I was trying my luck with a TIP120, but that seems to give me a voltage drop of 0,7v = 3,5v
I see people talking about p-channel mosfets, but I think I might face the same issue there.
Search for a MOSFET with low RDSon resistance (milli Ohms) and then the voltage drop will not be as great
A FET does not have the equivalent of a Collector to Emitter saturation voltage. Instead it has a turn on resistance. Therefore the voltage you drop depends on this resistance times the current you are drawing. Typically this resistance is much less than one ohm, and more likely in the tens of milli ohms or smaller. So the voltage you loose is going to be very small.
You sure of that? The TIP120 is a Darling transistor pair and according to the data sheet at 3A collector current you can waste 2V from collector to emitter. At the verey least I would expect 1.4V drop.
But why do all this to save so little? The sleep current is 0,7mA max. Okay, it's not nothing but a single led draws at least ten times that.
Thx for the tips guys! I'll look into the mosfets I think the sim800 will only use +2A when connencting to/sending over gprs, so amps will be much lower when starting up and conecting to network.
No, that's a bogus assumption. It may require 2+ amps at any time it chooses, that is somethingnegotiated on the fly between the module and the base station depending on reception conditions.
You need a _logic level_ pMOSFET, note, that's essential. 0.1 ohms or less is good or you'll need aheatsink. Remember that if you power down the module there must be no Arduino OUTPUT pins connected toit still sitting at a HIGH level, or you'll burn something out.
I know, but I'm still afraid OP is pissing away powera) with other componentsb) getting up and running once every 15min
For now I'm using a pro mini (sparkfun) with the low power jumper soldered. But In the end it should be just the chip, few capacitors and oscillator.