5V to 3.3V for analog input

I want to read a battery voltage so I can measure the remaining capacity (based on the battery voltage drop). The battery is closer to 5V than 3.3V.

I’ve been told I can use a circuit like this to drop the 5V to 3.3V without having to worry about small current changes. This is different than the other solutions I’ve seen which don’t seem to address the varying current issue.

Does this look right to anyone else?

That’s a 5.1K resistor on the top, a 10K resistor and 1uF cap on the bottom. And I realize this isn’t really NETMF specific, but I figure a lot of software guys like me could use this hardware trick (if it works).

Yes, that looks about right.

You might want to drop it a little lower than 3.3v so your not trying to use the ADC at its top limit.

The cap is a good idea to try and reduce noise.

1 Like

@ untitled - Without discussing if you are able to measure the capacity of the battery the circuit is right to adjust the voltage from 5 to 3.3 volt.

A disadvantage, if an issue, is that the circuit always consumes some power.

1 Like

Thanks, I did not realize. I will measure my loss, it might be ok. I figure the battery has some kind of protection so it will shutdown when it gets near empty.

What kind of battery are you using? Most don’t have any kind of shutoff like you mention. Also some chemistries have a very sudden voltage drop off at the end of their capacity.

How often do you need to take measurements?

Can you use a transistor, or the like, to turn the divider on periodically only to take a measurement?

It only consumes 0.3mA. That may or may not be a big deal, but unless you are using deep sleep states on the mainboard It’s a drop in the ocean.