(Off Topic) Battery Voltage Detector

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

I think this has been covered before, but I couldn't find the topic.

How does one control the power to a voltage divider to sense the battery voltage? This is a 4 AA powered app that runs at either 3 or 5V and needs to last as long as possible. So, the goal is to only power the voltage divider to the ADC when taking a reading.

The topology below is an obvious option. Simply set RA4 as an input to power it up. Take a reading and then turn it off.....

However, I remember there being some issues with the configuration....

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

When you put the RA4 into the high-Z state, +6V will appear on AN0 and RA4, and +6V is higher then the max allowed voltage. You could ground the bottom of the divider and add a small MOSFET (like 2N7000) in series with the top resistor. Or you could pick the higest possible values for the resistors, and have the smallest leakage. I remeber that the maximum value for the lower resistor is 2.5k, this was mentioned in the PIC's datasheet.

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Thanks Kender,

I can't believe that didn't dawn on me. I'll go the FET route and everything should workout.

John

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Wouldn't have caught that one. I've got other designs using FETs and they are always on the ground leg (low-side). Probably would have plowed right into the wall. Thanks for the correction.

I Googled and found this option:

http://www.micrel.com/product-info/products/mic2514.shtml

Printing out the data sheet now to read up on. Just another wild Friday night!