This is a bad idea, I know
Here's what I know about amplifying really tiny DC voltages:
- it's impractical to build your own for production
- there's going to be noise
- the signal is relative to the cold junction temperature
- it would be easier with an ASIC than using basic components
- most op amps aren't rail-to-rail (can't go to 0)
These are all acceptable to me. I want to learn how to do it anyway.
I've just started self-teaching and I thought that measuring the actual temperature in my toaster oven with what I have laying around (batteries, resistors, thermocouples, resistors, and op amps) would be a fairly simple and easy first project to design myself. Maybe I'm wrong, but I believe it's possible and I want to learn how.
Where I've been
I was able to measure between 0.2mV and 0.4mV while holding the K Type (41µV/˚C) thermocouple with my finger (37˚C supposedly) using my multimeter. That checks out: ~0.5mV = 41µV * (37 - 23).
Well, I can't read that in on 0-5v analog input...
At first I thought that perhaps I could use series of transistors, but since there's no current (and the voltage is well below the minimum necessary), that was a bust.
I remembered reading and hearing about op amps, so I thought perhaps that might be the right tool for the job. I watched Dave Jone's excellent EEVblog #600 - OpAmps Tutorial - What is an Operational Amplifier? and that I'd give it a try.
The local university has a plentiful supply LF347-N in a vending machine on the 4th floor of their college of EE, so I grabbed a few of those.
I was able to get a 10 µF capacitor to hold < 1V of charge (tapping to the 9v with a resistor and then draining with a resistor while measuring) and then created the suggested non-inverted feedback loop with each of a 5k1 and 2k resistor on a breadboard with a 9v battery and see the expected 3.55v (
1 + RF/R1). It seemed that the op amp was actually charging the cap, despite the no-in/no-out "guideline". Or it was just increasing from residual voltage as it settled.
Cool, I think I'm on the right path now.
I swapped out the 3.5x feedback (5k1, 2k) with a 101x feedback (100k, 1k) so that I could test with the thermocouple. I got an initial ~1.2V reading (maybe because I was below the rail limit?), but then it didn't change when I held my finger on it.
... and I could smell the Magic Smoke! Hot resistors. Hot op amp. I don't think it works anymore. I still get some readings, but they seem off and I don't trust it.
Tried a second time, similar result (gotta go back to that vending machine). At various points as I was experimenting I even tried touching my hot soldering iron to the tip, but never saw a response different from holding it with my finger through the op amp output.
What I'd like help with
- what might I have done to burn up the
- (it's rated +/- 25v input)
- how do I bias the thermocouple's 0v starting point?
- what happens if I don't? (it's not like the first 10˚C matter anyway)
- is battery "ground" compatible with the thermocouple "ground"?
- (they're both relative to themselves rather than ground ground)
I'm not ready to give up and use more specialized components like the MAX31855, LT1025 and LTC1049. I want to bang my head against this with "basic" components a little longer.
What else does a noob like me need to know to get on the right path? How might you go about this? What might I be missing?
P.S. Some related posts