[sdiy] Discharging electrolytics

[harrybissell]

Ok then lets play a little game :^P

I'll list as many reasons as I can think of to NOT
discharge a capacitor through a(n uncontrolled)
short circuit, and you respond with as many
reasons why a short circuit is the BETTER way
to discharge a cap.

If others want to play too... they can add to either
list as they like.

( I won't start the 'better' list myself
to avoid being more sarcastic than I'm already being...
if that is possible :^)

1) Sudden discharge may damage the foils connected
to the capacitor terminals. ESR may be permanently
degraded.

2) Arc might damage the capacitor terminal

3) Arc might damage your screwdriver

4) Arc might damage you (hope you have time to find your safety
glasses ;^)

5) Arc might leave a permanent metalization or carbon track on
the circuit board, making future voltage withstanding 'iffy'

6) Arc might generate EMI pulse and damage sensitive semiconductors
nearby  (which is why most folks are advised not to use transformer type
soldering guns like the old Weller high power pistol units).

7) Arc might 'ring up' with the stray inductance in the circuit and cause
voltage OVERSHOOT.  In this case, the capacitance is real, the inductance
might be large as well (transformer winding nearby ?) and the resistance
is out of your control.  The Arc is not likely to be one smooth steady discharge...
it is likely to be sporadic as your hand jitters on the screwdriver.  What happens when
the current is suddenly interrupted ?

(unofficially : there could be risk of legal trouble from someone follwing the short
circuit method and injuring themselves.  People posting in a public forum have a
responsibility to dissuade readers from following potentially dangerous practices.
By posting, you are in fact proclaiming yourself to be knowledgable. That could also
make you liable if your advice is contrary to accepted practice and you are unable to
prove that your advice is prudent.  I'm most concerned that you don't know what kind of
capacitors some reader may be encountering.  I play with capacitors that can and will
kill people given the chance.  A person who is wondering 'how' to discharge a capacitor
is probably not able to weigh the possible dangers.  So tell them how to do it safely.
Waste THEIR time, who cares.  Maybe they will make fewer repais an hour and you'll get
the extra business :^)

(actually the night was getting kind of boring - this could liven it up a bit :^)

<over>

H^) harry



Bob Weigel wrote:

> Gee harry...perhaps you forget.  I'm not ignorant or stupid on these
> matters.  You are speaking totally out of context of what i was talking
> about.  I forgot having my 'ass kicked'.  Perhaps your vivid imagination
> would find..better use in the studio? :-) -Bob
>
> harrybissell wrote:
>
> >LOL... when I saw this thread the first thing I though of was
> >"gee I hope I don't have to kick Bob Weigel's @ss again this time" but
> >it looks like the last one didn't take.
> >
> >If this one does not take... I have discussed this with my co-workers and
> >we're thinking of sending Bob a capacitor in the mail, minus the shorting wire
> >(to keep it discharged :^).   Choose your screwdriver Bob... anyone you try is the
> >wrong one.   ;^P    Its a 1500uF / 1000V with a VERY low ESR (its a film an foil
> >type)
> >Think "motorcycle battery" and you get the picture...
> >
> >Use a resistor to discharge the cap. Check the voltage with your meter.
> >Most caps are not suited for pulse discharge applications.  An Arc makes
> >a shitty (because you have no control over it) discharge.
> >
> >If you have a cap that IS rated for pulse discharge, you are in even more trouble.
> >
> >Play it safe and use a resistor.  (the Peasant likes 10K, probably a good start !)
> >
> >I disagree with the statement that the arc is going to begin before you touch
> >the conductor. At high voltages (a tube amp power supply for example) this is
> >true.  Its unlikely to happen below 100V... the arc starts when the screwdriver
> >TOUCHES and begins to burn back :^P
> >
> >Hey HERE'S an idea for you Bob... why not short out the cap with your multimeter ?
> >Just set it to AMPS instead of volts.  You will get the big arc that you like... and
> >if your
> >probes survive you can 'save time' because you can just switch to volts after that
> >and you don't have to look for another tool...
> >
> >... oops, maybe you WILL need to look for another tool - the meter might not like
> >that.
> >
> >(newbs beware -   Really, use a resistor that's my final advice to you. I'm just
> >having some fun playing with Bob :^)
> >
> >H^) harry
> >
> >Bob Weigel wrote:
> >
> >
> >
> >>Hehe.  The thread that wouldn't die.  People seemed all hacked off at me
> >>before about saying 'let 'er rip' but...  You've got to understand...I
> >>work on tons of these.  I have SO  MANY TOOLS and...it's often hard to
> >>find the right one at some point and you waste all your time looking for
> >>it.  And eventually waste all your time and you die...but at least you
> >>did it right.  :-)
> >>     I should have prefaced that in general, It's good to BE AWARE of
> >>what you are doing before getting into electrical equipment.
> >>That's...what those little notes are the back are for.  If you haven't
> >>studied every aspect of every part in there to where you are absolutely
> >>confident that it won't kill you...KEEP OUT is the basic idea behind
> >>that notice they make them put on just about everything now.
> >>     However once you begin to get the feel for things, you realize that
> >>anything 100uF 500V or less will do minor damage to a tool like a
> >>screwdriver at the worst.  It will not throw off body harming shrapnel
> >>however PROVIDE that you don't stick your face right in the arc and that
> >>you do what I do...which is get the driver almost there then close your
> >>eyes right as you..ahem.... 'let er rip'. :-).  (I hate having arcs
> >>burned into my retina)
> >>        But..you see most of the amps I work on...this is a worst case
> >>scenario because by the time I get to where I'm verifying that caps are
> >>discharged, the drain resistors have already take it down to less than
> >>50V and I'm not going to get a screwdriver damaging spark at all.
> >>BUT...just in case the resistors have failed...I dont' want to die.  So
> >>I throw my little 'let er rip' insurance policy on it just to be sure.
> >>       WERE I spending my time searching for my special discharge tool,
> >>(which is great to have for bigger scarier capacitors which are seldom
> >>found in guitar amps and stuff I normally work on) I know I'd get so
> >>frustrated looking for it I'd shave time off my life the way things work
> >>in my shop.  That's my reality.    That's how I deal with it.  I seldom
> >>have arcs but when I do I am thankful that I slightly damaged a 3 dollar
> >>screwdrier rather...than spending oh probably an accumulated hour and a
> >>half looking for the special tool amidst the sea of other tools that get
> >>misplaced occasionally.
> >>     So basically for those who want to employ disgression in this
> >>
> >>1) never assume that a small capacitor doesn't have a larger one in
> >>paralell with it.  Sometimes small caps are used to couple a higher
> >>frequency response with a huge one.
> >>
> >>2) But in general any capacitor below 100uF 100V, won't produce a spark
> >>capable of really damaging a normal tool worth mentioning.
> >>
> >>3) to extrapolate that to other values, rememer that we're talking about
> >>a couple factors here.  The amount of charge on the cap will produce a
> >>current when shorted.  That current we might think is 'instantaneously
> >>transported' but in reality it moves at a real rate which is in all
> >>likelihood not very constant.  Here's what happens
> >>
> >>   A) The current starts moving BEFORE the screwdriver actually does
> >>what we could call 'physically touching'.  That's right.  Remeber, air
> >>is rated according to it's composition in V/inch for what might be
> >>referred to as a 'dielectric strength'.  Vaccum is 20Kv/inch or like
> >>that.  I don't believe those values are perfectly linear as you get down
> >>to tiny distances but..my point is that charge starts moving when the
> >>screwdriver is like several thousandths of an inch away.
> >>Thus...dynamics like how fast you are moving the screwdriver for
> >>instance play a major role in how much damage the screwdriver receives.
> >>Because when the charge starts moving, a lot of heat is created at the
> >>interface and a plasma forms quickly there.  Material is spattered and
> >>oxidized and that's the black you see on the screwdriver if you don't
> >>use appropriate technique.
> >>
> >>B) Anyway over the course of that stuff happening the composition of the
> >>interface, and hence the perceived resistance of the interface changes
> >>significantly.  It goes from a high impdedence air gap to a plasma
> >>embodied conduction path of low impedence to an oxidized insulation
> >>boundary that charge how has to move around as that area rapidly
> >>expands.  To where
> >>
> >>C) Eventually we are looking at a higher impedence again.  Typical time
> >>it takes to go from the start of conduction for a 400V charged cap let's
> >>say to where there is actual 'contact' with ...whatever is left of the
> >>surface at the microscopic level... is on the order of oh a couple
> >>1/10ths of a msec or like that depending on how hard you push the tool.
> >>
> >>    So anyway you can see that there is complex and wonderfully complex
> >>interaction going on in that spark....now....can we sonically model it?
> >>Can you imagine synths that had along with VCO and Noise controls "Arc"?
> >>:-)    With great facilities so you can like model the sound of a piece
> >>of Bismuth being kerchonked into Cesium in an Argon/Chlorine atmosphere?
> >>hehe. -Bob
> >>
> >>The Peasant wrote:
> >>
> >>
> >>
> >>>Oh oh, not again...
> >>>
> >>>I recommend using an appropriate discharge resistor across each cap, around 10K
> >>>is a good start. This has been discussed before, there is info in the archives
> >>>as well...
> >>>
> >>>Take care,
> >>>Doug
> >>>______________________
> >>>The Electronic Peasant
> >>>
> >>>www.electronicpeasant.com
> >>>
> >>>
> >>>Quoting Karl Ekdahl <elektrodwarf at yahoo.se>:
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>Hi everyone, i just found this amp that seems to be a
> >>>>homebrewn thingy with prefabricated boards. it says
> >>>>82180 on the PCBs, if anyone has a thought.
> >>>>
> >>>>anyhow it's got two huge toroid transformers on it
> >>>>(about 15x7 cm) and two 1000uF 100V electrolytics
> >>>>which i'm a little bit unsure how to discharge,
> >>>>normaly i'd just shortcircuit it but these thingys are
> >>>>huge (6x12 cm) and looks like it could be bad news to
> >>>>do it. I was thinking of using a lightbulb, is that a
> >>>>good idea? How to do it otherwise?
> >>>>
> >>>>thanks
> >>>>
> >>>>Karl
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >
> >
> >
> >
> >