[sdiy] Discharging electrolytics

[Bob Weigel]

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>:
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>>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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