Hi Steve,
This simulation is based upon your last circuit and description.
I need to know the resistance of the meter and its specification.
In order to confirm the result, we also need the Diode voltage at 0C and 30C.
E

 

Hi Eric,

Getting the diode temps for 0°c and 30°c may prove tricky as it isnt easy to replicate those temperatures at home with any sort of stability. I have a fridge and some rooms. I could possibly see how far down i can get the bottle cooler to go and possibly get 30°c by building a little oven, ive got some 5w resistors and a cardboard box lol.
The meter coil resistance measures at 9.1 Ω after deducting the 0.1 Ω of fluke meter lead resistance.
Which other specs do you need? Ive recorded the voltage input to get the needle to every increment on the scale. 0 is 3.2mv through to 30 at 116mv.

kind regard steve

 

Hi S,
Do you have a domestic temperature thermometer , place the diode and thermometer in a plastic bag, pop it into your home freezer.
Observe the values at 0C..
Heat some water in a cup, use the bagged diode and thermometer to measure temperatures.
E

 

ive got a thermocouple for my fluke meter if we trust the accuracy of it?? ive also got a thermometer but it takes a long time to respond and settle

 

Sooooo, the readings taken at diode sensor cathode are thus.
2.446v at 0°c and 2.608v at 30°c. (i like the plastic bag trick, seems to stop the temp shooting up or down too fast to take a reading).

regards steve

 

I've had a little go myself, see the attached-

I have done away with OA4 and fed the sensor cathode voltages directly into OA2.
ZD1 has been dispensed with and OA1 now fed at 2.445v.
R6 changed from 39k to 1k, and R11 changed from 47k to 2k.
OA3 changed from inverting to non-inverting.

Does it stand a vague chance of working? I have a feeling I am about to learn how close I can get an op amp output towards + supply, but in this simulation OA2 output gets to 3.208v with sensor at 30°c. I am still trying to flog the horse of running it all on 3.7v!

Fingers crossed this time......

Regards Steve

 

hi S.
Is this one 1N4148 or 4?
E

 

its 4x bat43, same as before.

 

HI S
Your drawing shows one 1N4148, in order for me to help, you must give me accurate information.
E

 

Hi,
This is what I see, please compare my circuit with yours.
E

 

Readings at VO are a lot different to mine- 633.4mv to 1.994v? why such the difference? Both drawings match....
What do you get at the output of OA2?

Regards steve

 

hi,
This shows OA2 [vb], also vd and vc.
E

 

this is because of the op amp max output of 1.5v less VCC you told me about??

 

Hi Al,
The project will not work as expected until he uses at least 5V on the supply rail.

He is using LM324 OPA's which are not R2R.

In order to help him finalize his design for 0C through +30C, we need to know the VM2 voltages at these two limits.

At the moment the circuit is non-linear and clipping at 6C and 25C
E

Oh ok thanks Eric.

 

Hi S,
Consider this as an option.
Note the 5v for OPA supplies and the 3.67v for other parts of the circuit.
E

 

Interesting, and easily achieved.....
How stable will the op amps be with a fluctuating supply voltage?

 

How stable will the op amps be with a fluctuating supply voltage?

Hi S,
If you plan to use any type of OPA you should study the datasheet.
E

https://www.quora.com/What-is-the-PSRR-value-of-a-practical-opamp

 

Its hard for somebody new to the concept of op amps to really understand what this all means, but thank you for steering me in this direction. from what i can gather it depends on how high or low the gain is set to? but worse case we are talking in the range of a few millivolts?

 

Hi S,
May I ask, are you a student or hobbyist.
Do you understand what 60dB ratio means?
E

 

I am a hobbyist. I come from a railway electrical background but thats mostly medium to high voltage and lots of relay logic. Electronics although not entirely new to me, is a keen interest that i know little about currently especially when trying to create something from scratch.
60dB ratio? mathematics is one of my worst subjects so no, not really im afraid. keen to learn as always though...

kind regards