About the modules
SIM900 is a Quad band GSM/GPRS module, which you can use for Call, SMS, GPRS functionalities,
SIM908 is a Quad band GSM/GPRS/GPS combo module, which you can use for Call, SMS, GPRS, and GPS functionality. SIM908 has an inbuilt GPS which makes it useful for tracking related applications.
The image shown is a board i have designed for Vehicle tracking system based on SIM908 module. The board uses LM2576-ADJ regulator to provide 4V, 2A DC from a 12V to 37V DC source.
The circuit sets its output voltage based on a resistor voltage divider network.
You can contact me if you have a specific requirement other than what i have explained below.
Power supply requirements
SIM900 and SIM908 have the same power supply requirements and almost same power consumption,
Operating Voltage – 3.2V to 4.8V
Operating Current – Upto 2A
The module consumes around 300mA current while under normal operating conditions, and around 2A current during transmit bursts.
Recommended operating voltage is around 4V. I design all my circuits for 4V DC,2A.
Since we don’t get standard voltage regulators at this voltage, we need to use adjustable voltage regulators.
For calculating the resistor values in the voltage divider network, you can visit this link,
There are two cheapest options available for designing the power supply,
Using Adjustable Linear regulator LM317
LM317 adjustable voltage regulator provides output voltage starting from 1.2V to 37V DC at a maximum output current of 1.5A.
Yes, Yes, we need 2A current for SIM900/SIM908 to operate, we compensate the remaining 0.5A current from the 1000uF capacitor as shown in fig-(a). This capacitor should be placed as close as possible to the SIM900/SIM908 module VBAT pin. Whenever there is a Transmit surge, the energy stored in the capacitor is used to compensate the transient requirement.
Using Adjustable Switching regulator LM2576
LM2576 adjustable switching voltage regulator provides output voltage starting from 1.23V to 37V DC at a maximum output current of 3A. LM2576 can easily satisfy our 2A current for SIM900/SIM908 to operate, But we still place a 1000uF capacitor close to VBAT pin for better performance as shown in fig-(b).
If you want to order the parts used in the above circuits, Use this Search Engine.