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Requesting some input on this topic which has already been discussed (partially for my requirement) here: Negative to Positive Voltage Converter

I also have a positive-only display and I need to measure up to -32v DC. I need to convert the minus voltage to the positive equivalent in a simple manner and I am currently thinking of doing this in TWO op-amp stages as illustrated.

Questions: 1. Is there a more elegant & simple way to do this? Using only one op amp perhaps? The limit on +/- supply is a limiting factor hence my 2-stage design. 2. Does anyone see a problem doing it this way? This is a basic block diagram only.

Thanks for your input. Steve enter image description here

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  • \$\begingroup\$ Exactly what is your 'positive-only display', and what is producing the -32V? \$\endgroup\$ – Bruce Abbott Mar 7 '18 at 0:55
  • \$\begingroup\$ Basic block diagram or not - Take the first learning and drop the idea of trying to use an ancient relic of an 741 opamp. They are basic garbage compared to most modern types. \$\endgroup\$ – Michael Karas Mar 7 '18 at 1:23
  • \$\begingroup\$ Fair comment Michael. I will in fact use 2 x LM358s if I go with this current idea. \$\endgroup\$ – Steve Peck Mar 8 '18 at 1:33
  • \$\begingroup\$ Bruce, a link to the meter is here ebay.com.au/itm/…. I'm using 2 of them: one each for the pos & neg rails. The 32 v rails derive from this LM317 PSU: ebay.com.au/itm/… \$\endgroup\$ – Steve Peck Mar 8 '18 at 1:44
  • \$\begingroup\$ One more thing Bruce, these meters have the peculiarity of measuring current in the return 0v rail so I can't just "switch wires around" which would be oh so easy. And they need (+ve) power, too. \$\endgroup\$ – Steve Peck Mar 8 '18 at 2:37
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If your input ranges from say 0 volts down to -32 volts then you can use a simple inverting op-amp configuration: -

enter image description here

Consider the op-amp chosen will work well with -in and +in close to 0 volts. This is not unusual with rail-to-rail op-amps. Next, as a simple mind-experiment imagine that R1 is 32 x R2. So if Vin is -32 volts then Vout has to be 1 volt to drive -in to 0 volts. I use the word "drive" because this is what happens in an op-amp with negative feedback - the output drives the -in input to match the voltage at +in.

No negative voltages are required to be produced by this op-amp configuration. So, if Vcc is (say) 20 volts, you could make R1 twice as big as R2 and this would convert -32 volts at Vin to +16 volts at Vout. -16 volts at Vin would produce +8 volts at Vout.

There are a few op-amps that work very close to 0 volts but, as always, to widen your scope of choice, it might be better to make a small negative rail (-3 volts is all that is needed) to power the op-amp's negative supply pin: -

enter image description here

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  • \$\begingroup\$ That's very interesting Andy. Perhaps I was wrong in thinking that the rail supply voltage needs to be in excess of the + & - voltages on both the input and output as seems to be the case you've proposed - no? Hence thought it impractical to use (an expensive) opamp with +/- 35v supplies. \$\endgroup\$ – Steve Peck Mar 8 '18 at 1:57
  • \$\begingroup\$ I think I get what you're saying now: the actual voltage on the neg VIN pin will only ever be say 0v or a +ve voltage hence a neg supply rail to it isn't actually needed or as you suggest apply say a tiny neg supply "just in case". Right? \$\endgroup\$ – Steve Peck Mar 8 '18 at 2:02
  • \$\begingroup\$ One more thing Andy, must not lose sight that although your idea deals with converting -32v in to a +ve out, it's not achieving the exact equivalent +ve voltage out for the digital display to actually measure. \$\endgroup\$ – Steve Peck Mar 8 '18 at 2:16
  • \$\begingroup\$ @StevePeck oh, how do you work that out? What mapping of a negative voltage to a positive voltage do you need that cannot be accomplished by the right values of resistors R1 and R2? Maybe you should provide a link to your display or explain that a little more. \$\endgroup\$ – Andy aka Mar 8 '18 at 9:48
  • \$\begingroup\$ Hey Andy, put simply: I need a positive equivalent of the negative voltage for the display to measure. 100mv tolerance acceptable. How to do this without needing +/- 32v power to the opamp...in 1 stage? LM358 is my preferred choice, ideally. \$\endgroup\$ – Steve Peck Mar 8 '18 at 20:11

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