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Voltage divider/Comparator Resistance change on power up

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tiwari.sachin

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I am using the following circuit as a comparator

Comparator that I am using is MCP6561T-E/OT

Issue:

I am trying to change Rth by using a variable resistor

Initially when power is not supplied, Rth shows 18K for example

Once I turnon the power, this resistance changes to some 40K

This is effecting the design and how I want it to work.

The Vref changes as desired.

What could be causing Rth change when powered


1. There are no resistors in parallel that effect
2. Op-Amp connection I guess shouldnt be a concern

Circuit is as attached
 

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hi,
How are you measuring the Rth resistance when the 3.3v is connected across the the resistor.?
E
 

Hi,

There is no Rth in your circuit.
Do you mean Rh or the thermistor?

Klaus
 

I am sorry I forgot to mention, Rth is thermistor resistance

- - - Updated - - -

@esp1... I am connecting a multimeter across and checking it.

What I have want to do is output to be high when temparature crosses some 75deg cel... and holds at that till it comes down to 50 something... Calculations and circuit seem fine.

As said I am right now using a rheoshat... At 1.5K thermistor value, temparature is 75deg

At 12K it is 50 deg, hence I have considered a hysterisis for the same
 

You cannot measure an energized resistor using a ohm-meter. If you need to do this you have to measure the voltage across the resistor Vr and the current flowing in it Ir, then calculate R = Vr/Ir.
 

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