AO3400 N-MOSFET switch not turning off load

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
I was attempting to simply blink a red LED by switching a digital pin on my microcontroller High and LOW for 1second, connected to the gate of this Mosfet. The LED remained dim when I expect it to turn OFF completely

I removed the mosfet and tested like in the attached pic. I have tried with two new AO3400.

I must be doing something fundamentally wrong. I expected the LED to be OFF when the switch is open, only turning ON when the Gate is connected to 5V.
 

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peterdeco

Joined Oct 8, 2019
481
You said you expected the LED would shut off when the "switch is open." Does your microcontroller actually bring it low or just remove the B+? If this is the case you need a resistor (try 10K) from gate to source.
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
The Mosfets came from a reputable source and the circuit has a resistor from gate to source (as shown in the figure attached to my first post)
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
Here is a photo showing the test setup. I have tried with other AO3400 connected the same way. The gate is connected to gnd through 100K resistor; the grey wire is connected to 5V, I expected to have to touch the gate to light up the LED.


I have P-Channels AO3401 bought from the same source and they work properly.
 

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djsfantasi

Joined Apr 11, 2010
8,324
There's nothing wrong with the circuit. He just didn't draw it conventionally which made it more difficult to read.
<image omitted…>
Thanks, his drawings confounded me.

However, I think the pull down resistor is too large a value. I use a 100 ohm resistor, up to 1k ohms. A 100k ohm resistor may not provide enough current to pull down the input and turn off the MOSFET. Admittedly, I didn’t do the calculations.
 

dl324

Joined Mar 30, 2015
14,462
I have P-Channels AO3401 bought from the same source and they work properly.
If you bought from someplace like Ali Express, be aware that the counterfeiters sometimes throw in some good parts so you need to do 100% testing. If you buy from a reputable source, you don't need to do any incoming QA (in general).
 

dl324

Joined Mar 30, 2015
14,462
I breadboarded the circuit using 100k and then 10k for the pull-down resistor. There was no measurable difference.
 

dl324

Joined Mar 30, 2015
14,462
Previously I was playing with some 555 IC motor controller and successfuly used the AO3401 as shown in the attached photo:
It would be helpful for you, and everyone else, if you labeled pin function on the IC's. Not everyone has memorized pin function for the 555 timer.

At least your symbol is "better" than National Semiconductor's. They drew theirs backwards:
1638904226904.png
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
The problem was with the Mosfets, I tried with others from a different strip and they are working as expected.

Thank you all for your help.
 

dl324

Joined Mar 30, 2015
14,462
The problem was with the Mosfets, I tried with others from a different strip and they are working as expected.
So you should be asking yourself why they didn't work. Did you damage them? Were they defective? Are they counterfeit?

It doesn't take much voltage to damage them.
 
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