jayanth kumar

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8 years, 1 month ago.

mbed lpc1768 not running on external power source

I am using lpc1768 for my project. I need to run it on an external power source. I used a 9v battery. But when i connect it, mbed led glows for 10 seconds and then mbed turns off. I observed that the battery voltage is degrading when i connect it to mbed. So, how can i rectify this problem?

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Last updated: 30 Jun 2020
Rapid Prototyping for general microcontroller applications, Ethernet, USB and 32-bit ARM® Cortex™-M3 based designs

1 Answer

Wim Huiskamp
8 years, 1 month ago.

Looks like an overload condition. Possibly thermal shutdown of voltage regulator. How was the battery connected, what else is connected to mbed, in particular what current loads are on the pins and Vout. Are you using the 5V USB out pin on mbed? Note that this pin is unpowered when the USB cable is not hooked up to a host PC or other powersource..

Thanks for the reply Wim. I haven't connected anything else to the mbed. I haven't used USB out either. Vout reads 2.4-2.7v. current readings show -1 ma.

posted by jayanth kumar 20 Feb 2016

When battery is connected to gnd and Vin and the Vout drops that low within 10s even without additional loads you either have bad battery or there is something broken on the mbed. Is vout ok when you run on USB power.

posted by Wim Huiskamp 20 Feb 2016

Yes. It works perfect on USB power. I tried another battery. It worked well for a while, but it also started degrading.

posted by jayanth kumar 20 Feb 2016

A 9v battery has a capacity of 400-500mAh and the mbed typically consumes about 140mA. Based on the curve from Duracell's datasheet you would only get about a couple hours from a fresh 9V battery. http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1604_6LR61_US_CT.pdf

posted by Doug . 20 Feb 2016

Battery capacity should match the required current and expected duration, but less than 10s of operation means that battery is bad or something else is wrong. I would also recommend to use a different type of battery if possible. You waste a lot of energy in the linear voltage regulator when using a 9V battery for a controller operating on 3V3. Find a 4V5 or 6V battery instead or use a switched mode regulator between the battery and the mbed.

posted by Wim Huiskamp 20 Feb 2016