Discharging caps

Discussion in 'Amp Tech Center' started by sergiomajluf, Jun 15, 2019.

  1. sergiomajluf

    sergiomajluf Tele-Meister

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    Hello

    I need to discharge a 1500uF 200v cap, and have no power resistor around.

    If I parallel a lot (say, 10) of 1/4w resistors, to get a resistance of 500ohms, would this be safe?

    Asked differently, does power rating increase when using multiple resistors in parallel?

    Cheers,
    S
     
  2. RLee77

    RLee77 Friend of Leo's

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    To your last question — yes.
    To make things easy on yourself, and avoid the tedium of paralleling a bunch of resistors, just get a 100k 1/4w (or any value above 47k, really) and put that resistor, by itself, across the cap. It will take some seconds to fully discharge, but the resistor will handle the momentary pulse easily.
     
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  3. clintj

    clintj Friend of Leo's

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    Total power rating is the sum of all individual resistors' ratings, since each resistor sees its own individual voltage drop and current. My grounding jumper uses three 1M/1W resistors in parallel, so the overall rating is 333K/3W for example.

    You can also just use your multimeter set to DCV to drain caps; it's slow but it works.

    Sent from my Pixel 2 using Tapatalk
     
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  4. sergiomajluf

    sergiomajluf Tele-Meister

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    Thank you both!
     
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  5. D'tar

    D'tar Tele-Afflicted

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    Yes but the r value decreases with parallel resistors. Use your meter as recommended. You will be checking the volage any way right.:)
     
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  6. Whatizitman

    Whatizitman Tele-Afflicted

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    Plus it’s fun to watch the numbers. Kinda like plucking strings while you’re loosening the old ones when changing strings. I know I can’t be only person who does that.
     
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  7. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    With a fairly low voltage cap like that you'd be fine using the "BFS" method (Big F***ing Screwdrivers) - one well-insulated screwdriver touching the positive lead, another touching the negative - and cross them. If you get a spark there was a discharge.

    But did you measure the cap first? That value sounds like it's in a newer amp where the caps self-discharge. You have to measure after discharging...if necessary....so I suggest always measuring *first*. It may not be necessary
     
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  8. Southboundsuarez

    Southboundsuarez TDPRI Member

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    Yes agree with all the above. Just use your meter and watch voltage deflate and if it fails to discharge quickly then go ahead and use the screwdriver or a clip lead on the caps. if it is simultaneously being shunted with the meter, chances are you wont get a big ol zzap! But just be prepared that if you do directly short with the BFS or jumper without a load to soak the energy that there is a chance for a spark as the electricity arcs and discharges.
    Directly shorting is not the greatest way to discharge caps but it usually isnt going to be such a big deal. especially at lower voltages, however those caps do have alot of capacity.
     
  9. LightningPhil

    LightningPhil TDPRI Member

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    Don't use screwdrivers. While many caps survive, older ones can suffer dielectric recovery after being quickly discharged. Well, they can while slowly discharging them too, but since you're doing it slowly, whatever recovers voltage wiese, leaks away through the resistor.

    Essentially, it's a good idea to use a resistor that takes from several seconds to several minutes to drain the voltage down to a couple of volts - and leave it on for a bit longer than required. Rule of thumb for high voltage capacitor banks (big physics experiments) is 5CR <= 120. That means, 5 times the resistance in ohms, times the capacitance in farads is less than 2 minutes. The time constant CR is roughly equivalent to the time required to halve the voltage on the capacitor bank - so 5CR is 5 halvings of the voltage or dividing by 16. 400V devided by 16 is still 25V, but it won't kill you. But also, any charge that's oozed into the dielectric of the capacitor, that slowly oozes back, will have been bled away.

    I've seen capacitors regain a large fraction of their initial charge after being quickly discharged and been belted by it a few times. Slow is good. Minimum several seconds, maximum for sanity's sake is several minutes. It's a big window and doesn't require high power resistors.
     
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  10. LightningPhil

    LightningPhil TDPRI Member

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    For your 1500 uF cap, my rule of thumb would suggest a resistance of 800 kohms or less for a safe voltage within 2 minutes.

    If you're using 1/4 W resistors, then with 200 V, the minimum resistance would be 160 kohms.

    So a 470k resistor would do nicely and should be easy enough to find.
     
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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  12. schmee

    schmee Poster Extraordinaire

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    This. Or just clip a wire between them.
     
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