help..Mosfet problem

hi

i am using logic Mosfet to control high current motor ..about 10A

the Mosfet pass only 2.5A ! i am using 4.5v on the gate?

the Mosfet IRLZ24 17A datasheet

Post your fet circuit.
Hand drawn is ok.
Add supply voltage, gate resistor values, etc.
Leo..

Thanks for providing a datasheet, yes that is a logic level device which is good, but it
has far too high an on resistance - 0.1 ohm at 10A will drop 1V so that the effective
Vgs will be more like 3.5V than 4.5V if it were to carry that current.

The device is definitely not rated for Vgs 4.5V, but for 5.0V, and looking at the plateau
in the gate charge graph its clear why, the plateau is at about 4V, so really this device
is best driven from 7 or 8V than 5V.

Find a logic level device rated for < 0.01 ohm at 4.5V. Its max current rating is likely
to be more like 50A or more (you never expect a MOSFET to handle its rated current
continuously, it would need liquid cooling! - the key parameter is Rds(on)).

You want the power dissipation (I-squared-R) to be reasonable, and the on-voltage (I x R)
to be much less than the difference between Vgs and Vplateau (usually a fraction of a
volt).

The max that mosfet can dissipate without a heatsink is

150c/(62 c/w) =2.42 watts

You ate trying to dissipate 10w...Therefor you need a heatsink. Specifically a heatsink with a rating of 12 c/w which is a large heatsink.

The most you could carry without a heatsink is 4.9 amps but realistically 4 amps at which point the metal would be 125c which would hurt.

But as I said it will not work at 4.5V, its marginal at 5.0V. This is not quite a logic
level device in my opinion, logic level FETs need a plateau voltage more like <3V
than 4V

thanks all for reply :slight_smile:

i will buy some logic mosfet from digikey…what is the best Logic Mosfet that full open at 3v?

New Bitmap Image.jpg

R1 should connect to the side of R2 that connects to the Arduino, not to the gate of the FET.


You need a 15A rated diode across the motor, cathode end to the supply voltage.

Hi,
what is the specification of your motor?
The 10A spec is if you load the motor, are you running it with nothing attached to the drive shaft?
Measure the voltage across the drain and source with the motor running.
Also make R1 = 10K.

Tom...... :slight_smile:

Paul__B:
R1 should connect to the side of R2 that connects to the Arduino, not to the gate of the FET.

The way you have drawn it, you only get 90% of the 5V available on the gate, i.e 4.5V.
Alter your circuit in the way Paul_B suggests, and you will be able to get the full 5V on your gate.

Paul__B:
R1 should connect to the side of R2 that connects to the Arduino, not to the gate of the FET.

I think it should be there, because fets are usually not hardwired or mounted on the Arduino board.
The fet could be on a breadboard or on a heatsink, therefore it's best to have the bleed resistor also there.

The resistor value is the problem. It's a bleed resistor, and can be any high value.
100k is transparent enough. Even 1meg is not a problem.
I often see 10k used here. That is a common value for normal transistors, not mosfets.

110ohm gate current limiting resistor is a bit on the low side.
I suppose you're not switching the motor with 200Khz PWM.
So no problem to make that 220ohm.
Leo..

naser:
thanks all for reply :slight_smile:

i will buy some logic mosfet from digikey.....what is the best Logic Mosfet that full open at 3v?

Open circuit means unconnected - switches are on (closed) or off (open) in
English electronics - ie the opposite from water taps, so its best to say "on" and "off"
and avoid the confusion.

The is never a "best" device in the abstract, it all depends on the current level,
speed, package footprint, price, gate capacitance, on-resistance, availability.

List your requirements and find a device that meets them and minimizes cost/inconvenience.

Wawa:
I think it should be there, because FETs are usually not hardwired or mounted on the Arduino board.
The FET could be on a breadboard or on a heatsink, therefore it's best to have the bleed resistor also there.

I find that fuzzy thinking.

Why is the resistor there? Because the Arduino (or indeed, any other circuit) may be in a high-impedance state. OK, the "fault" is with the controlling circuit, not the FET, so the pull-down resistor should be on the side which is responsible for the problem, not on the FET side. :grin:

In any case, it is hardly difficult. The pull-down resistor will be wherever the series resistor is, so it will be perfectly easy to connect it on the correct (input) side.


Incidentally, there may be an argument for not making the pull-down very high, insofar as whenever it is needed (perhaps if the Arduino is reset), you do not want the FET to be operating any longer in its linear region than can be avoided.