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I'd like to replace a pair of NPN transistors and their associated base resistor with two small N-channel MOSFETs, such as the ESD protected 2N7002K.

I've drawn the two versions of the circuit (left side is known good, right side is what I'd like to do) but I have no idea if it's going to work. PCB fabrication takes time and I don't have the parts on hand to prototype...

Should my replacement circuit work like the original, or am I misunderstanding something important?

Thanks!

Left-side: original circuit, right-size: what I'd like to do

See also:

Work in progress: the project this is going to be used in,

Example of the circuit used on the NodeMCU devkit Networked Thermocouple Reader with Integrated Display

Okay, it took a while for me to receive the 2N7002K, but now I can confirm this modification actually works! All seems good on my modified WeMos D1. Now I'm waiting for my own circuit to come back from the PCB fab, and I'll test the modified circuit again and report on the results.

Original circuit Modified circuit

Replacement solder is Sn42Bi57Ag1 and it looks a lot shinier than the original lead-free alloy.

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    \$\begingroup\$ Why do you want to change it? Sometimes the desire to change a "known good" circuit arises from some kind of misconception or incorrect thinking. If the original is really known good, you should only change it if you have a clear reason to do so. \$\endgroup\$
    – user57037
    Commented Jan 6, 2018 at 3:15
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    \$\begingroup\$ This is a rather clever circuit, and if it was not known good I would have to stare at it for quite a long time to convince myself that it works. I would not change it if I were you. It is not just straightforward logic. The criss-cross connection of the two transistors makes it highly interdependent. It is not clear to me that it will work with NMOS. If component count is that critical, use a dual transistor package, and possibly a quad 12k resistor package, too (unless that makes the layout too complicated). Any small signal or logic NPN should work. \$\endgroup\$
    – user57037
    Commented Jan 6, 2018 at 3:50
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    \$\begingroup\$ I'm with @mkeith on this one. The difference between threshold voltages alone may cause this to not act the same way enough in practice to cause you all kinds of grief. The old adage "If it aint broke.. don't fix it!" springs to mind. \$\endgroup\$
    – Trevor_G
    Commented Jan 6, 2018 at 8:38

2 Answers 2

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One issue is that in your FET circuit the gates are exposed directly to static on the RTS and DTR lines, this may result in damage to the FETs. In this application BJT transistors are more robust.

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  • \$\begingroup\$ I have chosen the 2N7002K ESD-protected FET for this reason. I understand that a non-ESD protected FET would not be a good fit for this application. \$\endgroup\$
    – Hernick
    Commented Jan 6, 2018 at 17:03
  • \$\begingroup\$ It would be good if you put this in the question. \$\endgroup\$
    – RoyC
    Commented Jan 6, 2018 at 19:32
  • \$\begingroup\$ The other issue is that the Vgs(th) for the 2N7002 is a bit high for 3.3V circuits. Consider using BSS138 instead. \$\endgroup\$
    – user57037
    Commented Jan 6, 2018 at 19:45
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Yes, that mosfet circuit should work like the bjt one.

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  • \$\begingroup\$ Gate resistors should be used with mosfets. They tend to oscillate without them, and resistors protect the gates from an over voltage condition. \$\endgroup\$
    – user105652
    Commented Jan 6, 2018 at 1:32
  • \$\begingroup\$ Thanks for your answers! My understanding is that since the inputs of my two NMOS transistors (T1: DTR, T2: RTS) are provided by a powered 3.3V CMOS chip, the CH340G USB UART, I can rely on the output drivers of that chip to provide a stable, MOS-compatible 0 or 1. Are those two resistors really necessary? I am trying hard to reduce component count. – Hernick 7 mins ago \$\endgroup\$
    – Hernick
    Commented Jan 6, 2018 at 3:10
  • \$\begingroup\$ Yeah. My suggestion is to do the layout so that you can keep the gate resistors in case you need to go back to the BJT version. Use SOT-23 NMOS. Then you can change to SOT-23 BJT later with the same pinout. Gate resistors are not always needed (I often don't use them) but in this case I would just so you can switch back to a BJT circuit if needed. \$\endgroup\$
    – user57037
    Commented Jan 6, 2018 at 3:37
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    \$\begingroup\$ I just added a pair of 0-Ohm base resistors to the design. And I'm using SOT-23 NMOS. I'll remove the resistors in the next revision if it works without them. Further tests are required. \$\endgroup\$
    – Hernick
    Commented Jan 6, 2018 at 17:30
  • \$\begingroup\$ I think that is wise, Hernick. \$\endgroup\$
    – user57037
    Commented Jan 6, 2018 at 19:47

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