Hello,

I made a little change in the schematic.
With the modification, the voltage can be adjusted between ca. 5.00 and 5.26 Volts.

Bertus

Thanks.

With the help of everyone who posted (and reading the datasheet a little more closely,) I think I am getting there. However, I have decided to use an ADC reference of 4.096V derived from the 5V supply. I have modified your modification to achieve that.

It seems to work properly, but I still have one question regarding R3 in my schematic. The datasheet says that R3 should be sized to provide cathode current of at least 1mA at the lowest expected Vin. I tried your value of 2.7k and the TL431 would not regulate, so I scaled R3 down in the approximate ratio of 12 to 5, and arrived at 1k, which seems okay.

My question is how does one actually calculate the cathode current in this circuit? Thanks again to everyone, and please excuse my ignorance.

 

The current through R3 is is (5 -4.096)/1k = ~1ma

The current drawn by the Vref pin is small, so most of the 1mA go into the Kathode pin.

 

The current through R3 is is (5 -4.096)/1k = ~1ma

The current drawn by the Vref pin is small, so most of the 1mA go into the Kathode pin.

Thus, to follow the datasheet more closely, I should reduce R3 to 820Ω. Correct? Thanks.

 

Except that I forgot the current through VR1, R1 and R2, which would be ~4/15K = 266uA, so the current into the Kathode would be 734uA.

 

Here's what I hope will be the final solution (but I thought that last time. )

 

Hello,

Did you keep track of the input current of the Vref input of the PIC, as mentioned by #12?
This would be about 150 μA.
In that case R3 would be about 700 Ohms, to keep Ik at 1 mA.

Bertus

 

Hello,

Did you keep track of the input current of the Vref input of the PIC, as mentioned by #12?
This would be about 150 μA.
In that case R3 would be about 700 Ohms, to keep Ik at 1 mA.

Bertus

No, I overlooked that as well. Back to the drawing board...er, spreadsheet.

 

Here is a bad photo of my prototype at work. Now, I need to figure out the code for the PIC. Thanks to everyone for the help.