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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.

Argha Chakraborty
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2 Answers2

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

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

Edit: From your linked datasheet:

enter image description here

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.

Spehro Pefhany
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  • There are two Vgs in the datasheet, one in the subsection of electrical characteristics and the other in electrical ratings. Which one to check? https://www.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 '20 at 14:01
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    See edit, now that datasheet/part number is supplied. – Spehro Pefhany Jun 29 '20 at 14:30
  • Thanks for your reply. Its starting to make sense – Argha Chakraborty Jun 29 '20 at 15:55
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If you want to know how the drain source characteristic changes for different gate voltages try this graph on page 6: -

enter image description here

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.

Andy aka
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