PinkDuckJul 7, 2023

What's a simplest way to get -5v from +/-12v?

Hi! I'm working on an arduino project that has a -12v/0v/+12 psu. +12 is fed directly into arduinos voltage regulator, so there's also a +5v line available. I need to convert a -5v+5v signal to 0v+5v to feed it into arduinos analogue input and I've already mocked up a schematic in circuitjs based on an opamp that seems to do...

https://kbin.social/m/askelectronics@discuss.tchncs.de/t/154276

What's a simplest way to get -5v from +/-12v? - askelectronics - kbin.social

Hi! I'm working on an arduino project that has a -12v/0v/+12 psu. +12 is fed directly into arduinos voltage regulator, so there's also a +5v line available. I need to convert a -5v+5v signal to 0v+5v to feed it into arduinos analogue input and I've already mocked up a schematic in circuitjs based on an opamp that seems to do...

Show threadgrasib
Supply the OpAmp with +/-12V and use a Summing OpAmp Circuit to get your 0-5V signal. V1 is 5V, V2 your input signal and Rf/R1 = Rf/R2 = 1
Jul 7, 2023 at 8:45pmWeb
Show threadPinkDuckJul 7, 2023
thanks! gonna give it a try tomorrow. I'm pretty new to this, and never used opAmps in non-inverting configuration, so I missed this option
Show threadgrasibJul 8, 2023
Exiting! It has been some time for me too, let me know how you solved the problem at the end.
Show threadPinkDuckJul 8, 2023
it worked! (at least in the sim)
thanks a lot!
I'm not sure I got all the math right, since I get 0 - 5.05V on the output, but it's not a deal breaker for me.
I attached the schematics, but not sure that images in comments federate from kbin to lemmy, so if you don't see it, try opening an original link to this comment.
Show threadPinkDuckJul 8, 2023
ah, a little bit of thinking helped me realize that I don't need 1k in inverting loop and then I'll get 5v instead of 5.05v :) and 100k can also be replaced by 1k
Show threadgrasibJul 8, 2023

So what should happen if you replace the 1k in the inverting loop by 0 Ohm is, you should get a square wave.

Is that correct?

Show threadPinkDuckJul 9, 2023
No, with 0 Ohm I get a perfect 0+5v sine, just as I need. It's the other way round, it'll amplify the signal and clip off the top at about + supply voltage when you increase the resistance in the feedback loop, but it's not exactly square wave, as the bottom part of sine will remain intact. I use clipping schottky diodes to protect the arduino from voltage outside 0+5v range, this way if you increase input signal to say 20v peak to peak, you'll get more square-ish wave in 0+5v (I've attached the schematics)
Show threadgrasibJul 9, 2023

You’re right of course. Two more questions if you don’t mind:

  • what’s the 1k || 100k doing, and why add a 1k in series?

  • if you don’t want more than 5V at the output of the OpAmp, wouldn’t it be possible to just supply the OpAmp with 5V instead of adding a shottky diode which has a forward voltage of around 0.6V? As I understand the screenshot the voltage is 5.4V at that exact point.

    • Show threadPinkDuckJul 10, 2023
  • it does nothing 😅 I just forgot to remove it from the sim
  • you mean like power it with 0+5v? I think it won't work with negative input voltages then, right? and I'm using tl074 and I'm not sure that 0+5v is enough juice for it, and I use remaining 3 opamps for other stuff, so it's not really an option for me. the sim didn't have shottky diodes, so this measurements are not 100% accurate, but I used this approach to protect arduino inputs in another project, and it worked well. And yeah, I cranked up the input signal to +/-10v just to check the clipping, it'll normally stay within +/-5v

    • I already ordered PCB for the prototype. I hope it'll work fine. Thanks a lot for your help :)