Pro Mini - VCC pin strange behaviour

Hi all,

My understanding is that you can use the VCC pin on the Pro Mini to supply 5V to an external device (I've got the 5V 16 MHz version). Provided you power the Pro Mini itself from the RAW pin.

When I power the Pro Mini through the "USB" (PC -> FTDI converter -> pro mini) I indeed get 5.0V out of the VCC pin.

When I power the Pro Mini through the RAW pin using a battery I only get ~3.8V out of the VCC pin. I tried with both a 9V 'rectangular' battery and a 2cell (7.4V) LiPO battery. I thought at first the 9V battery might be the source of the problem but I get the same issue when using the LiPO.

The device I power through the VCC is a data logger shield that comprises a real time clock and SD card, should draw 200mA peak and much less most of the time.

I honestly have no idea what's going on.

Thanks for any input!

Sounds like it's got a fried regulator.?

What's the Vcc voltage without the datalogger? SDcard can take more than 200mA, it
all depends on the card. Have you checked what current the on-board regulator is able
to source? You might need external regulator if its very puny.

Thanks for your reply!

I just checked, Vcc is still 3.8V without the logger.

Also it appears that the regulator on the Pro Mini has a max current of 150mA so that might be it. I might have fried the regulator because of this.

I think I'll just use a 7805 to give 5v to the logger from the battery, with a digital pin+bjt (2n2222 is what I have available) to switch it on/off. Does that sound reasonable? (2n2222 goes up to a few hundred mA so should be fine there).

Dwacito:
I just checked, Vcc is still 3.8V without the logger.

But, most importantly, that's for the ProMini - any auxiliary use is a side issue.