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### tricky dc voltage problem

Posted: Mon Jun 17, 2002 1:30 am
I am after a circuit or some kind of IC which takes a low current 5V input and coverts it to 0V, the tricky part is I need the coversion done in real time and the coversion has to work for the entire 0-5V range, i.e. if the input voltage is 4.5 volts then the output is .5 if it's 4V then the output is 1V. So as the input voltage decreases the output voltage increases and vice versa.<p>Your help with this problem is most appreciated.

### Re: tricky dc voltage problem

Posted: Mon Jun 17, 2002 5:09 am
An Op-Amp could do this using the inverting input. Or were you thinking of using an ADC chip or uP?<p>bodgy

### Re: tricky dc voltage problem

Posted: Mon Jun 17, 2002 12:47 pm
No. An op-amp wouldn't work because of clipping.
A very stable voltage reference for the
output voltage would be a reverse-biased Zener
diode. You could then use a potentiometer (as
a voltage diver) to reduce it to the desired
voltage.

### Re: tricky dc voltage problem

Posted: Tue Jun 18, 2002 1:34 am

### Re: tricky dc voltage problem

Posted: Tue Jun 18, 2002 7:20 am
Go to http://www.poptronics.com/forums/electronic_bench/ and see the post entitled "Invert and Level Shift". A simplified schematic is posted there.<p>Ron H

### Re: tricky dc voltage problem

Posted: Tue Jun 18, 2002 5:25 pm
Thank alot RonH, I have posted some questions about the schematic on the poptronics forum, I hope you get time to answer them.<p>Your help has been greatly appreciated, and if the proposed circuit works, I might show my appreciation in a more tangible manner.<p>Thanks Heaps.

### Re: tricky dc voltage problem

Posted: Wed Jun 19, 2002 6:11 am
Nearly that exact application came up as a question in the "Q & A" column in Poptronics magazine, June 2001, page 44. The guy was wanting to interface a MAP sensor and needed the phase inversion. We used an Analog Devices AD8541 op amp, which is a rail-to-rail device. A general purpose op amp such as a 741 just won't work because it'll go nonlinear near the end points and will clip too early. Ron's schematic is similar. I'll post the skem on the same thread where Ron posted his. They're similar except that I've included the reference circuitry for the level shifting.<p>Dean

### Re: tricky dc voltage problem

Posted: Wed Jun 19, 2002 4:11 pm
What you describe is exactly what I am trying to do.<p>Thanks heaps for the info.