ESP8266 Community Forum

- Wed Sep 30, 2015 6:44 am #30186

Can you explain, in relatively simple terms (because that is likely all that I will understand), the advantage of a MOSFET over a bipolar in this case? I did some reading on IRL540s for power control applications but I am not sure I truly understand the advantage if you drive the bipolar to saturation.

BJTs are current controlled devices. For the transistor to switch on, there needs to be a current flowing through the base. In your application that means you waste 7-8mA, just to drive the transistor.
MosFETs are voltage controlled. They switch on or off, depending on the voltage difference between the source and gate pins. There is no current flowing through the gate (in praxis there is a few nA leakage, but it's so low, you usually don't need to worry about that). When switched on, the mosfet will basically behave like a resistor, whose value depends on the voltage difference between gate & source (that resistance is usually called R_DS(on)). This is usually more power efficient, since a) amount of power you waste in the transistor depends on the current your circuit draws (lower current drawn means less power wasted) and b) the resistance of good mosfets is usually very low (typically 100milliohm or less).

- Wed Sep 30, 2015 8:33 am #30191 The Espressif doc says the ESP only uses 0.5microamps when powered off, i.e. when CH_EN is low. So i wonder if an alternative approach is a simple inverter on the RTC MF pin to take CH_PD high when the alarm is triggered?

Agree with the comment about the MCP1702 maybe being not quite powerful enough. I've not had much success with them unless the input voltage is well over 6 volts, under that the ESP often crashes, i guess due to not enough current from the MCP1702. But here as the ESP is powered off for most of the time couldn't the regulator just be any old 3.3v LDO? Or even just have the ESP directly connected to a couple of AA batteries?

Also...if you just want to publish readings once an hour for a year then you might get away with just an ESP using deepSleep and powered by a couple of D batteries, and avoid all the RTC complexity

- Wed Sep 30, 2015 9:18 am #30194 Because there's no resistor to limit current in the base path of the 2N4403, you risk burning out the B-E junction. Also, it could be an excessive load on the RTC driving it, and fry that too. I'm surprised that hasn't happened yet.

As mentioned, MOSFETs are closer to 'switches' , and are much easier to turn on. You would need a small P-channel MOSFET capable of passing ~ 0.5 A. For regulator, I've been using '1117' type surface-mount regulators for 3.3v. They can regulate 5 or more V down to 3.3V and have no problem with the ESP load. I bought 10 for like US$1 from the 'bay.

- Wed Sep 30, 2015 11:59 am #30199 The 1117 has quite high quiescent current (5-10mA) for a battery powered circuits. It means that even if your ESP is in deep sleep mode, the 1117 will draw current. I think the better option would be XC6203 or MAX604.