Driving 2N7000 from Arduino

Thread Starter

djsfantasi

Joined Apr 11, 2010
8,256
I have successfully breadboarded and tested driving a 2N7000 from an Arduino digital pin. I used a 100k resistor to ground to help shut off the MOSFET.

The actual circuit uses one digital pin to drive 4 2N7000s. Actually, there are 7 MOSFETs on 7 I/O pins. Each MOSFET is on one of four PCBs.

I have installed one pull-down resistor (the 100k preciously memtioned) for each digital I/o pin - ON THE LAST PCB.

Anyone see any problems? I’m still wiring up the final installation.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
8,256
How can these two statements both be true of one circuit?
Confused of Cumbria.
There are seven independent pins driving identical but separate, identical loads. I am using one pin as an example.

So, each of the seven pins are connected to the base of four MOSFETs. These connections are chained and at the last gate connection, a 100k resistor ties the line to ground.

If there’s still confusion, I can get on my laptop and draw up a schematic. But really it is quite simple.

Here’s a text version
Pin_______________R
. |.....|........|.....|.....|
. G...G........G....G..Gnd

There are seven of these sub-circuits.
 

ebp

Joined Feb 8, 2018
2,332
It doesn't really matter where in the circuit the pulldown resistor is unless there is a possibility of the connections being broken (e.g. cable accidentally disconnected) while the FET and its load are powered. In a multi-board arrangement like that, the safest arrangement would be for each FET to have a local pulldown resistor, but if you are confident of the paths being intact at all times, it doesn't matter. Sometimes is is OK to just allow the FETs to turn on or partially turn on as they might without a pulldown, but you would need to consider the power dissipation in the FET and if it was OK for the load to get full current or partial current or be fully or partially switched at some unknown frequency according to noise that might be picked up.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
8,256
Thanks for your response. If a connection breaks, the entire device is useless. I can add one per FET, but do I have to change the value?
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
8,256
I'm attaching an annotated image of the PCB layout. Just to be completely clear. This was faster than drawing a schematic from scratch, so I hope that it is useful.

At the bottom of the image, is a comment referring to this post. As is, there are only pulldown resistors on the last board. Since the connections from the Arduino are replicated across all four modules, I assumed that one pulldown resistor per Arduino output would be all that is necessary. As @ebp mentioned, the "safest" arrangement would be to have multiple pulldowns for each module. But if I'm confident there won't be a breakage (which would render the entire project useless anyway), I only need the one. As a final note, I don't have enough 100K resistors to do this and could not get them in time anyway.

And yes, I know that the upper right resistor is a pullup, not a pulldown as annotated...

So it looks like this will work. Crossing my fingers!
Explanatory_image.jpg
 
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