what's up with this snubber?

Discussion in 'General Electronics Chat' started by m1ch43l, Oct 12, 2012.

  1. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    I've a snubber circuit/clamp circuit. Basically it's a cap and resistor in parallel.
    I have a supply of 23v and wish it to hover around 12-14v for my circuit that requires max 100mA.
    My circuit has 2 smaller circuits each supplied from the main line by a filter circuit
    resistor 100ohms and 16v 470uF cap.
    The problem is that the first snubber (which according to some equations from ehow.com needs a min of 125ohm 694pF clamp circuit (with or without diode) clamping an initial burst of 25v at 10KHz). Soldering them only ends up with a faint pop from my 555s. To temporarily counter this problem and create a vdrop of 10v from the supply, I run a fan parallel to my circuit and I end up with 14v for my circuit. That's good. However, I intend to run the circuit on its own. I use 1 3Kohm 10nF cap clamp circuit and according to the simulator that should have me 13v on the circuit supply. On the circuit, there's berely no change: 23v which is too high. I try with 33K resistor and still voltage is way up (after some time).
    What gives?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Without seeing you circuit I can only guess. Please post a schematic.
     
  3. mlog

    Member

    Feb 11, 2012
    276
    36
    Thank you. You took the words right out of my moth.
     
  4. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Here they r.
    Follow the naming. Regardless of the minimal values of resistance: 125ohms or 694pf for cap, nothing works until I get to resistance in the orders of 3k+.
    Note that my supply here can't be altered to give a dead 12 or 14v and hence the clamp attempt.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    If I understand you correctly, you want to reduce the supply current from 23V to around 12-14V for your circuit. For that you shouldn't use a series resistor since the voltage will vary with the circuit load (ehow is not giving you good information on that). Instead use a voltage regulator, such as an LM317 or a series resistor with a 12V zener to ground.
     
  6. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Will it take care of the initial spikes of v>25? Don want to burn more 555's: they aren't readily sourceable here...
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Yes, either solution will.
     
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  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I use 555 devices in auto and motorcycle electrics which get hit with nasty voltage spikes. The safe way to protect them is to use an R-C filter to the 555 power terminal and also a zener clamp on the 555 power pin to ground.
     
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  9. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Clamped a 1N4148 parallel to my circuit input to ground and pretty much burnt 1 555 I decided to sacrifice. I'm currently looking into the reverse breakdown v of this diode.
    BTW, where should the diode be...after the RC snubber or before it, relative to the voltage source?

    When you say 'power pin' are you suggesting I place this diode at pin8 to ground?
     
    Last edited: Oct 17, 2012
  10. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    My bad, 1N4148 has breakdown voltage of 75 and/or 100 (clarify?). Zeners are scarce here so the fan idea will have to do. Thanks gents.
     
  11. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Hadn't paid close attention to this...seems I need to learn as many zeners and their BV for my projects. Thanks alot cruts.:D
     
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