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Adding Short Circuit Protection

I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board. Anyone have a
suggestion to add short circuit protection to the existing board
already made? I'm adding the protection to my new boards, I am
concerned about the ones I already have in stock.

Thanks.
 
H

Homer J Simpson

Jan 1, 1970
0
I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board.

Isn't that a good thing?
 
M

maxfoo

Jan 1, 1970
0
I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board. Anyone have a
suggestion to add short circuit protection to the existing board
already made? I'm adding the protection to my new boards, I am
concerned about the ones I already have in stock.

Thanks.

If you're that worried about your design, add a over-voltage crowbar circuit.
Zener,fuse,scr, and a few resistors...
 
J

John Larkin

Jan 1, 1970
0
I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board. Anyone have a
suggestion to add short circuit protection to the existing board
already made? I'm adding the protection to my new boards, I am
concerned about the ones I already have in stock.

Thanks.

Kluge in some polyfuses, if you can afford a small voltage drop.

John
 
W

w_tom

Jan 1, 1970
0
I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board. Anyone have a
suggestion to add short circuit protection to the existing board
already made? ...

What creates the five volts? Most power supply ICs include short
circuit protection - called foldback current limiting. Most all power
supplies also have that feature.

Meanwhile, if your design has no short circuit protection, then an
overvoltage crowbar recommendation would only make things worse.
 
What creates the five volts? Most power supply ICs include short
circuit protection - called foldback current limiting. Most all power
supplies also have that feature.

Meanwhile, if your design has no short circuit protection, then an
overvoltage crowbar recommendation would only make things worse.

The power supply chips using MOS pass devices tend to only have
thermal shutdown since there is no SOA issue with the MOS device. If
so, the output oscillates under short circuit.
 
T

Tam/WB2TT

Jan 1, 1970
0
John Larkin said:
Kluge in some polyfuses, if you can afford a small voltage drop.

John
If he has a regulator on board, he could put the polyfuse at the input of
the regulator.

Tam
 
R

Rich Grise

Jan 1, 1970
0
What creates the five volts? Most power supply ICs include short
circuit protection - called foldback current limiting. Most all power
supplies also have that feature.

Meanwhile, if your design has no short circuit protection, then an
overvoltage crowbar recommendation would only make things worse.

That's the second time someone has suggested an "overvoltage crowbar",
which sounds like the exact opposite of what the OP was seeking - I
seriously doubt that a short circuit condition would cause an overvoltage!

What he needs is a _current sense_ that shuts down the supply over a
certain limit. There are as many ways to do this as there are designers;
I'd crank up the supply by, say, .7V, and put a current sense resistor
and a couple of transistors. Of course, you take your voltage sense
_after_ the current sense, and some kind of latching circuit to shut
down the supply - maybe a LED to indicate "I've been shorted and shut
myself down" or something.

I once did a short-circuit protector in firmware for an SCR
phase-controlled, 24V 40A battery charger thatused a 68HC11. It was pretty
cool watching that sucker pumping almost a kilowatt into a load, and when
you short it, the supply goes "Bup" and shuts down on the next half-cycle.
:)

Of course, we didn't actually short the battery terminals - that's a
whole different ball game. ;-)

Cheers
Rich
 
W

w_tom

Jan 1, 1970
0
That's the second time someone has suggested an "overvoltage crowbar",
which sounds like the exact opposite of what the OP was seeking - I
seriously doubt that a short circuit condition would cause an overvoltage!

What he needs is a _current sense_ that shuts down the supply over a
certain limit. There are as many ways to do this as there are designers;
...

The second time an overvoltage crowbar was mentioned was to disparge
the OVC recommendation. The second time OVC was mentioned was the
complete opposite of a recommendation.

Meanwhile, that current sense (which is standard in power supply
designs) is also called foldback current limiting.

Generally when a power supply oscillates under high load, then a
power supply is unstable. Power supply designers learn closed loop
theory, and the poles and zeroes in Nyquist stability criteria so that
the supply does not oscillate.
 
F

Fred Bloggs

Jan 1, 1970
0
I have a circuit board that provides 5 VDC power to some sensors,
reads the analog outputs of the sensors and converts that to an RS-232
signal. The problem is that I did not design in any type of short
circuit on the power supply output of my board. Anyone have a
suggestion to add short circuit protection to the existing board
already made? I'm adding the protection to my new boards, I am
concerned about the ones I already have in stock.

When you figure how to add ss protection to the new boards, put that
subcircuit on a daughter board and screw it onto the ones in stock.
 
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