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I am using SIM800L. It requires 2 amp current spike(very shortly). As it requires just 4Volts, I am powering it through Arduino. To sustain load spike of 2 amps, which cap should I use?

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According to Sim800l or Sim800C Datasheet page21 you will need a capacitor of the following value.

100uF tantalum capacitor here

33pF ceramic capacitor

10pF ceramic capacitor

And if you are using battery with less C rating then I recommend a large 1000uF electrolyte capacitor also so it can provide burst current that Sim800l requires.

And also make shure all capacitors are as close as possible to vcc pin of Sim800l.

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If we go to the Cap equations, we have the relation between the capacity and the current

enter image description here

If we resolve C, then we have:

This means that, the capacitance will be = C = I*t/(ΔV). If you have 2A peak, and following the SIM800L the time of the burst is about 600uS, and the maximum voltage drop from 4V is 1V, looks like the min voltage for SIM800 is 3V, then you have you will need 1200uF, but this is not taking in account the ESR of the capacitor..., so this is conservative calculation, and only taking in account that the capacitor is the only which is giving this current.

That is why, super-caps are used in this applications, or stronger power supplies.

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