Hello, I am using IRF830 MOSFET as a switch, I have 4 cooling fans . Each fan works at 5v DC and .3 A. I connect them in parallel and used the mosfet to switch them on and off. I am not too familiar with MOSFETs, and I am facing a situation that I colud not understand, the problem is:
I connect the MOSFET as follw:
The gate is connected to arduino to switch fans on and off,
the source is connected to ground of the 5 v power supply,
The drain is connected to the negative of the fans
The positive of the fans is connected to thepositive of the power supply.
When fans are switched on with the MOSFET connected, they don’t run as fast as if they are connected without it “ they run faster when connected to the supply directly” ,, so what is the reason for that..? And how can I make them run at the same speed??

 

Schematic?

 

The gate is connected to arduino to switch fans on and off,

I suppose you are driving the gate directly from the arduino port pin which is 5V around 10-15mA,
The IRF830 is not a logic level MOSFET it is a standard Type MOSFET which requires min 10V on the gate to properly switch ON
If my assumpsion is correct from your given info then I want to say you need a logic level MOSFET or an N Channel MOSFET Driver such as the TC1411N

 

I suppose you are driving the gate directly from the arduino port pin which is 5V around 10-15mA,
The IRF830 is not a logic level MOSFET it is a standard Type MOSFET which requires min 10V on the gate to properly switch ON
If my assumpsion is correct from your given info then I want to say you need a logic level MOSFET or an N Channel MOSFET Driver such as the TC1411N

First, thank you for your reply. And second, yes your assumption is right, however the mosfet that I am using is an N channel mosfet, as mentioned in the datasheet, also it’s gate threshold is max of4 v .
Here is the datasheet http://www.irf.com/product-info/datasheets/data/irf830.pdf
I would be grateful if you could take a look on the data sheet and tell me what can I do

 

in the datasheet, also it’s gate threshold is max of4 v

Hi.
In my opinion the Vgs threshold is the min were the gate will start to switch ON but very far from on ,I always use the Vgs value which for non logic level MOSFETS are +/-20V and at 10V they normally are full on and out of the FETS linear regen .
Then you need a Driver
Secondly if you do not use frequent switching like PWM then you can use a Transistor circuit to switch the gate to 10V or even 12 V ,or use a logic level MOSFET. IRF has Logic Level MOSFETS

 

As noted, that is not a logic-level MOSFET.
As see below, a Vgs of 10V is used to specify the MOSFET ON resistance.
That is the voltage you should apply when using that MOSFET as a switch.
A logic-level MOSFET has a Vgs threshold voltage of 2V or less so that it is fully turned on with a Vgs of 5V.

 

...the mosfet that I am using is an N channel mosfet, as mentioned in the datasheet, also it’s gate threshold is max of 4 v .

That's the gate threshold, the gate-to-source voltage at which the part barely begins to conduct (Id = 250 μA). It is NOT the gate voltage needed to allow the mosfet to pass 1.2 amps, which is what you need to power your four fans.

To properly drive the mosfet from an Arduino you need to either use a higher gate drive voltage, such as through a mosfet driver chip such as an MCP1407, or use a mosfet with a lower gate threshold such as an IRLZ34 or equivalent.

Using an IRLZ34 has another advantage, as it has a much lower Rds(on) of 0.046Ω, in contrast to the 1.5Ω of the IRF830 which @crutschow noted. Even if driven fully ON by a high gate voltage, the IRF830's Rds(on) will cause it to have a voltage drop of

Vds = Id * Rds(on) = 1.2A * 1.5Ω = 1.8V,​

which will reduce your fan speed.

My advice: use a more appropriate mosfet.

 

As see below, a Vgs of 10V is used to specify the MOSFET ON resistance.
That is the voltage you should apply when using that MOSFET as a switch.

Thank you for this I learned something important .

 

Thank you all for your help, The transistors you suggested were not available, however I solved the problem by adding a resistor between the drain and the power source and it worked fine