Display Vdd

[Archangel]

That might not be as easy as you state. 30 volts A/C is an RMS value, which is an average of all the voltage, the peak value of an AC 30v will be much higher, and you will see the rectified voltage will result in a much higher DC voltage than 30 VDC.

[davesatu]

Thanks Joe.

Yes of course you are right. The rectified voltage will be 1.414 times the AC voltage. And it measures accordingly. That's why I have a voltage divider built in and probably a zener after that to cover any cases where the AC goes over 30 volts.

Any suggestions on how to do display Vdd/Vref ? Dave.

[HenrikOlsson]

I may still be missing the point but if you want to measure and display Vdd, ie the PICs supply voltage, you MUST use an external, stable, source for Vref that does NOT "move" with Vdd, otherwise you will simply get the same reading all the time.

Let's say your Vdd is 4.85V and therefor your Vref is 4.85V. A voltage of 4.85V on the analog inputs will now give you a full scale result (1023) from the ADC because 4.85V is what Vref is set to. Whatever you set Vref to is what will give you a full scale reading so as long as Vref=Vdd you can not measure and display Vdd with the ADC.

/Henrik.

[davesatu]

Thjanks again.

All received and well understood. But the original question remains : "how do you physically display the current value of Vdd on an LCD?"

[HenrikOlsson]

Hi,

But the original question remains

And has been answered IMO.....
You use some kind of voltage reference chip that produces a stable reference voltage over the whole operating range of your batteries. Let's say that voltage is 2.048V. You connect this reference voltage to the Vref pin on the PIC and set the ADC up to use that pin as Vref+ instead of Vdd.

Now you create a resistor network between Vdd and Vss and choose the resisitor values so that the voltage over the lower resistor is as close to 2.048V as possible when the batteries are fully charged. You connect this "signal" to the analog input of your choise.

Now the ADC will give you a reading of 2mV/count.

There is no way to internally "connect" Vdd to an analog input - except to the Vref+ pin. And there is no way to do measure Vdd without using an external reference voltage.

/Henrik.