Protection of 1uF capacitor against short circuit

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   I'm looking for a component to protect against overcurrent
if the output of my Teletype current loop driver is shorted.
The output end of this system is a 1uF aluminum electrolytic
capacitor charged up to 120VDC, with the output switched by
an Omron G3VM401E optoisolator.  Normally, this drives
an electromagnet with 55 ohm resistance and 4 H inductance.
With that load, the maximum current is about 60mA, which is
what it's supposed to be.

   But what if someone shorts the output?  It could easily happen;
the output is to an ancient long-frame phone jack, which is
what old Teletypes use. If someone plugs a keyboard into the
printer jack, the output will be shorted.

   If shorted, the capacitor discharges into a dead short.
The only resistance in the circuit is the equivalent series
resistance of the capacitor.  There will be a huge current
for a few microseconds, and 0.005J of energy will be dissipated.

   Is this likely to blow the capacitor or the optoisolator?
It's a high-current transient that lasts a few microseconds,
without much total energy.  It could repeat at about 5Hz as
the charging circuit recharges the capacitor.

   Both regular fuses and polyfuses are too slow.  There are
current-limiting ICs, but I haven't found any rated for more
than 40V.  Any ideas?  Is protection even necessary?

   Here's the whole board design in KiCAD, if anyone needs
the details.

    https://github.com/John-Nagle/ttyloopdriver

                John Nagle

Re: Protection of 1uF capacitor against short circuit
wrote:

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There are self-protecting SSRs, like the Ixys CPC1540. We sometimes
use them always-on, as current limiters.

Teletypes are really blasts from the past. I used to hate them...
slow, noisy, oily, always breaking. Then came dot-matrix printers,
like Decwriters, slightly better.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Protection of 1uF capacitor against short circuit
On 10/23/2016 12:02 PM, John Larkin wrote:
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...

   Great idea! It turns out that availability of the CPC1540G (the DIP
version) is poor, but the CPC1510G is readily available in DIP.
The current limit is higher, but the goal here is for the device to
protect itself.  I'm also going to use a ceramic 1uF cap instead of
the electrolytic.   Those are less fragile.  That should do it.
I don't even have to change the PC board, which is currently
on its way to me from the board house.  (Seeed, in China.)

   Someone wanted to see the circuit without having to install KiCAD.
Here it is:

  
http://www.aetherltd.com/public/misc/loopdriverv1.0.png

This is for antique Teletype hobbyists.  Here's how we use it:

   https://vimeo.com/124065314

I restore those machines as a hobby. See "http://www.aetherltd.com ".
Right now, I have five of them.

                John Nagle



Re: Protection of 1uF capacitor against short circuit
On Sunday, 23 October 2016 19:10:07 UTC+1, John Nagle  wrote:
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What's wrong with a resistor?


NT

Re: Protection of 1uF capacitor against short circuit
John Nagle wrote...
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 NSC's LM317Hv rated to 60V, and better: TI's TL783C
 rated to 125V and 500mA.  That should be fine.

 Reminder, this info and more in Table 9.2, power-control
 Chapter 9.  Also 108 pages free, sample AoE-III chapter.
 http://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf


--  
 Thanks,
    - Win

Re: Protection of 1uF capacitor against short circuit
On Sun, 23 Oct 2016 11:10:01 -0700, the renowned John Nagle

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1uF regular aluminum electrolytic will have an ESR of a couple hundred
ohms, so the current will be limited to < 1A. The optoisolator would
also have an Idss which will limit the current. I don't see a spec for
the maximum current.  

You could add the classic two-BJT current limiting circuit, ensuring
the SOA of the BJTs are not exceeded.  

http://electronicdesign.com/site-files/electronicdesign.com/files/uploads/2015/02/IFD2634_F1.gif

You lose a bit of voltage in the ccircuit but you have 120V to play
with..  

--sp  

--  
Best regards,  
Spehro Pefhany
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Re: Protection of 1uF capacitor against short circuit
On 23/10/16 22:26, Spehro Pefhany wrote:
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[snip]
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What? A couple of hundred milliohms surely? The only time I
bothered to measure one I got about 1 Ohm, and I think *that*
was bad!

Jeroen Belleman

Re: Protection of 1uF capacitor against short circuit
On Sunday, 23 October 2016 22:57:54 UTC+1, Jeroen Belleman  wrote:
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I'll throw my hat in with 10 ohms.


NT

Re: Protection of 1uF capacitor against short circuit
On Sun, 23 Oct 2016 23:57:50 +0200, the renowned Jeroen Belleman

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That might be a bit on the high side. It's not listed on most of the
datasheets. I see 17 ohms for a 100V/1uF part from Nichicon that's
claimed to be low impedance. I would expect a 250V part to be quite a
bit higher, but it's not listed at all in the datasheet:  

http://nichicon-us.com/english/products/pdfs/e-upm.pdf

--sp  


--  
Best regards,  
Spehro Pefhany
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Re: Protection of 1uF capacitor against short circuit
On Sun, 23 Oct 2016 21:11:20 -0400, Spehro Pefhany

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The small alum cap data sheets rarely mention ESR. I have found a weak
correlation: for given C, higher voltage is lower ESR.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Protection of 1uF capacitor against short circuit
On Sun, 23 Oct 2016 20:55:52 -0700, the renowned John Larkin

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Two data points only at 1uF from the datasheet I linked.  

1uF 5 x 11 50V  11 ohms  
1uF 5 x 11 100V 17 ohms


Lots of points for bigger caps that support your weak correlation.  

47uF 5 x 11 16V 0.8 ohms
47uF 6.3 x 11 25V 0.55 ohms
47uF 6.3 x 11 35V 0.34 ohms
47uF 6.3 x 15 50V  0.3 ohms
47uF 8 x 11.5 63V  0.25 ohms
47uF 8 x 15 80V  0.39 ohms  
47uF 10 x 20 100V 0.28 ohms

But back to the oddball 1uF high voltage caps..  

Then we have the Vishay MALREKA00PB110P00K which is 265 ohms ESR for
1uF/450V.  

And the Illinois capacitor 105TTA450M which is a whopping 414 ohms for
1uF/450V.  

And many other similar ones that fit my old experience with it being
hundreds of ohms.  

That's a pretty wide range!  

--sp


--  
Best regards,  
Spehro Pefhany
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Re: Protection of 1uF capacitor against short circuit
Spehro Pefhany wrote...
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 I think you'll find better correlation to package volume.


--  
 Thanks,
    - Win

Re: Protection of 1uF capacitor against short circuit
On Mon, 24 Oct 2016 08:07:05 -0400, Spehro Pefhany

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414 ohms gives it a 380 Hz internal time constant!  

That's close to useless.

--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Protection of 1uF capacitor against short circuit
On Monday, October 24, 2016 at 7:43:34 AM UTC-7, John Larkin wrote:
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The application here, though, is an (ASR33?) teletype mechanism; those
operated at 110 baud, or 120 Hz.  The typical drive was intended to be
a high impedance (current source), and the receiver was a short.

Re: Protection of 1uF capacitor against short circuit
On Monday, October 24, 2016 at 5:07:07 AM UTC-7, Spehro Pefhany wrote:
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There are electrolytic caps rated for shorted outputs (so-called photoflash capacitors),
but that rating isn't seen on smaller capacitors.
Since the resistive nature of the capacitor isn't known, the possibility of local
heating (and physical damage) can be limited by putting a few ohms in series
with the capacitor.   5 ohms isn't going to impede normal operation, nor would
a small inductor (5 ohms at 1 kHz would come to 27uH).

The modern affinity for low-ESR in capacitors is useless in this application.

Re: Protection of 1uF capacitor against short circuit
On Sun, 23 Oct 2016 20:55:52 -0700, John Larkin

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For a constant body size and type, esr can be expected to rise with
voltage rating for Al electrolytics.

Degrading or 'stabilizing' esr range with an external series R is a
known method of getting predictable results from Al electrolytics, in
small signal or pulsed applications.

A more common problem is obtaining a low enough worst case esr in an
application where high esr doesn't work, usually in power conversion.

Getting anything below 10R in a <7.5mm diameter can was once pretty
near impossible, even over the consumer env temp range.

Anything over 63V typically suffers from further limitations in the
higher voltage Al dielectric formation.

RL

Re: Protection of 1uF capacitor against short circuit

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And for a given C, a higher voltage cap will be bigger.  


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But that wrecks the pulse current capability!

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Polymer aluminum caps are great. Low and stable ESR... too low
sometimes. They're available at over 100 volts now.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Protection of 1uF capacitor against short circuit
On Mon, 24 Oct 2016 08:38:50 -0700, John Larkin

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I've been unable to view the OP's gerbers, for some reason, but the
reasons people use electrolytics often include one size and cost. When
troubleshooting performance in a fixed layout, one of the things that
is seldom available is extra room. It is therefor more apt to consider
radial electrolytic component solutions with the OD limitation in
mind.
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The OP is looking for 60mA pulse, with considerable headroom to
provide it. His issue is predictable, non damaging, single fault
behavior, in the event of load shorts.

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A lower ESR will not solve the OP's problem.

RL

Re: Protection of 1uF capacitor against short circuit

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Shorting a charged electrolytic cap won't damage the cap. I used to
build huge banks of caps, charge them, and short them, and make
impressive explosions.

A few-turn coil of heavy wire would magnetize anything.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Protection of 1uF capacitor against short circuit
On Mon, 24 Oct 2016 13:07:33 -0700, John Larkin

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Ripple current?
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