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Op amp temp control

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zipdogso

New Member
Op amp temp control - help wanted....

What I wanted was a thermostat of sorts to control the cooling of a heatpipe,

The actual temps are irrelevant what I need is the difference between 2 sensors (one cold, one hot.) the hot end of a heatpipe is in a variable temp environment (so it gets hot and cooler.) I need to make sure the cold end is always colder. so if the hot cools to within x degrees of the cold i want operate a relay to start a cooling circuit.

I really only wanted one unit ( a prototype , I suppose.) But quotes to get this working varied greatly, some running into thousands.

However one person did point in a direction...though not necessarily the right one ! He said :-"My guess is that yoiu would get close enough putting the thermisters in series from a 5v rail and comparing the mid voltage to reference using an OP amp - and drive the relay from that. I suggest you experiment with this."

Sounds simple enough...except my knowledge of Op amps is about the same as brain surgery !!!

I only want to use simple NTC thermisters 10k±1% 3435°K –50 to 90°C

The relay ideally needs to control an output of 16v 8.5 amps but if it's easier a 240v 10amp can be used so long as it can be triggered.

I only need a small control range 0º-10º or maybe 12º between hot and cold sensors.

I really would like to solve this if I can....any ideas gratefully welcomed.
 
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MikeMl

Well-Known Member
Most Helpful Member
So let's see if I get this? You tried to get somebody to do it for you and you were not willing to pay consulting rates. So now you want us to do it for you for free?
 

zipdogso

New Member
NOPE.....

If someone would like to tender an idea I am actually quite happy to pay...just not £800-£2000

Otherwise I am looking for some help with op amps so I can build this thing myself....
 

MikeMl

Well-Known Member
Most Helpful Member
I was just yanking your chain. I'm working on it...:D
 

aljamri

Member
2 temp sensors ( NTC or PTC ) , single Dual-Comparator circuit ( Basic circuit ) and small Microcontroller 12FXXX will do this easily.

How much you will pay for it ?:D
 

MikeMl

Well-Known Member
Most Helpful Member
Here is a proof of concept. I'm simulating when the output of the (opamp used as a) comparator switches as a function of the resistance of the two "thermistors", R1 and R3.

Note that the two track each other, and to a first order, the comparator switches when the two resistances are equal. You could build-in a slight offset by slightly increasing or decreasing R2. The supply voltage is not critical; it could be anything from 5V to 12V. Lower might be better so as not to cause self-heating of the thermistors.

I didn't show how to switch a relay, but you could connect the OUT signal to the gate of an NFET, and use that to switch a relay...

I'm leaving it up to you as to which thermistor R1 or R3 is the hotter one. That will determine the direction you want to offset the trip point, and the polarity of the OUT signal...

The opamp/comparator could be an LM358 or a TL072.
 

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zipdogso

New Member
Cheers Mike...
I will breadboard it and play with it, see what hysteresis I can get for efficiency I really need the relay tripping a few degrees before the sensors are the same temp....once I have the bits...

I think I can handle the relay trigger.

PM me a valid email and I'll send a small reward your way via paypal ....
especially since you replied so quick...Sterling work in my book.

zipdogso
 
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