Unable to switch with a MOSFET

Thread Starter

ebolisa

Joined Jun 8, 2020
12
Hi,

I used a 2N3819 TO-92 FET to switch a 3V supply for a digital ckt on my breadboard and worked just fine. I then ordered from China few boards designed by me and switch the 2N3819 to FDN340P SMD to formfit.

To my surprise, the FDN340P is not switching. Its gate is fed by a 5V signal coming from the output of a ATTiny85 chip.

Where did I go wrong?

Any help is appreciated.
TIA
 

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Danko

Joined Nov 22, 2017
939
2N3819 is JFET but FDN340P is MOSFET.
Now +3.3V going right through body diode of MOSFET:(.
Maybe you need to swap drain and source leads?
And control MOSFET by connecting gate to ground?
 
Last edited:

Thread Starter

ebolisa

Joined Jun 8, 2020
12
Maybe you need to swap drain and source leads?
And control MOSFET by connecting gate to ground?
Thanks for the reply.

Unfortunatelly, cannot change the PCB's pads but I can replace the component. How about a SST201?
 

dl324

Joined Mar 30, 2015
10,987
I used a 2N3819 TO-92 FET to switch a 3V supply for a digital ckt on my breadboard and worked just fine. I then ordered from China few boards designed by me and switch the 2N3819 to FDN340P SMD to formfit.

To my surprise, the FDN340P is not switching. Its gate is fed by a 5V signal coming from the output of a ATTiny85 chip.
So which is it? Are you using the N channel JFET or a P channel MOSFET?

The way you have the MOSFET connected, the body is forward biased.
 

Thread Starter

ebolisa

Joined Jun 8, 2020
12
So which is it? Are you using the N channel JFET or a P channel MOSFET?

The way you have the MOSFET connected, the body is forward biased.
Because I cannot make any physical changes to the board, I should go for a replacement. So, what I'm saying is to replace the FDN340P (which works better with 5V and not 3.3V) with a SST201 which has the same pinout. The image shows the M1 on the board.
 

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dl324

Joined Mar 30, 2015
10,987
Because I cannot make any physical changes to the board, I should go for a replacement. So, what I'm saying is to replace the FDN340P (which works better with 5V and not 3.3V) with a SST201 which has the same pinout. The image shows the M1 on the board.
The board picture offers no useful information.

A P channel enhancement mode MOSFET is not a suitable drop-in replacement for an N channel JFET. A JFET is on with no gate bias. For the JFET you mention, you need as much as -8V on the gate to turn the device off.

SST201 isn't a drop-in replacement for 2N3819.
 

dl324

Joined Mar 30, 2015
10,987
Ok, I didn't mean for the SST201 to be a drop-in replacement for 2N3819 but for the FDN340P as shown in the diagram below.
You have the MOSFET source and drain connected backwards. In addition to that, it isn't a drop-in replacement for the JFET.

Is this some circuit that you have too much money invested in blank boards to prevent you from just doing a redesign if you can no longer get the JFET? Post the schematic that uses the JFET. Unless you had a -8V supply, you could never turn the JFET off.
 

Thread Starter

ebolisa

Joined Jun 8, 2020
12
Sorry for the delay, I'm on a different Timezone.
I'm ending this thread as it's going on a tangent and starting a new one.
 
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