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Microcontroller Programming » 2N7000 work with open drain?

March 06, 2012
by RogerFL
RogerFL's Avatar

I toggle the output from microcontroller to the gate once per second. I see the gate pin of the 2N7000 toggle between 5v and 0v every second as expected. I see the source pin at a constant ~9v (9v battery). However, the drain pin also never changes. It shows about a half volt lower than source pin all the time.

There is nothing connected to the drain pin yet.

Is my testing valid? Is some load on drain required to make it work?

March 06, 2012
by pcbolt
pcbolt's Avatar

Hi Roger -

Try hooking up the source to ground and placing an led (correct polarity) and resistor between 9V and drain. The see if the MCU can control the led by toggling the gate pin.

March 07, 2012
by RogerFL
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Thanks pcbolt. I tried that but saw a weird problem. At first it did not work, but when I touched the gate pin with the multimeter it surprised me and started working - only when the multimeter was touching the gate pin. Not really understanding why I needed to do this, just trying to duplicate the effect the multimeter had, I added a large resistor between gate and ground and the LED started blinking OK without the assistance of the multimeter. It's as if MCU couldn't make the output pin low enough for this MOSFET without an external pull-down?

Next I want to make that gate pin controlled by PWM output.

March 07, 2012
by hevans
(NerdKits Staff)

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Hi RogerFL,

That is a bit confusing the MCU should be able to easily shut off a 2n7000. My guess is that something isn't hooked up quite right in your circuit. Make sure your circuit is hooked up the way pcbolt described above. Link to see a circuit lab diagram here.

Notice the drain of the mosfet should be connected to the LED/resistor on the high side, and the source is connected to ground. In your earlier description you said that you had the high side hooked up to the source.

Humberto

March 08, 2012
by RogerFL
RogerFL's Avatar

I'll revisit that issue again soon. Meanwhile I finished my project of making a smoker controller - thermocouple, PID, PWM controlled duty cycle to a 12v fan. Can't wait to try it out on my friend's green egg.

Btw, love circuit lab. Awesome!

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