arduinocircuit-designmosfetswitching
I have a circuit that requires to switch on/off the load using arduino control signal. The load uses 5V DC and upto 3A current. MOSFET or relay must be used to switch the load supply on/off. Input to my circuit is continuous 5V DC.
Which should I use either a relay or MOSFET?
I have used IRF540 MOSFET which works but after it switches on the load voltage is only 3V, which is very less power to the load.
If I have to use a MOSFET what type of MOSFET is preferrable?
This should work. You may want another resistor in series with the gate (in between the MOSFET gate and processor). Maybe 1k in this application. When you pick a MOSFET make sure it is rated for Vds of more than 12V. I think 20 is enough, but I see the other answer suggested 30.
I don't know what is the IO voltage of arduino. Is it 5V? If so, well, most MOSFET's will probably be ON when gate is 5V. But if it is 3.3V, double check that the MOSFET can be fully turned on at 3.3V.
Your solenoid says the power is 4.8W. That implies a current of 400 mA at 12V. So you want a MOSFET that can handle Id of 400mA without getting hot. I would suggest that you look for a MOSFET that lists Rds at of 100mOhm or less at Vgs = 3.3V or lower (or 5V or lower if you have a 5V IO voltage).
There are lots of MOSFET's out there that can do this, and they shouldn't be terribly expensive.
The other important thing is you need to add a diode in parallel with the solenoid. This diode should be pointed so it is reverse biased when the solenoid is on. Otherwise you will blow-up your MOSFET when you turn off the solenoid.
For the type of application you mention in your question you could think of an N channel MOSFET as a relay (but without isolation between contact and coil). In other words the "relay contact" is between drain and source and the "coil" is between gate and source.
In your application, the source connects to 0 volts and the gate connects to your digital control voltage. The voltage applied between gate and source turns the "relay" on and connects drain to source. As this imaginery device is equivalent to an N channel MOSFET current should only flow between drain and source.
As you apply a gate voltage the "contact" doesn't suddenly change from open to closed - it happens gradually as gate voltage increases gradually. If gate voltage increased rapidly then the "contact" resistance changes from "open" to "closed" rapidly.
Lowest contact resistance is achieved (for an N channel MOSFET) when gate voltage is highest with respect to source AND, like a normal relay, if you put too much voltage across the coil (gate to source) you will damage your MOSFET.
If you don't apply enough voltage between gate and source you get a half-on "contact" of anything between mega ohms and a few tens of ohms. You must choose a MOSFET that can near-fully turn on with your logic control voltage of 1.8 volts. This will usually mean that the gate threshold voltage in the data sheet will need to be sub 1 volt.
So, it's like a three terminal relay with coil between gate and source and contact between drain and source.
Best Answer
If you want to use a MOSFET, my preference, you need to chose one suitable for the application. This includes current, voltage, load, etc. With the Arduino you need a Logic Level preferably a UIS rated mosfet. The MOSFET must be properly enhanced (see graph on data sheet) to determine the Vgs voltage required to do this. Keep your load and allowable voltage drop in mind. Be sure the grounds are connected. When it is on measure the voltage between the source and drain, it should be less than 0.2 volts, if more you are not properly driving it. The IRF540 is not designed for logic level inputs. Its threshold can be as high as 4 volts. With the Arduino it is wise to put a pull down or pull up resistor to guarantee the output state while it is in reset and your code takes over. This will prevent output glitches.