Zener TVS

Discussion in 'The Projects Forum' started by Schniz2, Sep 10, 2011.

  1. Schniz2

    Thread Starter New Member

    Apr 15, 2010
    I need to place a ~5V limit on the output of a current sensor which could potentially spike to 8.3V for a short period of time (~18msec) (manufacturer part number ACS712ELCTR-05B-T).

    As far as i understand, i just connect the cathode to the positive voltage and the anode to ground...

    I need help selecting the zener diode, i was looking at the following "1N5337BG" (characteristics in attached image zenerdata).
    It says the Zener impedance Z_ZT @ reverse current I_ZT is 2.Ohms. Is this the correct reasoning?>>> if there was a 8.3V voltage spike, and the current was diverted through the zener it will see an impedance of 2.Ohms (or will it see a lot less than this?)... , so the zener will need to dissipate P=V^2/R = (8.3^2)/2 = 34.4Watts? It says the maximum steady state power dissipation is 5W and the surge current rating is 15.3A (current would be 8.3/2 = 4.15A)

    The main thing i am worried about is what impedance the current will see at voltages greater than the zener voltage??
  2. vrainom

    Active Member

    Sep 8, 2011
    The surge current rating is for a non repetitive short pulse (8ms) so you should not rely on that for normal duty. The safest would be to put a limiting resistor from the sensor to the zener as the output to mantain it within its electrical limits.

    For a maximum input of 8.3v a resistance as low as 3.9ohms would do: (Vin max - Vz min) / Izm = R (8.3 - 4.7 ) / .930 =~3.9 ohm

    However if you only need to read the sensor's voltage drop as a signal this is way overkill.
    Last edited: Sep 11, 2011
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  3. Schniz2

    Thread Starter New Member

    Apr 15, 2010
    Is this what you mean? - see attachment (note there will be a ground reference at the bottom.. i forgot to put it in)
  4. bertus


    Apr 5, 2008

    You still will have the full voltage on the measurement part.

    Here is a "corrected" corcuit:


    Now the voltage on the measurement part will not go higher as the zener voltage.
    The 3.9 Ohms resistor may be larger, as the microcontroller will not take much current.

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