Transient voltage suppressor

Discussion in 'The Projects Forum' started by RodneyB, Jan 31, 2015.

  1. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    680
    15
    I have been using the k7812 to run a 12 Volt tracking unit from 24 Volts. Out of 5 units installed 4 failed in the first week.

    When I contacted my component supplier they said that it was a possible transient and supplied a drawing (attached) and the components.

    having built and connected the unit the resistor gets really hot . VERY quickly. the unit is drawing 150mA.

    Some advice will be appreciated
     
  2. #12

    Expert

    Nov 30, 2010
    17,840
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    You are dissipating 3/4 of a watt in a 1 watt resistor. That's why we commonly use oversized resistors. The manufacturers of the resistors tell us what power the resistor can survive, but make them so small that they unsolder themselves. :mad:
     
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  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    3,234
    565
    hi Rodney,
    Ref the 33R resistor passing a current of 0.15A Watts = I^2 * R,, thats (0.15 *0.15) * 33, giving a figure of 0.75Watt, so it will get hot.
    E
    See that #12, beat me too it while compiling the post.:(
     
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  4. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    680
    15
    Thanks very much for the help and advice. I will change the resistor and try again. The regulator as rated at 1 Amp. Ideally the resistor should be able to handle this. However a 33Watt Resistor is just not Practical.
     
  5. #12

    Expert

    Nov 30, 2010
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    You don't have to rate the resistor for the regulator spec. You can rate it for the known load.
     
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  6. crutschow

    Expert

    Mar 14, 2008
    16,209
    4,333
    So a 2W resistor should be adequate for your load (or two 1W 16 ohm resistors in series).
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Wait a minute. That is a switcher with ~90% efficiency. If the load is 12v*0.15A, that is 1.8W. The power input is ~1.8/0.9 = 2W, so the input current is only 2/24 = 83mA. The power in the 33Ω resistor is 33* 0.083^2 =230mW.

    On the Transient suppressor, the max input on the switcher is allowed to go to 32V. Just use a 30V Zener, with a series resistor. No current will flow into the Zener unless a transient happens.
     
  8. #12

    Expert

    Nov 30, 2010
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    Fooled me!
    The schematic looks just like a linear regulator. Only the datasheet betrays the switching aspect.
     
  9. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    680
    15
    The Zener and series resistor will that be across the input without A series resistor into the load
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Just like you showed on your hand-drawn schematic. Replace the transzorb with a 30V 1W Zener, or use the 1.5KE30. You can leave out the 1N4937 input diode because the Zener will clamp both positive and negative input transients...
     
    Last edited: Jan 31, 2015
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