[sdiy] Discharging electrolytics

[Bob Weigel]

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