some nice benefits.

Because of MOSFET features and right choice of parts this circuit gives stable and flexible timing.

That is why it very helpful in electrical projects where long delay time is needed.

References:

Delayed start with RC and MOSFET