Author Topic: Power Shield A7 range  (Read 1461 times)

scottpenrose

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Power Shield A7 range
« on: February 06, 2015, 08:07:15 AM »
Good evening

I am just trying to get the voltage from the A7 pin, and the documentation talks about being a 1M over 470K, which would be 100/47. This should allow a range of approximately 6.6 volts maximum before getting about the 3.3 volt analog input.

Long story short. I have a battery currently measuring 4.0 volts, and it appears as 2.8 volts at A7. Which sounded quite high. This is confirmed by the raw reading of 817. This adds up to 5.2V using 100/47 (it is < 4V)

There seems to be some indication you can measure a 9 volt battery, which would suggest a higher resistor. What am I doing wrong?

Thanks

Scott

Felix

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Re: Power Shield A7 range
« Reply #1 on: February 06, 2015, 09:40:57 AM »
I know there has been an early batch of PShields where the resistors on the V monitor were inverted. Normally they have to be like this:

VBAT - 470K -(A7)- 1MEG - GND

That should allow a range up to about 6V like you said before it goes over 3.3V in the divider. If you have the older reversed batch that should divide it even further, which may correspond to the figure you're getting for the 4.0 V. To confirm you can measure the resistors, and if they are indeed reversed you can change your division ratio or if you're brave you can exchange them.

scottpenrose

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Re: Power Shield A7 range
« Reply #2 on: February 08, 2015, 06:24:40 PM »
Thanks Felix. I actually just got these only a week ago, so they are not likely the old boards, but I will find the resistors and measure them today.

Scott

scottpenrose

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Re: Power Shield A7 range
« Reply #3 on: February 08, 2015, 06:31:06 PM »
OK that is indeed the case. The Ground to A7 is 0.9 meg, and the VBat is 0.38meg (I suspect the value difference is inaccuracy of my meter, will check with another later). But they are definitely reversed, and they are new, not old batch (1 week old).


OK. Looks like it is just the first board I tested is backwards. Next board I tested is the right way (I wrote the wrong way above).
Will check the other 3 I bought too.

Scott
« Last Edit: February 08, 2015, 06:32:53 PM by scottpenrose »

scottpenrose

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Re: Power Shield A7 range
« Reply #4 on: February 08, 2015, 06:40:34 PM »
All the rest appear fine. Maximum of about 5.2 volts can be put in on A7 which is good because really a lithium shouldn't go above 4.2

Thanks.

scott