# Understanding gate threshold voltage of logic level mosfets from datasheet

I want to build an Arduino Project to control a DC load(DC 5V water pump ampere rating is not known) and using a transistor with PWM to control speed. There is a need to shrink down the project and I have chosen Digistump ATTiny85. But read that Digistump board can't source enough current in the transistor to turn it on. An extensive search reveals people suggesting to use a logic level MOSFET instead for the purpose.

Even Digistump sells a MOSFET shield which has a logic level MOSFET(FQP30N06L). But this part is unavailable in nearby stores.

I am trying to read the datasheet of other available MOSFET(STP90NF03l) and understand whether it would suffice my purpose. What should I look for in the datasheet?

Digital IO pins in Digistump outputs 3.3V-5V. So the voltage threshold for the MOSFET to turn on would be less than 3.3V line(~2.5V say) so that I may test it with both my Arduino and Digistump board.

Look for the Vgs at which Rds(on) is guaranteed.

Vgs(th) is the voltage at which it is mostly off.

If you have 5V drive you can count on 12m$$\\Omega\$$ maximum Rds(on) with the MOSFET at room temperature (maybe 50% higher with it hot).

If you have 10V drive, it is lower by almost half.

There is no guarantee for 3.3V so I would suggest not using that particular MOSFET with 3.3V drive. It will turn on somewhat, for sure. Typical curves in the datasheet should not be depended upon.

• There are two Vgs in the datasheet, one in the subsection of electrical characteristics and the other in electrical ratings. Which one to check? st.com/resource/en/datasheet/stp90nf03l.pdf So if at Vgs threshold it is mostly off does this imply that the gate is fully open at any value above 2.5V(say 2.6V) ? Or does it open fully at abs max rating? – Argha Chakraborty Jun 29 at 14:01
• See edit, now that datasheet/part number is supplied. – Spehro Pefhany Jun 29 at 14:30
• Thanks for your reply. Its starting to make sense – Argha Chakraborty Jun 29 at 15:55

If you want to know how the drain source characteristic changes for different gate voltages try this graph on page 6: -

With $$\V_{GS}\$$ at (say) 3.5 volts, the drain can conduct (for example) about 35 amps with a volt drop ($$\V_{DS}\$$) of around 2.5 volts i.e. a power dissipation of 87.5 watts (not fantastic). $$\R_{DS(ON)}\$$ will be 2.5/35 = 0.07 ohms.

However, if $$\V_{GS}\$$ is 5 volts, the voltage dropped will be about 0.5 volts at 35 amps (a power dissipation of 17.5 watts). $$\R_{DS(ON)}\$$ will be 0.5/35 = 0.014 ohms.

I just used 35 amps as an example. If your water pump is only (say) 10 amps, power will be proportionately smaller but $$\R_{DS(ON)}\$$ will be approximately the same.

There are better MOSFETs of course but whether you can get them is a different story.

• Nice reply. I will vote this up once I have 15reps. – Argha Chakraborty Jun 29 at 15:54
• Can you name some other alternatives? Or how to shortlist such alternatives – Argha Chakraborty Jun 29 at 15:57
• Graphs on a datasheet are only for a "typical" one. But you cannot buy a typical one, some need a much higher Vgs. – Audioguru Jun 29 at 16:41
• I didn't downvote .This is so strange.My post got downvoted too. – Argha Chakraborty Jun 30 at 13:19
• @ArghaChakraborty it'll be some idiot thinking he's clever or sneaky. – Andy aka Jun 30 at 14:05