Working on a project that would sequence 320 LEDs
using an Arduino Uno and shift register 74HC595
based on
http://bildr.org/2011/08/74hc595-breakout-arduino/

Have run into problems with the max current that the Arduino Uno
can handle.

Note figure says "18 more SprkFun BOB10860" but the design is now 38 more.

So it was reasoned that a BJT transistor could be used with a separate, higher voltage source to reduce the amps
required by the 75HC595s.
But even with the reduced current required to saturate
the transistors the microcontrollers max was still
getting in the way.

Then I ran onto MOSFETS. Cool. The transistor is controlled by voltage instead of
current so the current hassle should go away.

So looking to acquire 320 MOSFETs research was done at DigiKey and Mouser. The Arduino can supply either
3 or 5 volts. A MOSFET seen mentioned in several articles and tutorials is the Fairchild 2N7000.
( I have read everything I could find and understand including Mr. Kiem's excellent Understanding MOSFET On-State Drain-to-Source Resistance
but am still getting my head wrapped around it.)
It seems an important spec for the MOSFETs is the Gate Threshold Voltage (Vgs). The Vgs for
the 2N7000 is .8 3 volts min. and 3 v max. So it would seem that even using the Arduino's lower power output would
be bumping the max of the MOSFET. (I believe the Vgs is the point at which the MOSFET would allow the higher voltage source to
flow to the LEDs, right?)

If that is so, then using even the lower Arduino power source
would be constantly bumping the 2N7000 against it's max 3Vgs.
Another MOSFET is the ZVP2106A which, if the data sheet is
read correctly has a 1.5 min Vgs and max of 3.5 v.

I believe this is the MOSFET to get but would welcome
input before I send Digikey my hard earned dollars.

Thanks.

Allen Pitts, Dallas Texas

 

So it would seem that even using the Arduino's lower power output would
be bumping the max of the MOSFET.

If that is so, then using even the lower Arduino power source
would be constantly bumping the 2N7000 against it's max 3Vgs.

That Vgs(th) specification does not express the maximum gate voltage the 2N7000 can tolerate; when the data sheet lists Vgs(th) minimum and maximum, it means the range of gate-to-source voltage to begin turning the 2N7000 on. In other words, it says the part will begin to turn on at a gate-to-source voltage of somewhere between 0.8V and 3V. Any more voltage than that, merely assures the MOSFET will achieve a lower Rds(on).

The maximum gate-to-source voltage the part will tolerate is something else entirely; it is given in the Absolute Maximum Ratings section of the data sheet, as ±20V.

So no problem.