SV: Re: [sdiy] Discharging electrolytics

[Karl Ekdahl]

Sorry people, i haven't been on the list for a while
and didn't know you've discussed this recently. i
allways forget about the archives. But okay, i have
death to face now...

Karl

--- Bob Weigel <sounddoctorin at imt.net> skrev:

> 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
> >>
> >>    
> >>
> >
> >
> >
> >
> >  
> >
> 
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