[SOLVED] Selection of Bypass(decoupling) capacitor for high spike currents,voltage drop probem

[Jahanzeb1]

I am using 3.6v cell 19Ah (EVE - ER34615) to power the SIM900D module. The problem is I need help in the selection of proper bypass capacitor. According to battery's datasheet, it can provide max continuous current of 400mA and maximum continuous current of 230mA while SIM900D requires the spike current of 2A(for 100us) and continuous current of more than 500mA while sending data or registration of module with the nework.

Here is the link of datasheet of battery : "http://www.evebattery.com/asp_bin/upimg/pdf/20111210114340.pdf"

I searched a lot of forums for proper selection of bypass capacitor but not satisfied with answers. What happens in my project is without bypass capacitor the GSM module starts and after some seconds it turns off because of voltage drop across its power terminals as it is consuming more current. SIM900D requires the stable voltage of 3.2V with 2A of current according to SIM900D datasheet.

So in order to solve this problem i added a big 2200uF capacitor but I need to understand the properly mathematics of this problem and its solution.

I red that its is good practice to add 1uF and 10uF capacitors in parallel with every IC. I also know that small value capacitors removes noise while large value capacitors solves the voltage drop problem.

So basically i need to understand the following things:

1. Mathematics behind selection of bypass capacitor for GSM module in order to properly work the GSM module with EVE 3.6v battery ?
2. What will happen if capacitor value is small or large from the desired value ?
3. Whats the mathematics behind adding too many capacitors in parallel ? For example i added 2 bypass capacitors in parallel with power supply for microcontroller and a large capacitor of 2200uF for GSM module ? What if i add more sensors in my design, Do i need to insert more capacitors in parallel according to their datasheet ?
4. What are ESR capacitors ? If possible can i add 2200uF ESR capacitor with GSM ? Whats the difference ?


Same problem is faced by me in another project. I designed an automatic school bell automator device. School bell(220V) is attached with micro controller and relay. When relay is turned on after 2-3 secs the device gets restarted. I then inserted a big 2200uF capacitor with the power supply of microcontroller and it solved the problem but I am not sure with the value of capacitor ?

Best Regards

 

[FvM]

Presumed the battery can't source the GSM pulse current, the bypass capacitor must be able to. If you refer to the current waveform in SIM900 hardware design manual, you can calculated that 2200 µF is still below the required capacitance for 0.4 V voltage drop. 577 µs*2A / 0.4V = 2900 µF.

Average current can't be moderated by the capacitor. If the battery can't deliver it, choose a different battery type.

 

[Jahanzeb1]

Thank you FvM for replying. What if i connect two 2200uF capacitor across GSM or one 2200uF cap and one 1000uF cap ? What about low ESR capacitor ?

 

[FvM]

Low ESR is already presumed for the ectrolytic capacitors

 

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Looking at the Discharge characteristics graph, the battery output is only 3.0V for 230mA continous current. You'll want way more than the capacitance that FvM calculated as you probably don't want the battery discharge current to be over 100mA when the SIM900D is drawing it's average and peak currents, otherwise you'll be below your 3.2V supply requirement of the SIM900D.

This looks like the wrong battery for this application. Given the amount of bulk capacitance you will require to supply the average 500mA @ 3.2V for ?? length of time, it's likely you'll regularly be exceeding the maximum continuous current specification of the battery due to inrush currents every time the bulk capacitance is charged. Probably not the best scenario for battery longevity.