Sunday, April 7, 2013

Simple Mosfet Switch Circuit with Delay Timer

The following article discusses the use of a mosfet as a switch for toggling high current loads efficiently. The circuit can be also transformed into a delay OFF circuit with simple modifications. The design was requested by Mr.Roderel Masibay.


A field effect transistor or mosfet can be compared with a bjt or the ordinary transistors, except one significant difference.

A mosfet is a voltage dependent device unlike BJTs which are current dependent devices, meaning a mosfet would switch ON fully in response to a voltage above 5V at virtually zero current across its gate and source, whereas an ordinary transistor would ask for relatively higher current for switching ON.

Moreover this current requirement grows higher proportionately as the connected load current increases across its collector. Mosfets on the other hand would switch any specified load irrespective of gate current level which may be maintained at the lowest possible levels.

Another good thing about mosfet switching is they conduct fully offering very low resistance across the current path to the load.

Additionally a mosfet wouldn't require a resistor for gate triggering and may be switched directly with the available supply voltage provided it's not far too beyond the 12V mark

All these properties associated with mosfets makes it a clear winner when compared to BJTs, especially when it's used like a switch for operating powerful loads such as high current incandescent lamps, halogen lamps, motors, solenoids etc.

As requested here we'll see how a mosfet may be used as a switch for toggling a car wiper system. A car wiper motor consumes considerable amount of current and is usually switched through a buffer stage such as  relays, SSRs etc. However relays can be prone to wear and tear while SSRs can be too costly.

A simpler option can be in the form of a mosfet switch, Let's learn the circuit details of the same.

As shown in the given circuit  diagram the mosfet forms the main controlling device with practically no complications around it.

A switch at its gate which can be used for switching ON the mosfet and a resistor for keeping the mosfet gate to a negative logic when the switch is in the OFF position.

Pressing the switch provides the mosfet with the required gate voltage relative to its source which is at zero potential.

The trigger instantly switches ON the mosfet so that the load connected at its drain arm becomes fully ON and operative.

With a wiper device attached to this point would make it wipe for so long the switched remains depressed.

A wiper system sometimes requires a delay feature for enabling a few minutes of wiping action before stopping.

With a small modification, the above circuit can be simply turned into a delay OFF circuit.

As shown in the diagram below, a capacitor is added just after the switch and across the 1M resistor.

When the switch is momentarily turned ON, the load switches ON and also the capacitor charges up and stores the charge in it.

When the switch is toggled OFF, the load continues to receive the power since the stored voltage in the capacitor sustains the gate voltage and keeps it switched ON.

However the capacitor gradually discharges via the 1M resistor and when the voltage drop below 3V, the mosfet is no longer able to hold, and the complete system switches OFF.

The delay period depends on the value of the capacitor and the resistor values, increasing any one of them or both increases the delay period proportionately.

Making a Long Duration Timer

A relatively long duration timer may be designed using the above explained mosfet concept for switching heavier loads.

The following diagram depicts the procedures of implementing it.

The inclusion of a extra PNP transistor and a few other passive components enables the circuit to produce higher duration of delay period. The timings may be suitably adjusted by varying the capacitor and resistor connected across the base of the transistor.







14 comments:

  1. thanks sir swagatam for your quik response for my resquest your the man hehe

    ReplyDelete
  2. sir swagatam ihave mosfet IRFZ44n this mosfet work desame IRF540n,or not ?

    ReplyDelete
    Replies
    1. you are welcome Roderel, yes your msfet will also work here.

      Delete
  3. very very thanks sir swagatam yah its deferent in ampere and volts max right?

    ReplyDelete
  4. sir i want a high current automatic solar charge controller ,battery full charge cutoff,low and battery full indicator.please sent me my email egenfarid29@gmail.com

    ReplyDelete
    Replies
    1. check out the last circuit in this article, it's a over/low charge controller:

      http://homemadecircuitsandschematics.blogspot.in/2011/12/how-to-make-simple-low-battery-voltage.html

      also checkout this high current charger circuit:

      http://homemadecircuitsandschematics.blogspot.in/2012/11/high-current-transistor-tip36-datasheet.html

      combine the two circuits for your application.

      Delete
  5. I have a small problem and I am trying to use IRFZ44N.. Since the starter is manufacture connected to the ground, and p-mostet will not be able to handle such a large current to drive the motor to drive, that is why I am trying to make this n-channel mosfet to be connected to the motor to drive properly. But can I know the near current of gate for each mosfet. So that I will start to use the current limiting circuits for it. Well I have not described that I am using for my bike starter motor in which the relay refuse to return to the original position (off/no connection) after releasing the push button switch, thus draining all the current to the motor resulting to wastage of fuel on the run and damaged the battery.
    Reply

    ReplyDelete
    Replies
    1. who said P-channel cannot handle large currents? N and P are complementary devices, both types are assigned to handle currents as per their specs.

      mosfets are not current dependent, you can drive them even with currents as low as 1mA...the voltage should be above 5V, though... that's the only criterion.

      the relay issue may be some internal problem of the starter, get it checked from a qualified mechanic.

      Delete
  6. Hi,

    Can you please explain why the the 1M ohm resistor is needed? In other mosfet switch sample circuits, people are using 2.2K or 10K or even 100K resistors!

    Thanks in advance

    ReplyDelete
    Replies
    1. Here we have tried to apply a mosfet to work as a cheap, simple timer therefore we had to employ a 1M resistor, however in the first circuit you can use any other smaller value resistor

      Delete
  7. Hi Swagatam

    Please modify the 1st circuit replacing -12V with 0V.

    ReplyDelete
    Replies
    1. Hi Abu-Hafss, the (-) sign is relative to the (+) sign, just as we have marked in batteries.

      A (0) sign is also correct, any of the signs may be used for indicating the negative line, but only as long as the supply is not a dual type...

      Delete
    2. Oh, got it.
      I thought it is designed for dual supply.

      Delete

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Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.