# Enhanced N-Channel Mosfet selection

can someone help me choose a good mosfet, i need a enhanced n-channel mosfet for driving a solenoid, id like to send a high signal to the mosfet with an arduino, i already have the circuits built but im having trouble finding the right mosfet. im going to say it needs to be atleast 10a/9v. i will been charging a 6800uf capacitor from a 9v battery. it would be nice to only have to send 3.3 volts to the fet to turn it fully on but 5v would be okay too. can someone help me please, thankyou!!

There are lots available for this sort of job.

I often use the FQP30N06L.

Don't forget the flywheel diode.

Allan

hi, thankyou. can you tell me what Vgss Gate-Source Voltage +20v means? this is on its data sheet. also what exactly does this mean,

32A, 60V, Rds(on)=35 mOHM(max) @ Vgs=10V, id=16a. ,

thanks, im not sure what they mean when they are talking about the gate?

long story short, i bought a fqpf13n50c and tried to sue that, but the output signal from the arduino was not enough current to fully activate the mosfet, im not sure how to look for hat im looking for lol

The fqpf13n50c is only rated 6.5A with an on resistance of 450mOhm. Not good enough.

The 20v is the absolute max gate-source voltage - not relevant.

The FQP30N06L has an on resistance of <50 mOhm at 5v gate drive. So with 10 amps Id it will dissipate about 1/2 watt

It’ll work fine.

Allan

okay so the on resistance will determine how much current the gate will need to turn on? thanks again

"The on-resistance determines the conduction power dissipation and increases with
increasing temperature"

but can you tell me what they mean when they say "conduction power dissipation" do the just mean its going to want to use about that much current when fully on at said temp?

The on resisance at a given gate source voltage dissipates power exactly as a resistor would, and obeys Ohm's law.

Things vary with temperature, so you may need to fit a heatsink.

Since you're only switching a short pulse this is probably not necessary.

Allan

okay so the on resistance will determine how much current the gate will need to turn on? thanks again

No. Just to repeat things for clarity, a fet/mosfet is a voltage controlled device, transistors are current controlled. As Allan said, the drain to source resistance varies in proportion to the gate voltage. The rated resistance is given with the symbol Rds. With the datasheet in hand, you know the on resistance and when you know the current flowing through that resistance, it is a simple matter to calculate the power dissipated with Ohms law which gives us I2R.

so then maybe the moafets i have tried to use have too much on resistance and thats why i need to much input at the gate to turn it on enough to work? i need to learn to read the data sheets better.

The usual problem is that many mosfets need 10 volts or more on the gate to reach their rated (lowest) resistance. That's why we have "logic level" mosfets that turn fully on with as low as 2.5 volts. The logic level devices usually have an "L" in their part number.

can someone tell me exactly what the resistance does in a simple-er way please!!!! lol thankyou

okay i get how the resistance changes when you apply voltage to the gate, but what i dont get is how all this current can flow through a 50Mohm resistor from source to drain. i dont get where they are coming up with these values and what they actually mean,

but what i dont get is how all this current can flow through a 50Mohm resistor from source to drain.

That's is 50 mOhm, or 0.050 Ohm, not 50MOhm (50,000,000 Ohm). With an Rds of 50MOhm, not much current is going to pass through the channel. If you had a Vds of 10V, it would have a current of 200 nanoAmps. It is Off.

You have to take notice of the the letters actually used. In anything technical "m" and "M" don't mean the same thing.

mOhm means milliOhm or thousanths of an Ohm. 1 mOhm = 0.001 Ohm.

MOhm means megaOhms or millions of Ohms. 1 MOhm = 1,000,000 Ohms.

So what the specification actually says is 0.050 Ohms and what you seem to be reading instead is 50,000,000 Ohms. Not the same thing at all.

Steve

notsolowki:
okay i get how the resistance changes when you apply voltage to the gate, but what i dont get is how all this current can flow through a 50Mohm resistor from source to drain. i dont get where they are coming up with these values and what they actually mean,

So first learn your SI prefixes, a prerequisite for electronics where pico (10^-12) to giga (10^9) are routinely
used…

In any enhancement FET (which the vast majority of power MOSFETs are), the charge on the gate builds
up as the gate voltage increases, and at a certain point this starts to create a conducting channel between
source and drain - the gate and the channel behave rather like a (very non-linear) capacitor. The channel itself acts as a conducting path between source and drain.

The bigger the channel the lower the resistance between source and drain. A typical logic level MOSFET
will start to form its channel at 0.5 to 1.0V Vgs, but it only becomes large at 2 to 4V, so its important
to drive the gate voltage to its recommended value. For non logic-level MOSFETs these voltages are
typically 2 to 4V for the start of channel formation, ~7V for it to be fully formed. Checkout the graphs
of gate charge v. gate voltage and you’ll see a plateau voltage which is where most of the channel growth
happens.

MOSFETs have a lot of variability between devices for the gate voltages, a volt or more is common, so
its again important to stick to the datasheet Vgs value so that your circuit works well with every device
of a given type, not just some of them. Gate voltage can also drift with time (especially older devices).

Basically if the spec is 0.05 ohm at Vgs=4.5V, don’t expect it to work with Vgs < 4.5V unless there
is another listing in the datasheet expressly saying what the on-resistance is at a lower voltage.

Hi,

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom...