wierd voltage divider issue

[aruna1]

Hi
I'm working on a power supply based on a smps and a linear regulator. idea is to use buck converter to drop high voltage to a value 3 volts above the required value, and use a linear regulator to get the output. I'm using LM2596 as buck converter and LM317 as linear regulator.
I wanted to measure the output voltage using a PC16F877A and display on a LCD. so I used a voltage divider at LM317's out put to reduce output voltage ito safe value.

now comes the question. power supply is designed to give variable voltage and 21.24V is the maximum. circuit is fine and voltages are regulating correctly. I used a dummy load to test power supplies ability to with stand current requirements. so far I tried up to 1A and regulator works fine. how ever there is a problem at voltage divider. I'm using 10k trimpot as divide to set divide point voltage. so I have tuned it so that when power supply give 21.25v divide point output is 5V.
under no load condition divide points voltage is correct and varies as i change the output voltage. but when I load the circuit and begin to draw current voltage at divide point increases. actually when I load the regulator, regulator's output drops slightly. I can see this on multimeter. but voltage of dividing point increases rather than decreasing. its funny, just like ohms law got inversed. that is as voltage across trimpot drops voltage on divide point increases. (it should be decreasing right?). Huh

got any idea what the heck is going on?
i have attached the linear regulator circuit here.

PS. I tried changing the trimot but results were same

 

[Pommie]

How are you measuring the output from the pot?

Mike.

 

[aruna1]

using a multimeter. between voltage dividing point and ground

 

[Pommie]

Then it makes no sense. There must be something else that you're not mentioning. What else is connected to the wiper?

Mike.

 

[aruna1]

nothing is connected, just a piece of wire to connect multimeter.

 

[Pommie]

I quote, "Ya cannae change the laws of physics Jim".

Mike.

 

[aruna1]

so since voltage divider is not going to do any good, is there any oter methods to drop the voltage 0-21.24V to 0-5V range?

 

[Pommie]

The voltage divider should work fine and if it doesn't then you have somehow changed the laws of physics.

Mike.

 

[aruna1]

lol, i knew the ohm guy is a lire

 

[crutschow]

If it doesn't work, then there's something wrong with your wiring.

 

[aruna1]

it seems problem is with the resistance of the current paths. I added parallel paths and seems problem is gone

 

[aruna1]

its still remains, I thought its gone but now its there

 

[ronv]

Turn everything off and measure the resistance across the pot (not what you think is the wiper) it should not change when you adjust the pot. If it does you have switched the wiper with one of the ends of the pot.

 

[Ziddik]

how did u connect the wires to pot? R you sure about the exact pins of pot?

 

[mdorian]

It's no magic , just ground problems , the Pic GND and load GND must be connected to R2 GND using distinct wires and also R3 GND , (like a star configuration).If you have a common GND for load and PIC the voltage drop on id is added to the adc input.