0-5v voltage clamp and pass thru?

Discussion in 'General Electronics Chat' started by elRey, Jul 6, 2009.

  1. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    0
    0-5v voltage clamp and pass thru?

    DC regulated 5v.

    Hello, I need a simple circuit that takes a 0-5v input from a sensor and has 2 outputs.
    1) a straight 0-5v pass thru (same as input). I could just use input
    2) an adjustable clamp with range 3v to full 5v (no clamping).

    I'm currently using a zener diode, but have no way to get the non-clamped voltage.

    Thanks,
    Rey
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The secret is to use a "variable" zener where one can set what voltage the zener diode become active(conducting), via a variable resistor or POT.

    The part name is TL431. The attached image shows how to use it. Vref is always 2.5V.

    [​IMG]
     
    Last edited: Jul 7, 2009
  3. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    Thanks for the reply.

    So, with above circuit can I also get the true input voltage along with the clamped voltage?
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    As you have said, you can get the true input voltage using the input as is.

    The above circuit only provide a variable level clamped output.
     
  5. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    Great! Another quick question. With the zener diode I'm using now, I have no resistor between input source (sensor) and diode. And any reading I take between the sensor and diode is clamped. If I add a resistor between sensor and diode, can I get a true (non-clamped) reading before the resistor and a clamped reading after the resistor?

    This of course lacks adjustability.

    Thanks,
    Rey
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I would say you probably can.

    Here is a quick simulation. Note that the clamp voltage is lower than the zener rated voltage because the zener current is small.

    [​IMG]
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You don't mention the impedance of your sensor.

    Placing a voltage clamp across the sensor's output without limiting the current may damage it.

    You may need a buffer circuit between the sensor and the clamping circuit to prevent the sensor's signal from being overloaded.
     
  8. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    I greatly appreciate the help!!!!
    Sgt, I and many others have been running 4.3v and 4.7 zener diodes and no resistor on our MAP sensors for a while now without killing the MAP. Is that any indication of the impedance in relation to the sitiuation you mention?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    As far as I know, the power for the MAP comes from the ECU.

    If you're clamping the output voltage to a certain level, you might risk engine damage when running near WOT (wide open throttle).

    When at lower throttle settings (below perhaps 70%), the ECU will attempt to maintain a stochiometric (14.7/1) air/fuel mix by looking at the O2 sensor output.

    But since I'm not sure what you're trying to do, go ahead and 'splain it to us.
     
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