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| [Solved] Differential OP-Amp unexpected output voltage |
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| 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: --- End quote --- 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 voilĂ it works on all 7 Channels. --- Quote from: duak on January 07, 2019, 05:38:35 pm ---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. --- End quote --- 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: --- +------+ | VCC | | + | | |\| | +-|-\ | | >-+ +-|+/ | |/| === - GND VCC --- End code --- 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. |
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