ESP8266 Community Forum

- Thu Feb 16, 2017 10:02 am #62644 An issue to consider is the voltage to be switched. A relay can switch almost any voltage so long as its coil can be driven. For a relay the coil voltage can be 3v and yet it can switch say 100v. Now a FET can only switch well if the gate voltage is as high as the voltage to be switched so a FET switching say 12v needs to have its gate voltage at close to 12v otherwise it is not fully on and will have a meaningful resistance between the source and the drain. There are solid state solutions but they involve things like high side drivers.

- Thu Feb 16, 2017 10:49 am #62646 I think it is not correct to state that the gate voltage for a mosfet needs to be as high as the voltage to be switched.

For 2N7002, maximum switching (drain-source) voltage is 60V and maximum continuous gate (gate-source) voltage is 20V. Applying 60V to the gate of the device would destroy it.

However, it is true that the on state resistance of mosfets is normally lower with 12V gate drive than with 3.3V gate drive, but "logic" drive mosfets are available which have lower gate threshold voltage and which, if selected correctly, have sufficiently low on state resistance with a gate drive of 3.3V.

- Thu Feb 16, 2017 11:08 am #62647 Right ! the importance is to check the specs/graphs about Rdson vs Vgs.

- Fri Feb 17, 2017 9:53 pm #62721

If that is all correct then I would suggest that you try using the Temp pin (pin 8) of the TP4056 to disable charging. - See more at: viewtopic.php?f=160&t=13745&start=4#sthash.kFnkw8eG.dpuf

Thats correct. And thanks for such a detailed response this sounds like a good options. Just to note the LTC4150 can be set to provide 3.3v output to the ESP8266. The solar panel will output 5V to the TP4056

As an alternative could I also use a 2N7000 or similar to do the switch?

Thanks I have BS170 in the box, I'll give that a go.