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'[EE]: Overvoltage protection and Resetable fuses?'
2003\05\19@115800 by

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 = resistor (limits inrush to charge the mosfet gate)
F = fuse (lowest low trip currnet available)
Z = zener diode (around 5.3+V)
M = 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.

--BobG

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.
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.
Regards Steve...

{Original Message removed}
Is this what you described?

+Ve ------F---,------,-------
|      |
Z      |
|      a
'---g-SCR
|      c
R      |
|      |
Gnd ----------'------'-------

So the SCR draws the current to short the fuse so that the zener does not
have to have a high current rating.  My circuit has a surge requirement of
2A, so the fuse is pretty big. This seems to make it so that a small zener
can crowbar a large current.  I am guessing that you would size the resistor
so that at the maximun supported overvoltage, the zener would see its
maximum rated current (Ve-5.1)/R = Izmax.

What happens when a reverse polarity is applied? I am not up on SCR's. Would
it turn on in this configuration and allow a reverse current to flow through
it?  Since the zener would be forward biased, almost the whole -Ve would
drop across the resistor so R would have to be bigger to keep the zener from
blowing up.  But if the SCR does not conduct in this direction, it all for
not because the fuse would not blow.

Maybe you would add the mosfet from the previous circuit to protect against
reverse polarity making...

+Ve ---,--F---,------,-------
|      |      |
|      Z      |
|      |      a
|      '---g-SCR
|      |      c
|      R      |
|      |      |
Gnd ---M------'------'-------

Does this make sense?

--BobG

{Original Message removed}
I have a circuit that uses two coin batteries and a voltage regulator to
drop the ~6.2 V down to 5V.  However, I noticed that everything works
fine without the regulator, and it also allows the batteries to last
longer because the regulator seemed to drop out at about 5.7V input.
The circuit has resistors, an IR decoder, and visible and IR LEDs.

Now, intuitively I want to assume that the circuit will consume less
current if I don't have the regulator, but I know intuition is not
always correct.  Can anyone help me out here?

Thanks!

Alex

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.
Simpler than that to reverse polarity protect the circuit add a shockty
diode across the SCR then if the supply is backwards the diode forward
biases and blows the fuse. Same thing with the AsqS for the diode should be
greater than the fuse prearcing value. 2A is fine the same circuit can be
used to blow fuses of hundreds of amps with the corect size of SCR and a 1k0
pull down res is ok as the input voltage above 5v is only applied for the
time it takes to get the SCR on as then the voltage colapses due to the scr
being on shorting the supply and blowing the fuse. See dwg for additional
diode. The mosfet would be in circuit all the time and disapate some wattage given
by I load x RDS on and would need to be rated to withstand the peak current
during fuse blowing the diode is not only cheeper but will not disapate any
power when connected corectly hence Idiot Diode (I made loads of money in
1980 replacing 1n4000 diodes in the back of CB sets when blowing the 4A fuse
also made the diode short circuit added pratt fee of a minimum of £10 for 7p
in parts)Wish I had a full time job with the same profit to parts ratio now

Steve....

{Original Message removed}
If the max voltage input to the active componants is > 6v2 then don't use
regulator or use small shunt regulator just on the active parts and leave
the rest of the circuit to run on the unregulated supply keep the v zener as
high as possible to reduce the quessent current consumption when the device
is not active.

Steve...

{Original Message removed}
Thanks for the advice and explanations, Steve.

--BobG

-----Original Message-----
From: pic microcontroller discussion list [PICLISTMITVMA.MIT.EDU]On
Behalf Of Steve Smith
Sent: Monday, May 19, 2003 4:06 PM
To: PICLISTMITVMA.MIT.EDU
Subject: Re: [EE]: Overvoltage protection and Resetable fuses?

Simpler than that to reverse polarity protect the circuit add a shockty
diode across the SCR then if the supply is backwards the diode forward
biases and blows the fuse. Same thing with the AsqS for the diode should be
greater than the fuse prearcing value. 2A is fine the same circuit can be
used to blow fuses of hundreds of amps with the corect size of SCR and a 1k0
pull down res is ok as the input voltage above 5v is only applied for the
time it takes to get the SCR on as then the voltage colapses due to the scr
being on shorting the supply and blowing the fuse. See dwg for additional
diode.
The mosfet would be in circuit all the time and disapate some wattage given
by I load x RDS on and would need to be rated to withstand the peak current
during fuse blowing the diode is not only cheeper but will not disapate any
power when connected corectly hence Idiot Diode (I made loads of money in
1980 replacing 1n4000 diodes in the back of CB sets when blowing the 4A fuse
also made the diode short circuit added pratt fee of a minimum of £10 for 7p
in parts)Wish I had a full time job with the same profit to parts ratio now

Steve....

-----Original Message-----
From: pic microcontroller discussion list [PICLISTMITVMA.MIT.EDU] On
Behalf Of Robert E. Griffith
Sent: 19 May 2003 20:16
To: PICLISTMITVMA.MIT.EDU
Subject: Re: [EE]: Overvoltage protection and Resetable fuses?

Exactly...
Is this what you described?

+Ve ------F---,------,-------
|      |          |
Z      |          |
|      a          c
'---g-SCR         D
|      c          a
R      |          |
|      |          |
Gnd ----------'------'-------

So the SCR draws the current to short the fuse so that the zener does not
have to have a high current rating.  My circuit has a surge requirement of
2A, so the fuse is pretty big. This seems to make it so that a small zener
can crowbar a large current.  I am guessing that you would size the resistor
so that at the maximun supported overvoltage, the zener would see its
maximum rated current (Ve-5.1)/R = Izmax.

What happens when a reverse polarity is applied? I am not up on SCR's. Would
it turn on in this configuration and allow a reverse current to flow through
it?  Since the zener would be forward biased, almost the whole -Ve would
drop across the resistor so R would have to be bigger to keep the zener from
blowing up.  But if the SCR does not conduct in this direction, it all for
not because the fuse would not blow.

Maybe you would add the mosfet from the previous circuit to protect against
reverse polarity making...

+Ve ---,--F---,------,-------
|      |      |
|      Z      |
|      |      a
|      '---g-SCR
|      |      c
|      R      |
|      |      |
Gnd ---M------'------'-------

Does this make sense?

--BobG

-----Original Message-----
From: pic microcontroller discussion list [PICLISTMITVMA.MIT.EDU]On
Behalf Of Steve Smith
Sent: Monday, May 19, 2003 2:15 PM
To: PICLISTMITVMA.MIT.EDU
Subject: Re: [EE]: Overvoltage protection and Resetable fuses?

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.

Regards Steve...

-----Original Message-----
From: pic microcontroller discussion list [PICLISTMITVMA.MIT.EDU] On
Behalf Of Robert E. Griffith
Sent: 19 May 2003 16:57
To: PICLISTMITVMA.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 = resistor (limits inrush to charge the mosfet gate)
F = fuse (lowest low trip currnet available)
Z = zener diode (around 5.3+V)
M = 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.

--BobG

--
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See http://www.piclist.com/#archives for details.

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See http://www.piclist.com/#archives for details.

--
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See http://www.piclist.com/#archives for details.

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