exploding wirewound resistors

On Sun, 17 Sep 2017 09:05:21 -0700, John Larkin wrote:

> I don't think so.

Don't you think you should find out for sure? Maybe you'll be fine doing 
a one-off, but multiple tests I'm not so confident about.



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On Sunday, September 17, 2017 at 6:05:34 PM UTC+2, John Larkin wrote:
> On Sun, 17 Sep 2017 14:21:45 -0000 (UTC), Cursitor Doom
> <curd@notformail.com> wrote:
> 
> >On Sat, 16 Sep 2017 11:01:31 -0700, John Larkin wrote:
> >
> >
> >Isn't such a sudden discharge likely to damage the caps?
> 
> I don't think so. You can fully charge an electrolytic cap and then
> short it with a screwdriver (or a flashtube) and it doesn't seem to
> mind.
> 
> I'd have to use a lot of caps in parallel, to get enough coulombs and
> low ESR, so no one cap would see all the load current.
> 
> A cap bank like this can magnetize anything ferrous. Just a few turns
> of fat wire, and a couple thousand amps, makes a lot of field.
> 
Shorting the caps are normally not a problem, just buy the best you can find

Cheaper manufactors can have problems. We have seen that in motor drives, that failed when the powermodule discharged the caps to fast

To create you bank, use leaded caps with long wires. Mount the cap into an PCB with unbroken copper foil, with one of the leads into an oversized hold, so it has no contact. Then add another PCB with the same holes and solder only the lead that was not soldered in the first PCB. Then you get two PCBs with very low inductance

Cheers

Klaus

>To create you bank, use leaded caps with long wires. Mount the cap 
>into an PCB with unbroken copper foil, with one of the leads into
> an oversized hold, so it has no contact. Then add another PCB with 
>the same holes and solder only the lead that was not soldered in 
>the first PCB. Then you get two PCBs with very low inductance 

Nowadays we have this newfangled "two-layer" thing. ;)

Cheers

Phil Hobbs

Den søndag den 17. september 2017 kl. 21.59.46 UTC+2 skrev pcdh...@gmail.com:
> >To create you bank, use leaded caps with long wires. Mount the cap 
> >into an PCB with unbroken copper foil, with one of the leads into
> > an oversized hold, so it has no contact. Then add another PCB with 
> >the same holes and solder only the lead that was not soldered in 
> >the first PCB. Then you get two PCBs with very low inductance 
> 
> Nowadays we have this newfangled "two-layer" thing. ;)
> 

it is hard to solder the pin connected to the bottom side unless you get 
pcbs made with plated holes ;)

and with a string of caps connect to opposite ends of the string 
to get even similar pcb resistance to all the caps

it is hard to solder the pin connected to the bottom side unless you get 
pcbs made with plated holes ;) 

Well, so you do know about some of the modern developments, I see. 

Seriously, if you have top and bottom pours, why on earth do you need two boards?

Cheers

Phil Hobbs

On Sun, 17 Sep 2017 18:32:36 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sun, 17 Sep 2017 09:05:21 -0700, John Larkin wrote:
>
>> I don't think so.
>
>Don't you think you should find out for sure? Maybe you'll be fine doing 
>a one-off, but multiple tests I'm not so confident about.

I've been shorting charged electrolytic caps, and banks of same, since
I was a kid, and never broke one. I'm suggesting a one-off setup to
see how many joules various parts can absorb, not a production unit.

We have sold lots of NMR gradient amplifiers that output up to 120
amp/17KW pulses, from energy stored in electrolytic caps. I don't
think we had any caps fail.






-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Sun, 17 Sep 2017 13:17:35 -0700 (PDT), pcdhobbs@gmail.com wrote:

>it is hard to solder the pin connected to the bottom side unless you get 
>pcbs made with plated holes ;) 
>
>Well, so you do know about some of the modern developments, I see. 
>
>Seriously, if you have top and bottom pours, why on earth do you need two boards?
>
>Cheers
>
>Phil Hobbs

I'm going to Dremel my cap board from double-side copperclad FR4. I
think I'll just carve an interdigitated pattern on the bottom side. I
could use both sides, but I can't solder the radial caps on the top
very well, so I'd have to make some little islands on the bottom and
via them up to the top side. A nut+bolt makes a pretty fat via.
Soldering a bunch of little pieces of wire as vias is a nuisance.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

Den søndag den 17. september 2017 kl. 22.17.42 UTC+2 skrev pcdh...@gmail.com:
> it is hard to solder the pin connected to the bottom side unless you get 
> pcbs made with plated holes ;) 
> 
> Well, so you do know about some of the modern developments, I see. 

;)

> 
> Seriously, if you have top and bottom pours, why on earth do you need two boards?
> 

if you have PCBs made you don't, I was guessing that John would just drill some holes in a piece of FR4

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:glgtrc9rd3gqbmb65p6n821ro4tavl3681@4ax.com...
> I've been shorting charged electrolytic caps, and banks of same, since
> I was a kid, and never broke one. I'm suggesting a one-off setup to
> see how many joules various parts can absorb, not a production unit.
>
> We have sold lots of NMR gradient amplifiers that output up to 120
> amp/17KW pulses, from energy stored in electrolytic caps. I don't
> think we had any caps fail.

Gotta love the "I haven't seen it so it can't exist" fallacy.

120 amperes?  That's cute.  Shorting a cap can produce kiloamperes and 
megawatts.  Enough force from the current alone to cause internal damage, 
let alone transient heating.  Plenty of reason to believe things can happen.

And the manufacturer, let's not leave them out.  Y'think they might know a 
thing or two about their product?

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

On Sun, 17 Sep 2017 21:24:33 -0500, "Tim Williams"
<tmoranwms@gmail.com> wrote:

>"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
>news:glgtrc9rd3gqbmb65p6n821ro4tavl3681@4ax.com...
>> I've been shorting charged electrolytic caps, and banks of same, since
>> I was a kid, and never broke one. I'm suggesting a one-off setup to
>> see how many joules various parts can absorb, not a production unit.
>>
>> We have sold lots of NMR gradient amplifiers that output up to 120
>> amp/17KW pulses, from energy stored in electrolytic caps. I don't
>> think we had any caps fail.
>
>Gotta love the "I haven't seen it so it can't exist" fallacy.

When you build thousands of high power things, over several decades,
and no elec caps fail from discharge pulses, that's a pretty good
sign. 

>
>120 amperes?  That's cute.  Shorting a cap can produce kiloamperes and 
>megawatts.  Enough force from the current alone to cause internal damage, 
>let alone transient heating.  Plenty of reason to believe things can happen.


Did you notice, on my sketch, the parts labeled  CURRENT LIMITERS  ?

>
>And the manufacturer, let's not leave them out.  Y'think they might know a 
>thing or two about their product?

Why are you being such a smarmy jerk?


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics