Inline Resistor for ADC read

[bpowell]

Hello All,

I'm looking to read the value of a single-cell LiPo battery using ADC input. the battery voltage will range from 4.2 Volts (fully charged) to 3 Volts (fully discharged).

The 20x2 will be running off of 5 volts (through a power boost PCB).

Given the LiPo voltage swing is < 5V, I'm thinking a voltage divider is not necessary. However, I'm wondering if it might be wise to put a 1k resistor between the battery and the ADC pin?

I'm thinking...just of the off chance the pin is changed to an OUTPUT and switched to ground...a 1k resistor would limit the current through that pin to 5mA...thereby saving the PICAXE.

Any thoughts on this?

Is a resistor necessary? Is it overkill?

Thanks!

Brendan

 

[premelec]

I just do it... the ADC input is quite high impedance and the 1K is insurance against some inadvertent stupidity - resistors are cheap - PICAXEs not so much...

 

[bpowell]

You may not know this Premelec, but "Inadvertent stupidity" is actually my middle name!

 

[AllyCat]

Hi,

Probably not an issue, but beware that if the 5 volts is turned off, then the PICaxe may continue to run from "phantom power" via the ADC input pin and the ESD "protection" diode to the Vdd rail. Less likely with 10k, which is still within the ADC specification.

Cheers, Alan.

 

[bpowell]

Hi,

Probably not an issue, but beware that if the 5 volts is turned off, then the PICaxe may continue to run from "phantom power" via the ADC input pin and the ESD "protection" diode to the Vdd rail. Less likely with 10k, which is still within the ADC specification.

Cheers, Alan.

Thanks for the tip AllyCat!

I have a SPST switch on the battery, when I turn the circuit off, I'm actually disconnecting the battery from the circuit (and connecting it to the LiPo charging circuit)...so there is no way to phantom power the circuit...also no way to run the circuit while the LiPo is charging...which is also by design.