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I have read that usually a gsm module requires a power supply capable to provide up to 2A. But how can 3g modems sticks work properly when they are powered through usb port which provides only up to 1A(?)

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    \$\begingroup\$ They only require high current in intermittent bursts. Capacitors solve that problem. \$\endgroup\$ – brhans Jul 24 '17 at 22:08
  • \$\begingroup\$ But capacitors need to refresh their energy in between of each burst. \$\endgroup\$ – MrBit Jul 24 '17 at 22:11
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    \$\begingroup\$ Yes - and they do - between the gsm module's high-current bursts, the caps recharge. \$\endgroup\$ – brhans Jul 24 '17 at 22:13
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    \$\begingroup\$ Well, yes. After all, the devices do work. \$\endgroup\$ – Wesley Lee Jul 24 '17 at 22:29
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    \$\begingroup\$ Another possibility is that the dongles use newer signalling modes that simply don't demand as much power. Experientially, old GSM phones used to really "thump" nearby speakers (especially right before they were about to ring, to the point where that became my effective ringtone), but modern ones don't. The UART modules are often old 2G designs, while the dongles are more oriented towards achieving useful data speeds. \$\endgroup\$ – Chris Stratton Jul 24 '17 at 23:07
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Here is a very related question. This modem has 25% duty cycle, so 2A peak translates into 500 mA average. And yes, the voltage drops noticeably when RF transmits, so certain care must be taken to bypass this drop. Other people indicate the amount low-ESR ceramic caps of the order of 2,000 uF, some recommendations go up to 12,000 uF.

Obviously, for the bus-powered USB dongle to meet inrush current requirements, a special gradually-working power management is needed.

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