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GSM GPRS Modem Power Supply From Alkaline Batteries

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-Polym er 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 r unning the modem from 3x cells 4.5V nominal, 4.8V max with new cells on lig ht 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

TomH

Reply to
Tom Harris
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I think you'd be much happier with an LDO regulator. Into a honkin' big cap, given that 1.5A pulse.

A decent one won't have much drop once it goes out of regulation.

Keep in mind that there's still quite a bit of useful charge in a dry cell even after the cell voltage has dropped to 1V. See if you can find a data sheet or three; plotting energy left vs. cell voltage should be educational. You may decide that four cells are better.

I'd be tempted to do a switcher; it may be the best way to wring the maximum amount of energy out of the batteries, given the wide voltage range you see from dry cells.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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.

Reply to
miso

There are boost convertor ICs specifically desined to make 3.3V from one alcaline cell. Very efficient, too.

Vladimir Vassilevsky DSP and Mixed Signal Designs

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Reply to
Vladimir Vassilevsky

s a little unusual, as they are designed to run straight from a Lithium-Pol ymer 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.

running the modem from 3x cells 4.5V nominal, 4.8V max with new cells on l ight load via a silicon diode to drop .7V to keep the volts below the maxim um for the modem. When the cells discharge a bit below 3.8V say, the diode is switched out with a latching relay.

d on only for a few seconds every week, so the power lost is minimal.

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

Reply to
Lasse Langwadt Christensen

Compare the internal resistance of alkaline batteries to the LiPo batteries that are recommended.

Pay attention to battery terminals & the interconnects & the losses they incur during tx. Use GOOD quality battery holders.

Use 3 or 4 x 1500uF 4V caps in // across the output supply to modem

Reply to
Rodwell

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