Over Voltage Protection Crowbar Circuit

On Sun, 02 Dec 2012 00:46:59 -0500, legg wrote:

> Applying the crowbar to the output is the most effective way of reducing
> potential damage to the load - which is it's intended function.

Yup.

Two-hundred-buck PSU toast - pick up the phone.

Ten thousand buck load toast - fifteen aspirin headache.

-- 
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

On Sat, 01 Dec 2012 15:46:58 -0700, Jim Thompson wrote:

> Sounds like a couple of amateurs when it comes to computing I^2*t >:-}

Pas du tout.

The purpose of the crowbar is to protect the load. When the SCR fires, the
load voltage drops to the SCR's Vt in a few microseconds. The load is protected.

How long after, or whether at all, the fuse ruptures, depends on the PSU.
If it has foldback protection, the fuse (selected for full-load current)
will handle the reduced current. If there is no such protection,or it has
failed, the fuse will rupture after t>I^2t/I^2. What happens to the PSU
afterward depends on its design and how well its input is protected.

The PSU is the sacrificial item, not the load. We must assume that it has
been designed or selected with all possible fault conditions of itself
considered.

-- 
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

On Sunday, December 9, 2012 1:10:37 PM UTC-5, Fred Abse wrote:

> The purpose of the crowbar is to protect the load.=20

The purpose of the crowbar is to protect the load by 1) reducing output to =
voltage to zero AND 2) activating the overcurrent protection.

If you don't blow a fuse or trip a breaker, you don't have a crowbar. And f=
or each of those devices you need a crowbar current in excess of 10x pass c=
urrent rating unless you want to wait the better part of a second in the ca=
se of a fuse and minutes in the case of a breaker. If your source does not =
supply that kind of current then you need an energy storage device to do it=
.

On Sun, 09 Dec 2012 10:10:37 -0800, Fred Abse
<excretatauris@invalid.invalid> wrote:

>
>The PSU is the sacrificial item, not the load. We must assume that it =
has
>been designed or selected with all possible fault conditions of itself
>considered.

Your work may be of that caliber and some others around here, but to
project that competence on all other engineers and their management is
surely specious.

?-)