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I have a MAP(manifold absolute pressure) sensor, which has 3 pins: 5V, GND, OUTPUT. It acts like potentiometer and output's voltage varies between 0(0 bar pressure) and 5 volts(2.5 bar pressure). I would like to make the simplest possible circuit to limit the output voltage from the sensor to 4.5 volts, instead of 5v:

  • INPUT - OUTPUT
  • 0.5v - 0.5v
  • 3v - 3v
  • 4.5v - 4.5v
  • 5v - 4.5v

What circuit should I be looking into?

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    \$\begingroup\$ Zener and resistor? \$\endgroup\$ – Christian Oct 5 '17 at 11:40
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    \$\begingroup\$ Why do you need to do this in the first place? The best approach is probably dealing with this at a higher level. \$\endgroup\$ – Olin Lathrop Oct 5 '17 at 12:19
  • \$\begingroup\$ Background information needed. What is the signal controlling? Is it fed into a microcontroller (why not have the µC handle these values?)? Does it control an actuator? Do you actuall need to lineary translate the voltage from 0-5V range to 0-4.5V? Be aware of the X/Y problem. Downvote until clarified. \$\endgroup\$ – try-catch-finally Oct 6 '17 at 15:51
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According to your description, the output can't exceed the supply voltage since the sensor is just a resistor divider. Therefore, supply it with 4.5 V instead of 5 V, and the output won't be able to exceed 4.5 V.

You didn't supply a link to the sensor datasheet, so I'll leave it to you to check that it can operate on 4.5 V. If it is something that varies resistance internally, then it probably can.

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Use a diode voltage clamp like this:

enter image description here

Due to the diode forward voltage drop the output voltage stays below 4.6V which is more or less what you want. Use a 4volt supply voltage. You can make the reference 4V using a simple voltage divider.

Also I'm not sure why you're going to do this. The output voltage depends on the amount of the pressure it receives. It can be 0 bars or 2.5 bars. So that doesn't make sense to limit the output at 4.5V. Simply try to avoid measuring pressures greater than 2 bars to stay within the safe limit.

Since you haven't provided us a datasheet it's not possible to say if it works with a 4.5V input voltage. Read the datasheet and see what it says.

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  • \$\begingroup\$ I believe the reason to do this is to prevent a car's ECU from detecting an "overboost" condition and panicking. \$\endgroup\$ – Frosty Oct 5 '17 at 17:21
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Perhaps oversimplifying your problem, but can you do something like this?

schematic

simulate this circuit – Schematic created using CircuitLab

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If a bit more output impedance doesn't matter, you can do it with just two resistors.

For instance a 1KΩ in series with the output and a 9KΩ to ground (after the 1KΩ) will rescale your output from 0 - 5V to 0 - 4.5V.

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  • \$\begingroup\$ This does not do what the asker wanted \$\endgroup\$ – Christian Oct 5 '17 at 17:04

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