WHY are you using TL084's? They are noisier versions of the TL074's, which I told you before were not good to use in your application.

They do not have rail-to-rail inputs and outputs, and use more power than the other opamps I suggested.

Yes, I knw SgtWookie, but I had a made a working circuit with TL084,
so, I didn't want to disturb that...

Also, previous to this circuit, I had another prototype which had 21 TL084 and it ran on 1A power supply...

Now, I have hardly 9 TL084, so, I was thinking that I could run with the NMH chip power supply....

 

Try disconnecting a different set and reconnecting the set that won't work.

If the set that won't work now works, you have a power supply loading problem.

Seems to me that you were marginal to begin with.

Yes... will try that out...

 

Check the datasheet for the NMH DC-DC converter again.

It's input current is 494mA, and the output is 111mA for each rail.

You are overloading the NMH DC-DC converter. You have exceeded your power budget.

You will either have to spend more for another DC-DC converter, or figure out a way to use less current without disturbing the functionality of the circuit.

You will find that if you look in the datasheet, you are using up almost your entire power budget in the quiescent current of the nine TL084 opamps. Quiescent current is max 2.8mA per amplifier. 2.8mA x 4 amps/package x 9 amplifiers = 100.8 mA.

Had you use the LM6134 quad opamp like I first suggested in post #59, your quiescent current/amplifier would have been 450uA, for a total of 16.2mA.

The MC33174 has a max quiescent current of 300uA/amplifier.
The MCP6024 has a max quiescent current of 1.35mA/amplifier.

Had you used ANY of the opamps I had suggested, you would not be in the mess that you have created.

Were I your professor, I would give you a failing grade. You obviously need to repeat the course, as you will not be able to do a good job for an employer, even if your fellow employees hold your hand every step of the way.

I gave you good advice, and you failed to follow it. You did not keep track of your power budget, and worse than that, you did not know that you had exceeded it, nor what to do about it.

How do you think an employer is going to react to such foolishness?

[eta]
You may think that I am being unkind. If you think that, you are mistaken.

An employer would hand you a pink slip while you were being escorted to the door; and you may not even be told what you did wrong.

I am telling you exactly where you went wrong. You should pay attention.

 

If this kind of brutal honesty is a blow to the ego, take a few minutes and read through the thread again. At every point you start to go wrong, you are not only put on a proper course, but you are explained why it was needed.

Having ignored warnings this time only leads to a poorly designed, under powered, non working product. In the future, with higher voltages and more complex systems, ignoring warnings can have much worse consequences.

In the begining phases of designing any circuit, the datasheets will be key to planning on power requirements. SgtWookie didn't build your circuit to find the problem..he didnt have to. All he needed was 5th grade multiplication skills and the datasheet.

This will make you better. You most likely wont repeat the same mistakes.

 

@SgtWookie and Retched, thanks a lot for the advice... esp SgtWookie, I would
never be hurt with the reactions you have shown... In fact I would like to thank you for telling me such directions that I would probably have never thought of...

And yea, by the way, the circuit never got loaded, the last column of the aluminum foil sensor was getting short to the ground plane I have connected, hence it did not show up on the amplifier output...

However, I would like to thank you for opening my eyes....
I appreciate it...

 

Hello All,
I know this isn't the write place to ask, but i have already posted in other place and didn't get a response...

I have a telephonic interview coming Thursday for the position of Entry Level Electrical Engineer... I was curious as to what i should brush up...

What would you have done if you were in my position...


Note: @SgtWookie, I replaced the TL084 with MCP604, everything works great now and I still have a buffer of 50mA in case the circuit pulls some extra current from unknown sources... thank you indeed...

 

Glad it's working better - too bad you didn't start off with them, as it would have made your project much less frustrating.

Regarding your interview Q's - that's a completely different subject from this thread.

If people what to respond to that, they should do so in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=38488