[Solved] Differential OP-Amp unexpected output voltage - Page 1
Forum section: electronics / projects

author: SevenSins
author_rank: Newbie
timestamp: January 07, 2019, 05:08:42 pm
content: Circuit problem with differential op-ampHello,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 arduino2nd cell 8.4V -> differntal amplifier: 8.4V-4.2V = 4.2V to the arduino3rd cell 12.6V -> differntal amplifier: 12.6V-8.4V = 4.2V to the arduino4th cell 16.8V -> differntal amplifier: 16.8V-12.6V = 4.2V to the arduino...And so onI 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 3.5V2nd cell 4.2V3rd cell 5.5V4th cell 5.5V5th cell 5.5V6th cell 5.5V7th cell 5.5VSchematic:I have no clue what I did wrong.

author: glarsson
author_rank: Frequent Contributor
timestamp: January 07, 2019, 05:25:08 pm
content: The op amplifier does not like input voltages outside a supported range. This range is most likely well inside the power supply voltage.

author: StillTrying
author_rank: Super Contributor
timestamp: January 07, 2019, 05:32:42 pm
content: With up to 12V on the TL074's inputs they'll need a supply of 14+V, using the 24V supplymightwork.

author: duak
author_rank: Super Contributor
timestamp: January 07, 2019, 05:38:35 pm
content: 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,

author: SevenSins
author_rank: Newbie
timestamp: January 07, 2019, 08:23:07 pm
content: It works now.I put both of the OP Amps on 24V supply Voltage und voilà it works on all 7 Channels.Quote from: duak on January 07, 2019, 05:38:35 pmThe 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 VCCThat'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.

author: duak
author_rank: Super Contributor
timestamp: January 08, 2019, 01:32:20 am
content: 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!