Capacitor but no resistor in chainsaw DPST switch

Discussion in 'General Electronics Chat' started by Jardov, Apr 18, 2015.

  1. Jardov

    Thread Starter New Member

    Apr 18, 2015
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    Hi - It's a long time since I studied electronics but I'm confused by something I've recently come across. My chainsaw stopped working and after dismantling I found that the DPST switch was faulty (one of the contacts was burnt out). I noticed it has a capacitor (0.15 microfarad) wired across it, I understand (sort of) that this is there to reduce potential damage caused by the arc however I always understood that it would need to be wired in series with a resistor, however there is no resistor - surely this would allow the capacitor to discharge rapidly and actually lead to more damage (maybe that's why the DPST burnt out) - or am I missing something?

    If I replace the DPST, can I leave out the capacitor, is there any danger or is it just a case that the DPST life will be shortened?

    thanks

    J
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC
    You need to solder the cap back to the switch.
    It is used to sorta minimize arcing when the switch opens
    Such caps do not hold charge that can destroy the contact but it helps prolong it
     
  3. WBahn

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    Mar 31, 2012
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    You want to put the cap back (if you can identify the cap, you might want to replace it with a new one). It prevents arcing when the switch is opened (so it might be damaged and need replacing since it appears the switch has been arcing). It won't stay charged and will bleed down through the circuitry in the saw after it stops.
     
  4. ian field

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    Oct 27, 2012
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    You're probably right about a resistor - think about what happens on the occasions you just happen to close the switch at the peak of the mains cycle.

    Something in the range about 15 - 33R in series with the capacitor should stop the contacts going "splat" when they close, and shouldn't have too much effect on suppression.
     
  5. WBahn

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    The peak of the mains cycle? In a chain saw? You close the switch when the saw isn't running (just before you attempt to start the saw).
     
  6. ian field

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    Because the TS made a reference to electronics, I assumed it was a mains operated electric chainsaw, I have one too so it didn't seem all that unlikely.
     
  7. WBahn

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    I've got an electric saw as well, but it only has the normal trigger switch (no toggle switch of any flavor). Gas saws have an ignition cutoff switch that typically grounds the magneto. I've never opened up one of my gas saws so I don't know if it's SPST or DPST. Can't really think of what the other pole would be used for. I've looked online for schematics for chainsaws and have largely come up empty -- haven't found anything with the right kind of detail.

    @Jardov : What kind of saw are you taking about?
     
  8. profbuxton

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    Feb 21, 2014
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    Capacitor needs to connected. Thats why they put a capacitor (condensor) across auto points in the "old" Kettering ignition systems. To help stop points burning out too quickly. Doesn't need a resistor in series.
     
  9. Jardov

    Thread Starter New Member

    Apr 18, 2015
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    Thanks for the replies, I should have been more clear about the chainsaw - it is an electric chainsaw.
     
  10. ian field

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    Some makes have (preferably a sbubber) or a capacitor across the switch because the inductance in the motor lets loose a hefty kick when yo break the current.

    Should be less of a problem with something on a long extension lead, out in the back garden - but they're still pretty good at wrecking mains in rectifiers on anything plugged into the same ring main.
     
  11. ian field

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    The capacitor on auto points merely slows the dv/dt so the points parting is quick enough to quench the arc before too much energy is dissipated there instead of at the plug gap.

    The capacitor still charges up to the LT supply and goes "splat" when the points close - the damage from that is just insignificant compared to the pitting caused by arcing.
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Thats part of the reason for the points cap. The other big one is to supply the resulting ~400V from the last discharge to the next closing of the points, resulting in a higher voltage in the spark plug the next time the coil cycles. When the engine is running there is only ~6V on the coil input, due to the ballast resistor. The coil and points only see the full 12V during cranking/starting.
     
  13. WBahn

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    Since the points short out the condenser when they close, how does this result in a higher voltage in the spark plug?
     
  14. shortbus

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    From Wikipedia, but available in many books -
    "The capacitor has two functions. Its main function is to form a series resonant circuit with the ignition coil. During resonance, energy is repeatedly transferred to the secondary side until the energy is exhausted.[2] As a result of this resonance the duration of the spark is sustained and so implements a good flame front in the air/fuel mixture. The capacitor, by default minimizes arcing at the contacts at the point of opening. This reduces contact burning and maximizes point life. The Kettering system became the primary ignition system for many years in the automotive industry due to its lower cost, higher reliability and relative simplicity."
     
  15. ian field

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    The energy transfer variety that used to be common on mopeds produced the spark at the time the points closed, AFAICR: they also had a points capacitor, but I'm not certain what function it served.
     
  16. WBahn

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    Can you recall what the circuit looked like?
     
  17. ian field

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    Everything was in parallel: The generator winding, the points, the capacitor and the IGN coil primary.

    In the even more distant past, it was common practice to wind the HT secondary directly over the IGN winding on the generator - heat from the engine and possible oil splash made them less than reliable.
     
  18. WBahn

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    I don't know if I'm picturing things correctly. If the points were in parallel with the coil, how did closing the points cause the spark?
     
  19. ian field

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    The poles of the generator have to be synchronised to the timing point, so maximum amplitude at the instant the points close.

    AFAIK: Shorting the LT winding at the instant of maximum energy storage causes a huge current to flow until the field collapses - I assume that's where the "energy transfet" bit comes in, that energy has to go somewhere.

    A possible clue is described in a Tyco relay appnote on supressing back emf in relay coils. Tyco points out that if you simply clamp the coil with a back diode, that carries a large pulse of current as the field collapses - once again, that energy has to go somewhere. In the case of a relay coil; that energy causes a current to recirculate round the coil and diode, that slowly dissipating current causes a magnetic field itself - which slows parting of the contacts, and can draw a pretty nice arc if its switching an inductive load.
     
  20. shortbus

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    Opening the points causes the spark. The coil hot wire is on the positive terminal of the coil. The movable point is connected to both the coil negative terminal and the positive of the cap/condenser. The fixed point and negative (case) of the condenser are grounded/negative.
     
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