I have a Sim908 module. In the hardware design document, page 17, 4.1 Power Supply:

"The power supply range of SIM908 is from 3.2V to 4.8V. The transmitting burst will cause voltage drop and the power supply must be able to provide sufficient current up to 2A. For the VBAT input, a bypass capacitor (low ESR) such as a 100 µF is strongly recommended; this capacitor should be placed as close as possible to SIM908 VBAT pins."

On page 18:

"When battery is used, the total impedance between battery and VBAT pins should be less than 150mΩ."

From the information, I assume 3.3v voltage regulator and 100 µF are enough to power up the Sim908 module without damaging it.

Is my assumption right or do I need different components to power it safely?

Reference: http://www.simcom.us/act_admin/supportfile/SIM908_Hardware%20Design_V1.00(110715).DOC

Hi oshjdf that Sim908 is a interesting module.

I believe you are correct with your power supply but hopefully someone who actually knows what they are talking about will confirm.

What are you thinking of doing?

It might be fun to put one on a rocket or RC plane. I assume you could use a AT&T SIM card.

Ralph

Hi Ralph,

I am thinking of sending my room temperature when I am away from home. I wonder what is the temperature when all windows and doors are closed and no fans are on.

The statement that has taken me a step back is:

"The transmitting burst will cause voltage drop and the power supply must be able to provide sufficient current up to 2A.".

Based on this statement and power supply range 3.2V-4.8V, I assume that if 3.3v power (using regulator) is supplied, the module will run out of minimum power when it is in the state of (transmitting) sending text message or voice call. Is this right?

Page 18 of the documentation has 2 figures: 1. Figure 7: Reference circuit of the LDO power supply 2. Figure 8: Reference circuit of the DC-DC power supply

These figures are schematic to provide sufficient current up to 2A using +5v power supply without exceed/fall behind the power supply range. However, I am finding simpler alternative than building circuit based on those 2 figures because I am not knowledgeable enough thus may ending up damaging the module.

Hi oshjdf,

I am finding simpler alternative than building circuit based on those 2 figures

why bother?

How much are you going to save?

I would try to stick as close to their furnished power supply circuit as I could.

Other circuits "might" work BUT ...

and yes apparently there is a big current draw when transmitting.

Here is a Buck DC/DC converter off Ebay that "probable" would work.

That would cost $5.30 probable cheaper than you could buy the components.

You would have to compare this to their power supply circuit to see if there are any problems.

If I was going to build the power supply I would just match their schematic.

Ralph