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'[EE]: 2-tran SMPS , NOW: Transient suppressors'
2002\09\25@175619 by Morgan Olsson

picon face
>A TVS (transient voltage suppressor, goes by other names too) is
>faster than a zener, can absorb way more energy, not so sharp voltage
>clamping. You can get them in AC & DC  types.

Arnt´ theese a type of Zener with high peak handling?
And AC types dual antiseries?

>A VDR (voltage dependent resistor, goes by other names too) is I
>think slower than a zener, can absorb more energy than a TVS and is
>coarser voltage clamping than a TVS. These are AC.

IIRC turn-on is not notable, and theese also have some capacitance.
Can consume pretty much, i have once found one shorting, but mostly they blow open at overload.

I still think that for outputs SCR is best when affordable.
One problem is that when the SCR fires it consumes all the capacitor stored energy and that is a very high peak load i the SCR chip.  I seldom see series resistors, but series resistors of only few mohm (PCB track) will easy the life of the SCR.

/Morgan

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2002\09\26@134739 by Peter L. Peres

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On Wed, 25 Sep 2002, Morgan Olsson wrote:

*>I still think that for outputs SCR is best when affordable. One problem
*>is that when the SCR fires it consumes all the capacitor stored energy
*>and that is a very high peak load i the SCR chip.  I seldom see series
*>resistors, but series resistors of only few mohm (PCB track) will easy
*>the life of the SCR.

You can use an inexpensive large VFET (like BUKxxxx series 50A 50V) as
crowbar, with a pair of resistors setting the threshold. It is *much*
faster than any thyristor scheme and protects against reverse pulses too
(slower).

The point with miliohms is that amperes * miliohms = volts and at 100+A
short currents from modern caps the protective effect could be nulled. So
the crowbar should be at least between the load and the caps so the
voltage develops in front of it, not across the load.

Peter

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2002\09\26@135957 by Olin Lathrop

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> The point with miliohms is that amperes * miliohms = volts ...

Huh!!!?

(And "milli" has two "L"s in it.)


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2002\09\26@140411 by mpoulton

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face
His point is that inserting any significant resistance in
the crowbar circuit may well prevent it from conducting as
well as it needs to -- it must have super low resistance in
order to suppress voltage spikes from low impedance sources.

---- Original message ----
>Date: Thu, 26 Sep 2002 13:59:46 -0400
>From: Olin Lathrop <spam_OUTolin_piclistTakeThisOuTspamEMBEDINC.COM>  
>Subject: Re: [EE]:  2-tran SMPS , NOW: Transient
suppressors  
>To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
>
>> The point with miliohms is that amperes * miliohms =
volts ...
>
>Huh!!!?
>
>(And "milli" has two "L"s in it.)
--
Mike P.
MTP Technologies
KC0LLX

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2002\09\26@152121 by Morgan Olsson

picon face
Mike P.. wrote:
>His point is that inserting any significant resistance in
>the crowbar circuit may well prevent it from conducting as
>well as it needs to -- it must have super low resistance in
>order to suppress voltage spikes from low impedance sources.

The idea with *output* crowbar is to protect when the switcher go bananas (control cirquit/switch, etc)

Can´t normally make hundreds of amps, if it did, then just arrange the tracks intelligently and it pulls down correctly anyway.

For using fets, the turn on is not well enough defined, and also too soft/high resistance.

/Morgan

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2002\09\27@020211 by Peter L. Peres

picon face
On Thu, 26 Sep 2002, Olin Lathrop wrote:

*>> The point with miliohms is that amperes * miliohms = volts ...
*>
*>Huh!!!?
*>
*>(And "milli" has two "L"s in it.)

Ah I see that you have recovered, you are your old self again ;-)

Peter

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2002\09\27@030524 by Peter L. Peres

picon face
On Thu, 26 Sep 2002, Morgan Olsson wrote:

*>Can´t normally make hundreds of amps, if it did, then just arrange the
*>tracks intelligently and it pulls down correctly anyway.

That is exactly what I meant and wrote in my original posting. And the
lowly 1000uF low ESR smpsu output cap will cause current in excess of 100A
for microseconds when shorted by a crowbar.

*>For using fets, the turn on is not well enough defined, and also too
*>soft/high resistance.

Huh ?! 9 milliohms too high for you ? Turn on slope is very well defined
for a characterised FET. For an uncharacterised one use a comparator to
drive it, as you would for a thyristor.  Plus the FET circuit is self
restoring so the crowbar will not take out the whole circuit if a single
event transient is present. Thyristors in precise crowbar role are often
steered by a TL431 or equivalent anyway. The thyristor firing voltage is
also not well defined over temperature and spread between components, when
driven with a simple voltage divider. Or with a +/- 15% zener.

Peter

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2002\09\27@031805 by Peter L. Peres

picon face
On Thu, 26 Sep 2002, Mike P. wrote:

*>His point is that inserting any significant resistance in
*>the crowbar circuit may well prevent it from conducting as
*>well as it needs to -- it must have super low resistance in
*>order to suppress voltage spikes from low impedance sources.

Yes, but the trace inductivity can also get you.

Peter

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2002\09\27@053912 by Andy Kunz

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I've been off the list for 2 years, and come back and the first messages I
get show...

...some things never change.

Hi Guys!

Andy


At 07:59 AM 9/27/02 +0300, you wrote:
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2002\09\27@073233 by Russell McMahon

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> I've been off the list for 2 years, and come back and the first messages I
> get show...

> ...some things never change.

You mean Olin was on the list 2 years ago ? :-)

> Hi Guys!

Welcome back Andy. Have you built that water wheel yet? Time's a wasting.


       Russell McMahon

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