GSM GPRS Modem Power Supply From Alkaline Batteries

[Tom Harris]

Greetings,

Got a requirement for a GSM/GPRS modem to run off alkaline cells, which is a little unusual, as they are designed to run straight from a Lithium-Polymer battery at about 3.6V. A common power specification for modems is 3.3v -4.6V at a few mA standby, 50mA active with 500us bursts every 5ms up to 1.5A(!) when transmitting.

As I said I need alkaline cells to run this from. What do people think to running the modem from 3x cells 4.5V nominal, 4.8V max with new cells on light load via a silicon diode to drop .7V to keep the volts below the maximumfor the modem. When the cells discharge a bit below 3.8V say, the diode isswitched out with a latching relay.

I don't care about wasting the power in the diode as the modem is switched on only for a few seconds every week, so the power lost is minimal.

Thanks for any ideas

TomH

 

[miso]

Normally I like LDOs with PMOS pass devices, but if you go LDO in this
case, I'd probably go with a bipolar LDO. There isn't much voltage to
turn on the PFET when it goes out of regulation.

Alkalines have a linear drop to about half a volt a cell, but you really
need to buck/boost if you are only using a few cells.

 

[Lasse Langwadt Christensen]

Greetings,



Got a requirement for a GSM/GPRS modem to run off alkaline cells, which is a little unusual, as they are designed to run straight from a Lithium-Polymer battery at about 3.6V. A common power specification for modems is 3.3v- 4.6V at a few mA standby, 50mA active with 500us bursts every 5ms up to 1.5A(!) when transmitting.



As I said I need alkaline cells to run this from. What do people think torunning the modem from 3x cells 4.5V nominal, 4.8V max with new cells on light load via a silicon diode to drop .7V to keep the volts below the maximum for the modem. When the cells discharge a bit below 3.8V say, the diode is switched out with a latching relay.



I don't care about wasting the power in the diode as the modem is switched on only for a few seconds every week, so the power lost is minimal.



Thanks for any ideas

use an LDO, pick one with enable and you also have you on-off switch

you could try and find an example of a usb gsm mode, usb is 5V and
limited to 500mA so the supply must handle the voltage and the burst
current


-Lasse