Dirt Easy Thermistor Idea

bad BAD mistake ..._bugger

...as I was sipping a coffee and pondering over my post, I saw I made a rather slight (well, bloody serious error)... ...fixed up the connection for R1 and U2A pin 3, as they are not directly connected to the DC voltage source. I also made the R3 potentiometer in the incorrect orientation...the adjustment pin feeds into U2A pin 3. In simplifying the schematic for the web, I made the error so I fixed up the shortcut I took and resubmitted the correct version below...the green trace is the correction and the properly orientated R3...oops

...I reaaly am having a bad day

Hi,

Nevermind We learns from mistakes.

But I am still in doubts, As pin number two is directly connected to ground with thermistor source+270K Resistance and potential at pin 2 is not going to change regardless of any change in the thermistor value due to change in source temp.

As the Therm_SOURCE thermistor heats up relative to the Therm_AMB thermistor which is stable, resistance at the SOURCE thermistor (using NTC type) decreases which increases voltage into pin 2 of U2A. this creates a difference in voltage relative to that of the "stable" pin 3 and causes the OpAmp to switch.

The R3 27K resistor linking the 47K SOURCE thermistor with the ground mainly helpls reduce internal heating of the thermistor and any link with ground is needed for a thermistor to function correctly as a resistive component. R3 also constrains the range of change for the SOURCE thermistor and you are correct that if the R3 resistor was in fact a 270K resistor...change in value at pin 2 would be neglible when using a 47K thermistor...a lesson I took a day or two to sink into my brain.

I don't like to go in values, but As I undersatnd I think the end of 47k Thermister should go to ground,and end of 27 k should be connected to VCC(SUPPLY) Where as center point that is connetced to capcitor should go on Pin number 2.

you are in fact indeed correct...I am experiencing the early onset of dementia

Thanks -- you saved a lot of explaining by rakesh or me!

OMFG....yet another mistake....

rakesh was meaning about no change in value to resistance due to NO opposing positive....dam dam dam....I gave an answer to rakesh assuming I had the junction connected to the positive and then realized later that it wasn't actually connected at all....rofl

just link up the resistor/thermistor junction at the location in question to POSITIVE DC supply...I am a twit

My worst post in history and a funny one at that

Oh Ozzy Never mind,

By the way its my 500th post and I did lot of similar mistakes in past.

Any way Cheersssssssssss--, I am a Guru now.

I have doubts, Pin Number 3 (Input +) is shorted with Pin number 8(supply voltage)

I am not clear how can output flip from high to low if voltage at Pin 2 never crosses to voltage at pin 3?

I mean +VE input is always at the same voltage( = supply voltage) regardless of any change in thermistor value due to change in temp.

There are few more concern in mind but no point in talking unless above point is clear.

yeah...lol...just read your post after I fixed up my stupid mistake that corrected my other mistake. Having submitted a correction to my original mistake(R1), you were reading my earlier corrected version that also contained another mistake (R3)...so while I buggered off back and fixed my second error (R3) you were reading my corrected post for (R1) and made a comment picking up another mistake (R3) I was already correcting Now I submitted the final corrected version and saw that you too picked it up ...well picked up and an obvious one at that

Need more coffee intake before I post something on the WWW again

Nice to see you've used some positive feedback for good clean switching and hysteresis.
A lot of people don't realise the benefits.
Del

Yeah, thanks mate...one such as myself would have thought such a circuit (without my crazy sleepy errors) was common...but indeed is not.

By the way, here is the FINAL corrected circuit ...saw another mistake thanks to some sleep and a coffee ...my only hope is the moderators could either delete or fix this erratic post....

...will next time run my circuits through a simulator after I water them down for simple reading on the WWW

It is fairly common, you've just drawn it a bit funny.
Conventionally it's easier to read if the zero volt line goes straight across the bottom.
The potential dividers show up more readilly then.
E.G R2, R3, C2 would go down rather than across. The battery would be shown on the left side with it's negative end going straight down to the 0v line at the bottom.
Hope you don't mind me pointing this out.
Del

When I was doing my researching on the WWW, I really could not find similar use of a reference thermistor...

I used keywords on Google using "opamp" "comparator" "thermistor" and I found one only that was close to what I needed, but it really did not work as far as the comparator (311) is concerned in this example located at this LINK

I wanted to overcome the challenge of measuring ambient as a reference and compare it with another thermistor located at a variable heat source to trigger the OpAmp at a specified temperature difference between the two thermistors... without requiring constant calibration with each use to take into account the variable ambient temperature as is typical for the common model using a single thermistor compared against a fixed value resistor.

I ain't claiming what I figured out over the week to be original or complicated...but was a steep learning curve and very simple to the initiated in the end...and if you could locate some ideas similar to my application, it would be greatly appreciated.

...will try a Google search using the boolean "potential divider" (the wording you used) and "themistor". I will also learn the proper etique for circuit drawing and thanks for the heads up on that

Kudos for getting stuck in and playing with this stuff, it's the best way to learn, not necessarilly the quickest, but I reckon it's the best.
You can do some pretty cool stuff with a couple of op amps and a few components.

Del

"my only hope is the moderators could either delete or fix this erratic post...."

No success can be achieved without (failures)mistakes, I do admire your work. and Finally you corrected the post which is good enough.

I doubt anyone can challenge your understanding to the circuit, but may be you were in bit hurry.

Still not quite sure about that, if it was supposed to be symetrical?

You've got C1 directly across the supply, whereas C2 smooths the pot formed by R3 and Therm_SOURCE.

Did you really want C1 from 0V to the wiper of R2.

Or even the top of R2 AND C1 connected to the wiper instead of 6V: so that R2 becomes just a variable resistor instead of a potential divider.

I might have missed the point here.

Randall you are quite right, I would rather do it like this-

I can see the divider reasoning here...thanks for the heads up