0-90ohm input to pic, 25ma limit

Discussion in 'General Electronics Chat' started by Dragn Wagn, Jul 25, 2008.

  1. Dragn Wagn

    Thread Starter New Member

    Jul 25, 2008
    2
    0
    Hi. I'm working on my first pic automotive project and need to get a 0-90 ohm fuel sender to a 0-5v input. My thoughts were to use a 7805 regulator and resistor voltage divider to get the required voltage but I am stuck on the 25ma max input limit. Can anyone suggest an alternate arrangement that would meet both criteria? perhaps a noninverting op amp? I appreciate any advice!

    john
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    One solution could be a constant current source.
    The voltage over the sensor will be (the resistance of the sensor) X (current from the source).
    Example : when the current source is 10 mA, your range will be 0 - 0.9 Volt.
    Put this trough an amplifier that mutiplies it by 5 and your range is 0 - 4.5 Volts.

    Greetings,
    Bertus
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Nice idea, Bertus :)

    Tried it in a simulator, seems to work pretty well.

    The LT1006A I used is rated for inputs down to ground, but the offset gets skewed a bit when the input is down at zero; output is off by 50mV. Otherwise, it's good. You couldn't use a common opamp like a 741 or TL081 or LF353 without using a negative supply.

    I decided the current limiter needed a voltage clamp on it as well; you wouldn't want 13 or so volts making arcs inside the fuel tank :eek:

    I used a couple of diodes in series to clamp the sender signal level at around 1.4v. Little chance of arcing if the level sender is dirty or intermittent.
     
  4. Dragn Wagn

    Thread Starter New Member

    Jul 25, 2008
    2
    0
    Thanks guys. I have the parts on order and will let you know how they work once I get them assembled.
     
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