[Solved] Differential OP-Amp unexpected output voltage

[SevenSins]

Circuit problem with differential op-amp

Hello,

I'm working on a high power balancer for Li-Ion batteries.
It's designed for 7s systems around 24V.

I'd like to convert the individual cell voltage with a differential amplifier (TL074) to 0-5V signal (with Gain of 1):

1st cell 4.2V   -> differntal amplifier: 4.2V-0.0V = 4.2V to the arduino
2nd cell 8.4V   -> differntal amplifier: 8.4V-4.2V = 4.2V to the arduino
3rd cell 12.6V   -> differntal amplifier: 12.6V-8.4V = 4.2V to the arduino
4th cell 16.8V   -> differntal amplifier: 16.8V-12.6V = 4.2V to the arduino
...
And so on

I used 0.1% 33k resistors (not the 75k like in the shematic - but since all the resistors are roughly the same for my intended gain of 1 it shouldn't matter).
The TL074 has a supply voltage of 5V+ and GND, it's not a rail to rail amp so the output should be within 0.xx Volt and 4.xx Volt.


I build the circuit but it doesn't work like I thought it would.
The Batteries where charged up to around 4.2V and I measured this Values:

1st cell 3.5V
2nd cell 4.2V
3rd cell 5.5V
4th cell 5.5V
5th cell 5.5V
6th cell 5.5V
7th cell 5.5V

Schematic:

I have no clue what I did wrong.

[glarsson]

The op amplifier does not like input voltages outside a supported range. This range is most likely well inside the power supply voltage.

[StillTrying]

With up to 12V on the TL074's inputs they'll need a supply of 14+V, using the 24V supply might work.

[duak]

SevenSins, I see some problems:

1.) as glarsson points out, IC1 & IC2 require higher power supply voltages to operate correctly.  V+ should be +24 V and V- should be - 5V.  If you use a single supply opamp, you will not need the -5V supply.

2.) I think there should be current limiting resistors in series with the drain connections for Q1 - Q7 otherwise the discharge current will be much too high.

Cheers,

[SevenSins]

It works now.
I put both of the OP Amps on 24V supply Voltage und voilà it works on all 7 Channels.

2.) I think there should be current limiting resistors in series with the drain connections for Q1 - Q7 otherwise the discharge current will be much too high.

The Resistors go into the Screw terminals because they'll have around 10 Watts each-maybe more.

I also found another error in my schematic. The unused Amp IC2D is wrong (It got really hot next to those pins)

Code: [Select]

     +------+
     |  VCC |
     |   +  |
     | |\|  |
     +-|-\  |
       |  >-+
     +-|+/
     | |/|
    ===  -
    GND VCC

That's the how it is supposed to be.

Thanks for the quick help.

Can I buy you guys a beer?
I would've never guessed the reason why it didn't work was the supply voltage.

[duak]

Ja, ein weiss bier bitte!

About 20 years ago I went to Heidelberg to work with Heidelberg to help solve an EMI problem.  Marvelous people, excellent facilities and super results.  I took my wife and we had a great time.  I still remember how to order beer.

Tschüss!