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I recently found a bag of 3N128's at a yard sale, coincidentally, a couple days after I saw this video on YouTube about controlling a 12V motor with a mosfet, car battery, and arduino:

https://www.youtube.com/watch?v=Lzxuro0Z2Ew

Here's the schematic he drew: enter image description here

What I'm unsure about is if I can use these as a direct substitute for the mosfet he used. It has 4 legs and the symbol that I found on the datasheet looks a little different than what he used:

http://html.alldatasheet.com/html-pdf/108468/ETC/3N128/110/2/3N128.html enter image description here I'm tempted to just connect (4) to (2).

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No, no - the 3N128 is an ancient depletion mode MOSFET requiring a negative voltage to switch it "off" i.e. it is normally "on" with no gate voltage. It won't work as a substitute in the motor controller unless you can provide a negative voltage on the gate with respect to source (0V): -

enter image description here

It's only good for an on current of perhaps a few mA too which rules it out of all but nano-motor applications!

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It is not clear which mosfet is used in the YouTube video. However, the 3N128 mosfet that you have is not suitable for that type of application, ie controlling a motor. The 3N128 is a specialized high frequency amplifier for use in VHF radio's and such like. It has a maximum current rating of 60mA, whereas you will need several hundred mA (or even several amps) to control a motor. Lastly, it is a depletion mode mosfet which makes it very difficult to use in this type of application. Certainly, you cannot use the simple circuit that you have shown.

What you need is something like a STD12NF06L. This is an N-channel enhancement mode mosfet with a 12A current rating. Also, its Vgs(th) is low so it can be controlled by 3.3V powered logic.

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The 3N128 is a small signal Dual gate Mosfet.It was made before the Powermos Technology was invented.The 3N128 is only suitable for milliamps .The 4 legged device is like 2 Jfets in cascode .You can still use them today for RF or audio stuff where you want a valvey sound.

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  • \$\begingroup\$ Sorry, but the 3N128 is not a dual-gate mosfet (such as the 3N200). The extra pin is a substrate connection that is not connected to the source. \$\endgroup\$ – Tut May 3 '16 at 20:50

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