The most conventional form of this circuit is the crowbar (cant do =
ascii
art but heres a discription) the power +ve enters the circuit via a fuse =
to
the cathode of a small 5v1 zener and the anode of an scr. The gate of =
the
scr is connected to the anode of the zener and a low value resistor. The
cathode of the scr and the other end of the resistor is the gnd end.
It works the same and will rupture the fuse wen the scr is biased on =
(about
5.6v) it dependeds on the gate senisitivity of the scr but around 0.5v
higher than the zener voltage  (BT152) it then shorts out the logic line =
and
blows the fuse there are no circuit losses except the watts loss in the =
fuse
and the SCR will not require a heatsink. The only requirement is that =
the
AsqS of the SCR is greater than the prearcing AsqS of the fuse.=20

Regards Steve...

-----Original Message-----
From: pic microcontroller discussion list =
[mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of Robert E. Griffith
Sent: 19 May 2003 16:57
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Overvoltage protection and Resetable fuses?


I am considering the following circuit to protect against the wrong PS =
being
connected to my circuit as follows (it expects regulated +5V) .

Vin -----------.------------- Circuit Vcc (+5v)
               |
               R
               |
              Fuse
               |
            ,--,
            |  |
            Z  |
            |  |
Gnd --------'--M------------- Circuit Gnd

R =3D resistor (limits inrush to charge the mosfet gate)
F =3D fuse (lowest low trip currnet available)
Z =3D zener diode (around 5.3+V)
M =3D N-channel mosfet (FDN337N)

The mosfet protects against reverse polarity (an idea given to me on =
this
group a few months ago).  The idea with the zener, fuse, and resistor is
that if Vin exceeds the zener voltage, current will flow through the =
fuse,
tripping it.  If Vin is the specified +5v, no current flows through the
zener so the only current through the fuse is the inrush to charge the
mosfet's gate.  The resistor is there to limit this inrush so that it =
does
not trip the fuse.  Also, I am thinking resistor will help protect the =
zener
from over current in the time before the fuse blows (not sure if that is
needed).  That is why the resistor in on the common branch and not on =
the
mosfet lead, parallel to the zener.

So,
Question 1), is this a reasonable circuit?

Question 2), Would it be a good idea to add a 2nd zener across the =
Circuit
Vcc to clamp the voltage in the time before the fuse blows?

Question 3), Can I use a PTC resettable fuse such as Bourns MF-SM050-2?  =
The
datasheets for PTC resettable fuses list a trip current and a hold =
current.
I do not understand the hold current.  I am thinking that when a fuse =
blows,
it's a short and there is no current.  But resetable fuses must be more
complicated than this - how does the fuse know it can reset if it does =
not
sample the situation to see if the overcurrent still exists?  Maybe the =
PTC
fuse would just limit the current to hold current which would defeat the
circuit by clamping the mosfet's gate to the zener voltage which would =
keep
the mosfet turned on.

Thanks for your help.

--BobG

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