1
\$\begingroup\$

I have a WEMOS D1 mini board which I power with a 5V power supply. I need to switch 5V outputs with a few hundred mA load, so I use AO3400 N-channel MOSFET-s. My schematics are the following:

enter image description here

However, when I power the circuit, my outputs are all powered, their voltages are between 4-500mV and 4-5V, despite the fact that the gate-source voltage of the MOSFET-s are 0V.

What could be the problem?

\$\endgroup\$
  • \$\begingroup\$ Are you sure that you did not swap the drain with a source terminal? Also, can you measure the output voltage at a LOW state in WEMOS D1 board. \$\endgroup\$ – G36 Aug 19 '18 at 16:50
  • \$\begingroup\$ The problem persists even if I remove the WEMOS D1 from my prototype PCB completely and only connect the 5V input. The MOSFETs legs are connected like on the datasheet (aosmd.com/pdfs/datasheet/ao3400.pdf). Single top leg (drain) goes to output, bottom left (gate) goes to the WEMOS D1, and bottom right (source) is ground. Continuity testing shows these connections are OK. Interestingly, there is about 300k resistance between the output ground and input ground. \$\endgroup\$ – nXu Aug 19 '18 at 17:09
  • \$\begingroup\$ Where did you buy the MOSFETs? \$\endgroup\$ – SamGibson Aug 19 '18 at 17:16
  • \$\begingroup\$ But did you remove the pull-down resistors also? Try to short the gates directly to gnd. \$\endgroup\$ – G36 Aug 19 '18 at 17:19
  • \$\begingroup\$ I did remove the resistors (and the diodes). The problem is the same even with having only the DC input and the MOSFET. I have output voltage even when the Gate terminal is shorted to ground directly. \$\endgroup\$ – nXu Aug 19 '18 at 17:23
2
\$\begingroup\$

Although parts of some of your descriptions have been a little unclear, your recent statement was:

I have output voltage even when the Gate terminal is shorted to ground directly.

This only leaves faulty / damaged (e.g. by ESD) / counterfeit MOSFETs (or perhaps a problem with your PCB or other physical construction) as possible causes.

You could edit the question to add photos of your physical hardware, in case that shows something of interest. But the first thing I would do, is to get a suitable working, and known genuine MOSFET, and repeat your test.

Based on your information, I believe your MOSFETs are either not the type you believe, or are faulty / damaged in some way.

\$\endgroup\$
  • 1
    \$\begingroup\$ Thank you. I've tried some BSS138s I have laying around and everything seems to work. \$\endgroup\$ – nXu Aug 20 '18 at 12:06
1
\$\begingroup\$

However, when I power the circuit, my outputs are all powered, their voltages are between 4-500mV and 4-5V, despite the fact that the gate-source voltage of the MOSFET-s are 0V.

What could be the problem?

I suspect that you are not connecting the appropriate loads to your outputs and you are seeing the effect of drain-source leakage current: -

enter image description here

If all you have connected is a volt meter then the 1 uA flowing will certainly be enough to produce the effect you are seeeing. Try putting a 1 kohm resistor across the load terminals - it should result in maybe a millivolt shown on your meter maximum.

\$\endgroup\$
  • 1
    \$\begingroup\$ Thank you. While it was not the problem, it helped me to test the circuit with other MOSFETs. \$\endgroup\$ – nXu Aug 20 '18 at 12:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.