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.
Comparing Mosfet with BJT
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.
Why Mosfet is Better BJT
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.
Using Mosfet as a Switch
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.
Using Mosfet as a Delay Timer
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.
Video Demonstration
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.
Calculating the Delay
To calculate the delay produced by the RC constant we can use the following formula:
V = V0 x e(-t/RC)
- V is threshold Voltage at which the mosfet is supposed to just switch OFF or just begin turning ON.
- V0 is the supply voltage or the Vcc
- R is the discharge resistance (Ω) which is connected parallel to the capacitor.
- C (Capacitor Value (F) in the exemple 100uF)
- t (time of discharging that we want to calculate (s) )
we want to know the delay (t) = e(-t/RC) = V/V0
-t/RC = Ln(V/V0)
t = -Ln(V/V0) x R x C
Example Solution
If we select the threshold capacitance turn ON/OFF value of the mosfet as 2.1V, and supply voltage as 12V, resistance as 100K, and capacitor as 100uF the delay after which the mosfet will turn OFF could be approximately calculated by solving the equation as given below:
t = -Ln (2.1/12) x 100000 x 0.0001
t = 17.42 s
Thus from the results we find that the delay will be around 17 seconds
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.
About the Author
I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!
Pardon my ignorance, I know nothing about circuitry! I want to use a watch battery to power a mini vibration motor when a button is pressed and for the motor to stop after a certain number of seconds if the button remains pressed. Can your mosfet timer circuit be used for this purpose?
You can use the first diagram. Replace the switch with a 100uF capacitor or any value which suits the delay requirement. Now when you switch ON power through a pressed push button the mosfet will conduct for sometime and then switch OFF.
Make sure to add a 10K resistor across the circuit +/- supply lines so that the capacitor is able to discharge when switch is released.
I require,
A battery operated timer to illuminate USB lights in a cycle that automatically turn on at the same time every day 6 hours on 18 hours off.
Thanks.
I think you can try the following concept
https://homemade-circuits.com/how-to-make-simple-programmable-timer/
Hello, I'm trying to make a light turn on for 30 seconds to 1 minute every time a button gets press or a button gets disconnected what ever is easier. Would I be able to use this circuit for that. Thank you in advance.
Hello, yes the above circuit will work, you can also try the following concept which is cheaper and more reliable:
https://homemade-circuits.com/2014/11/long-duration-timer-circuit-using.html
Agree with you. Before this, I did approach some local universities & organizations (e.g SIRIM) to introduce my "idea", but none of them are interested. The reason they gave: there's already products out there in the market.
That one I do agree. But, mine is totally different (I would say) – just imagine you can build digital circuit (e.g 4-digit key code, sequencial controller, numerical display) without involve any software/programming. Only hardware involve, which give a tangible programming – easily to explain how the circuit "works".
Anyway, thank you so much for your valuable feedback.
The "digital" justification will not hold good especially after the advent of today's easy and highly advanced microcontroller applications such as Arduino which are specialists in digital execution.
as far as technology is concerned the human psychology has always been to go for the most advanced options and not the low-tech obsolete ones, therefore your gadget will not sustain in this era.
However microcontrolers are 'helpless' without the hardware stage or the power output stage, so you can think of some alternative board that would allow the students to get a quick access to all sorts of hardware stages for a given Arduino application
I'm currently working on one project, which I called "Digital Electronic LEGO" (DEL). This DEL consists of 2 sections: Board & Block. The block contains electronic components, and can be divided into 4 main groups: LOGIC gates (e.g AND, OR, NOT, NAND,Flip-Flop, etc.), Connector (e.g Straight, Tee, Cross, etc.), Input (e.g Switch, Pulse Generator, etc.) & Output (LED, Buzzer, 7-segment, etc.). Most blocks are of standard size with square shape (1×1). Others might be multiple of standard size (e.g 1×3, 1×4, etc.). These blocks can be arranged onto the board to build a desire digital electronic circuit, such as Up/down counter, timer, light chasing etc. easily, similar to LEGO concept. The board got 2 functions: to give power supply to the block, to provide a connection between two adjacent blocks. FYI, this DEL is an educational tools designed for young student to explore "possible" logic ideas.
At present, I don't see any products available in the market which utilize the concept as above. Have you came across of such product? I do understand that this DEL is still "on paper". Do you think it can be materialize??? Please advise.
Appreciate if you could provide your response by email to my Gmail: rahimanakmar@gmail.com
Tq.
The concept looks good and useful for the new hobbyists, however I am not sure whether or not this will attract many customers, because today with the presence of programmable boards like Arduno everybody is busy concentrating on these digital programmable boards which allow you to launch almost any desired application simply by compiling a a few lines of codes.
If I change the power supply voltage from 12v to 14.5v, will it affect the timer duration?
yes that might force the capacitor to charge faster reducing the delay period, you may have to increase the resistor value proportionately.
What is the maximum current in output ( load)
= to supply current
Should I use same 1m resistor ?
yes will do, the value decides the delay length.
Can i replace irf540n with irlb8721pbf ? And what value of resistor should i use?
yes you can..
Hi Swagatam
Please modify the 1st circuit replacing -12V with 0V.
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…
Oh, got it.
I thought it is designed for dual supply.
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
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