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SIM508 supply problem "caps on the supply"

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Jun 9, 2005
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sim508 evaluation board


I am using the SIM508 with its evaluation board.
I developed my controller board and test in lab and could send data to server.

When I try to test in CAR, the evalboard resets after command AT+CIICR

in the LAB the power supply was 2A@12V

in the car I use the 12 Volt direct form the battery but it's not working the car battery is 45A.

I face the same problem when I try to Power up the system from 9V Alkaline battery

So what's the problem?

Hossam Alzomor


This is the calssic symptom of a poor power supply design. You probably don't have the proper filter capacitors on your board.

problem sim508


I am not using my own power supply
I am using the eval board sim508eval as it is ,and supply my 12 Voltage from out side.
also I added 2 *100uF electrolytic caps on the regulator of the eval board.

what should i do more?

Hossam Alzomor

sim508 power supply

I've never used the eval board, so I don't know what its power supply looks like. From the symptom, I'd say you are probably seeing a voltage drop when the unit goes into GPRS mode and the current draw increases. As a simple fix you may want to try larger capacitors. I use a 4,700uF cap on the input to my regulator and a 2,200uF at the SIM power pins. I've power these devices from all type of power sources and never had a problem with my design.

2009 supply problems

Thanks GSM MAN for your Advice

But don't you think that 4700 uF is a huge cap?
on the other hand I have checked the HW SPECIFICATIONs and found the following regarding SIM supply
SIM Card Power supply, it can identify automatically the SIM
Card power mode,one is 2.85V±0.1V, another is 1.8V±0.1V.
Current is about 10mA
dose the 10mA requires a big cap as 2200 uF.

I am trying to minimize the cap values as possible to reduce board size.

Hossam Alzomor

sim508 eval

Yes, the 4,700uF is a Huge cap. But our products are used in a variety of industrial applications, some with very nasty power sources. For our applications, we've found that that size cap works for all situations, and many of our applications cannot tolerate failures.

The power reuquirements you quote are for the SIM card, not the radio. While connected to the network, the radio can draw almost 500mA, depending on the mode and band. During actual transmission it can draw as much as 3 Amps about every 4.6mS. The power supply has to provide clean power while transmitting. Look at section 3.4 of this manual: Design SIM300DZ_HD_V2.03.pdf

It details the power requirements. In particular look at the section regarding the VBAT voltage where it states:
Voltage must stay within the min/max values, including voltage drop, ripple, and spikes.
Depending on your application, you can probably reduce the size of the input capacitor significantly. However, our experience has been that the cap connected to the VBAT lines needs to be about 2,200uF for reliable power in all modes and under all conditions - even though SIMCom documentation may suggest otherwise.

I've consulted on a lot of GSM applicaitons, and by far the major problem with most designs is poor power supply design. I've seen systems that were in the field for months and worked fine, that then started experiencing failures because of marginal power supplies. I'd recommend that you test your supply in the production environment, under the worse case for current draw and temperature, and look at the VBAT line with a good scope to ensure that you are meeting the device requirements.


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Re: SIM508 supply problem


Thanks for Advice
we tested with 2200uF and it's working fine now
I'll test in worst case conditions to make sure it will be working.

Hossam Alzomor

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