dc switch snubbing questions

Discussion in 'General Electronics Chat' started by minkey01, Oct 20, 2015.

  1. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Hi.

    I have some questions about this yellow highlighted DPDT switch in the attached circuit jpg and contact snubbing.

    The top pole is switching 24 DC volt power on in the solenoid. Probably around 727 mA max. Should I be concerned with this switch? I've read that switching DC power can be a problem. Does it need a snubbing circuit in parallel across it?

    It looks like the second lower pole of the switch already has one (C409).

    Last question, will a new snubbing capacitor turn on the solenoid briefly while it charges up? It would seem like the switch in the off position would still be shorted while the capacitor is charging. I don't want the solenoid to flash activate when the AC power is first plugged in. I only want it to activate when I throw the switch.

    Thanks for the help!

    contact snubbing.jpg
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Upper switch pole already properly snubbed; D18.

    Lower switch pole snubber designer should be shot! Wiring capacitors directly across switch contacts is dumb! The peak capacitor discharge current as the contact closes is likely to do damage to the switch contact. The solenoid already has a RC snubber. Likely, they were trying to get rid of a "pop" in the audio.
     
    Last edited: Oct 20, 2015
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  3. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Now I'm more confused. That C409 design came out of a professional reel-to-reel that is over 30 years old and is still working. If it caused damage wouldn't it be dead by now?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How many times do you think that switch has been operated in 30 years?
    What is the value of C408 and C409?
     
  5. minkey01

    Thread Starter Member

    Jul 23, 2014
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    It operates when you start or stop the machine, so not too active. It activates the solenoid to pull in the pinch roller on the capstan. Maybe 4 or 5 times during a listening session. If they did that five days a week you would get 39,000? idk

    C408 Mylar .033 mfd
    C409 Mylar .1 mfd

    The bottom pole of highlighted switch turns on the relay called "D9 solenoid start." It is for triggering that K9. This gives extra capstan solenoid power boost in the beginning.

    The top pole of highlighted switch is the main power for the capstan solenoid.

    Here is a picture of the old switch. The reason I am replacing it is because I need a manual one you can flip with your hand.

    IMAG0154.jpg
     
  6. minkey01

    Thread Starter Member

    Jul 23, 2014
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    1) What if I add this snubber circuit to the top? Will the cap leak in the beginning and activate the solenoid?

    2) Do you think that old switch picture is anything special? I would just like to replace it with a new DPDT manual rocker switch and put the C409 across the bottom pole.

    Thanks for the help! Have a great day.

    new snubber.jpg
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Not needed as long as D18 is there. Adding unnecessary stuff just adds risk of failure...


    Both
    C408 and C409 need to have a low value resistor (10Ω to 22Ω) in series with each. Just like you showed the 120Ω resistor, above. That will cure the contact erosion issue with having the bare-assed capacitors wired directly across the contacts...
     
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  8. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Cool. Thanks, Mike!

    Will these new resistors in series mess with the timing of that D9 relay area?
     
  9. MikeML

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    Oct 2, 2009
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    No.
     
  10. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Hey, MikeML. Do you remember this thread? :) I was just wondering what wattage my resistors need to be for these snubbing circuits? Any idea?

    Thanks!
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    1/4W or 1/2W
     
  12. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Awesome! You da man. Thank you!!
     
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