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GSM GPRS Modem Power Supply From Alkaline Batteries

Discussion in 'Electronic Design' started by Tom Harris, Jul 10, 2013.

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  1. Tom Harris

    Tom Harris Guest


    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

  2. miso

    miso Guest

    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.
  3. 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

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