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Voltage limiting?

  1. Dec 6, 2009 #1
    Hi all,

    i found this forum while spending ages searching the net looking at limiting voltages, and it seems a good place to ask for advice.

    here is the problem i have,

    my car when it was normally aspirated it has a sensor that sensed engine load using a map sensor using 0-14.5psi, and then compared the throttle position sensor to work out timing and fuelling.

    basic 3 stages of the map sensor are roughly as follows but it is linear:
    0-5spi is engine braking 0-1.6v (shuts off or reduces fuelling due to engine braking/slowing down)
    5-10psi is normal engine load 1.6-3.32v (2.25-2.5v is tick over voltage when not driving)
    10-14.5 is heavy engine load 3.32-5v (full throttle / up hill or heavy weight in car)

    now i've turbocharged my car the standard sensor gets the wrong reading for when i'm driving in the normal range. say 0 boost (14.5psi) at 70mph so the engine is thinking it has a heavy load and increases fuelling and timing.

    i bought a 2bar map that spreads the 5v over 29psi but the voltage the engine looks for at tickover is too low (about 1v) and the ecu thinks i'm engine braking and shuts fuel off :rolleyes:

    so i did a line chart to compare different map sensors that use 5v and 12v


    i have a 3bar boost gauge on my car that tells me 0-43.5 psi working with an output from 0v-12v respective of pressure, i only use 10psi of boost (22psi) so the max voltage output is around 6v

    i need to take the 0-12v feed from this sensor and limit its output when it gets to 5v

    any one got any idea's

  2. jcsd
  3. Dec 6, 2009 #2
    What is the current level running in this feed?

    Here is a simple voltage divider using two resistors, but it works well if the current is low.
    Last edited by a moderator: May 8, 2017
  4. Dec 6, 2009 #3
    divider circuit wont work as the sensor output is only going up to 6v, plus if the pressure sensor is giving out 1v i need 1v into the ecu not .5v

    the current is very low as its only to a gauge that takes the reading and lights up cosponsoring LED's its self, the gauge has its own 12v supply to do this.

    i need a 5 volt regulator that will pass the input voltage from 0-5 through it but stop the output voltage from going above 5v, when the sensor goes higher, if that makes sense

  5. Dec 6, 2009 #4
    have you tried to clamp it with a zener?
  6. Dec 6, 2009 #5
    no as wasn't sure how and as i still need the full range of 0-7v to the gauge, using a zener would also limit the gauges reading and i think a resistor is required in series with the source which will also drop the voltage getting clamped, correct me if im wrong. http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm" [Broken]

    unless i split the output through two diodes to keep thing separate but then a drop of at least 0.6v will occur from what i'v read.

    could try this and see what readings i get,


    Attached Files:

    Last edited by a moderator: May 4, 2017
  7. Dec 6, 2009 #6
    well, if all you want is to limit voltage, you might try running a rail-to-rail op-amp as a follower, but only provide power up to the voltage you want to clamp at.
  8. Dec 6, 2009 #7


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    Science Advisor

    In the following diagram:

    voltage limiting.PNG

    the transistor will conduct and pull the relay in if the input is less than 5 volts.
    While this is the case, the input is connected directly to the output.

    When the voltage is greater than 5 volts, the relay drops out, and an alternative 5 volts is connected to the output.

    I haven't built it, of course, so it may not work or it may need some adjustment.

    The relay could be a quite small 5 volt relay capable of handling the small current required.
    Diodes and resistors are just a few cents each.
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