Why my Op-Amp is freezed? :)

Thread Starter

booboo

Joined Apr 25, 2015
168
Thanks for reply.
Half cycle = half period, as you stated :-
I didn't state it. it was quoted from a guy in this link that his forum name is 'sparkybg'. :

http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=5264#p50627

Can you post the schematic of that part of your circuit?
I don't use the 24V AC. mine is 24v DC. my project is based on this project as I said already:

http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=5264&start=180#p58264

Are you sure you have the + and - inputs the right way round?
My Op-Amp is MCP6V07T-E/SN. you can see its datasheet here:

http://html.alldatasheet.com/html-pdf/306080/MICROCHIP/MCP6V07-ESLASHSN/495/1/MCP6V07-ESLASHSN.html

Thus, due to the datasheet, the pinout is this:



I doubt that there would be any problem in the circuit that I wired up.

With respect to what are you measuring GND? Normally GND is the reference point.
Ok, let me show you an image of the circuit:



In this circuit from right to left you can see the 24v Dc switching power supply and then buck module to make around 5v Dc to supply circuit(everything save soldering iron). and also for MCU and Op-Amp I'm using two LDO (one for MCU and another for Op-Amp). everything in this circuit is working very well save Op-Amp. the 24v Dc switching power supply has two +V and two -V and one earth (and one L and one N for 220AC as input). now look at this image:



I connect the 'COM' probe of my DMM to number 1 (as GND). when I connect the red probe on my DMM to this point, it shows me 0v. when I connect it to the number 2 point, it shows me 0v again. when I connect it to num' 3, it shows me 40mv. for num' 4 and 5, it shows me -20mv.o_O
 

Alec_t

Joined Sep 17, 2013
14,280
Input pins 5 and 6 of your IC seem to be floating. That could upset its function.
Your voltage variations along the GND rail may well be due to interference pickup and voltage drops arising from the use of a breadboard and straggly wiring.
 

Thread Starter

booboo

Joined Apr 25, 2015
168
Input pins 5 and 6 of your IC seem to be floating. That could upset its function.
Grounded them but still nothing.
Your voltage variations along the GND rail may well be due to interference pickup and voltage drops arising from the use of a breadboard and straggly wiring.
I have checked the resistance of the aligator cables and some of them have a resistance around 1.5ohm and other have around 0.7ohm. I have changed the aligator cable that is connected between point 2 and 3 (it's a red aligator cable as you can see in the image) and now the output is 20mv at the output of Op-Amp.

I think the main problem is aligator cables. What do you think?

Thanks a lot for reply.
 

Thread Starter

booboo

Joined Apr 25, 2015
168
Oh! after a big change in ground, the output of the the Op-Amp is 0.3mv !!
I think the problem is in supply.
Now the + input of Op-Amp is around -6mv (why?) and the -input is -0v(as my DMM show me) and the output of the Op-Amp and the circuit is 0.3mv.

What's your opinion about the supply?(my 24v DC switching supply) maybe it's better to change it to a simple transformer-based supply!?
 

Thread Starter

booboo

Joined Apr 25, 2015
168
The thermocouple polarity may be wrong. Try swapping over the two wires going to the iron's heating element.
Nope! I changed it and as I expected nothing!

please note that When I say the -/+input of the Op-Amp I mean the inputs of the Op-Amp not the input of the circuit.

Thanks for reply
 

Thread Starter

booboo

Joined Apr 25, 2015
168
Could a Differential Amp or Instrumentation Amp work better for a high gain instead of non-inverting Amp?
You know my current circuit to read the output of the thermostate(thermocouple+heater) is this:


But a guy said me that probably you have a problem in ground plane. I don't know why but I feel he was right then I'm going to change this Amp circuit to a Differential Amp or Instrumentation Amp. I want to make something like this:



What's your opinion about it? also for first step of this amp I want to use this:


I don't know How to ground -HEATER. let me to explain it to you what's exactly my problem. with the above circuit I can measure the output of the thermostate. ok, so far so good. but how to turn the thermostate(heater in series with thermocouple) on? because when I apply 24V on +HEATER line, I have to ground it in -HEATER side. but I cannot do it because if I ground the -HEATER, I cannot read the output of thermocouple when thermostate is off (if I would know what's the tempereture of the heater(read it), I have to turn it off).
Can I put a couple 1N4148 for -HEATER after R1? (like as I have done for the +HEATER)
 

Thread Starter

booboo

Joined Apr 25, 2015
168
Thanks guys, please ignore the above post.
In that case logic suggests you have either a wiring error on your breadboard, or a dead opamp, or a faulty thermocouple in the iron.
Thanks buddy:)
I tested the Op-Amp and found out that it has gotten hurt. now my current problem is that the output doesn't give me any amount. I measured the input of the circuit and it was exactly the amount that I expected to get but it's negative. :( why? I changed +HEATER to -HEATER and it got worse( much more negative than earlier). this is the circuit:

 

Alec_t

Joined Sep 17, 2013
14,280
I measured the input of the circuit
Which point in the circuit shown is the 'input'?
Was the measurement done relative to GND?
Where is -HEATER now connected?
Are you switching +HEATER or -HEATER?
What is the specified maximum input offset voltage of your opamp?
 

Thread Starter

booboo

Joined Apr 25, 2015
168
Which point in the circuit shown is the 'input'?
HEATER+
Was the measurement done relative to GND?
Yes. and an interesting point about this circuit is that when I apply heat to the tip, it goes up. for example from -7mv to -2mv and when it gets cool, it goes down for example from -2mv to -7mv. but in fact it should give me a range between 0-10mv not a negative range.
Where is -HEATER now connected?
GND
Are you switching +HEATER or -HEATER?
No
What is the specified maximum input offset voltage of your opamp?
 

Alec_t

Joined Sep 17, 2013
14,280
I think your negative reading may be due to thermocouple cold junction effects.
Consider this:
Thermocouple.gif
You have one hot junction AB (within the soldering iron tip), consisting of two different metals A,B, neither of which is copper. Your connections to the soldering iron are via copper wire C (and also via meter probes which probably have plated tips of yet another metal). So you have at least two thermocouple cold junctions AC, BC. If the temperatures of the AC and BC (and any other) junctions differ then an unwanted voltage is added to the wanted thermocouple voltage. The voltage you measure is the sum of all the voltages produced because of the various junctions.
You can add a small positive offset voltage to make your readings positive.
 
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