Short Circuit Protection

R

Rileyesi

Guest
I have a product that I want to add short circuit protection to. The system
measures analog voltages from several sensors. Some of the sensors are self
powered and some require external power. There is a PCB that provides the
power to the sensors that need it and reads the signals from both styles of
sensors. I am using ADC on a micro to read the signals.

The power supply scheme I am using has one source to power the sensors, micro,
and it also acts as the reference for the ADC. I did this to be sure that
there was never a time when the signal is a greater value than the reference
for the ADC.

My world collapsed when one customer shorted the power supply driving the
sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection without
sacrificing the accuracy of the system. Ideally, I would like this to be
external to the PCB so I can have all my customers retrofit the systems that
are already in operation.

I hope this is clear!

Any ideas would be appreciated.

Thanks.
 
"Rileyesi" <rileyesi@aol.com.gov> wrote in message
news:20040427120456.19729.00000411@mb-m10.aol.com...
I have a product that I want to add short circuit protection to. The
system
measures analog voltages from several sensors. Some of the sensors are
self
powered and some require external power. There is a PCB that provides the
power to the sensors that need it and reads the signals from both styles
of
sensors. I am using ADC on a micro to read the signals. [...]

My world collapsed when one customer shorted the power supply driving the
sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection without
sacrificing the accuracy of the system. [...]
Click to expand...

Ah, now you start to understand (or your customers do) why commercial data
acquisition boards cost so much.

Is there a good reason, other than cost, not to use one? Because I can
guarantee you that by the time you've solved all the same problems they
have, your boards will cost even more.
 
Ah, now you start to understand (or your customers do) why commercial data
acquisition boards cost so much.

Is there a good reason, other than cost, not to use one? Because I can
guarantee you that by the time you've solved all the same problems they
have, your boards will cost even more.
Click to expand...

There are some very "application specific" requirements and I could not find an
off the shelf solution.

Sometimes life works out that way!
 
Rileyesi wrote...
I have a product that I want to add short circuit protection to. The system
measures analog voltages from several sensors. Some of the sensors are self
powered and some require external power. There is a PCB that provides the
power to the sensors that need it and reads the signals from both styles of
sensors. I am using ADC on a micro to read the signals.

The power supply scheme I am using has one source to power the sensors, micro,
and it also acts as the reference for the ADC. I did this to be sure that
there was never a time when the signal is a greater value than the reference
for the ADC.

My world collapsed when one customer shorted the power supply driving the
sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection without
sacrificing the accuracy of the system. Ideally, I would like this to be
external to the PCB so I can have all my customers retrofit the systems that
are already in operation.
Click to expand...
Any time a power-supply voltage is made available externally, there
should be adequate short-circuit protection. For example, with an
ordinary +5V power line, an easy way to obtain this is by adding a
low-dropout-voltage (LDO) regulator IC in series, like an LM2940,
etc. This will add a small extra voltage drop, e.g., 50mV for 50mA
load, or 200mV for 300mA load, etc., which will acceptable because
it's less than the poor tolerance of the 5V supply to begin with,
but it'll provide you with a protective short-circuit current limit.

If the sensor power provided must be precise, like for a ratiometric
sensor such as a strain gauge, the LM2940-5.0 is accurate to 5%, but
other parts are better, such as an LP2950A at 0.5%. However, to be
safe you'll need to provide this with a higher input voltage (the 5V
source might actually be lower than 5V, creating a problem), even as
much as 12 or 15V if that's all that you have available. In this
case, to keep the precision-regulator dissipation low, you may want
to pre-regulate to say 6V with another IC like a 7806.

Thanks,
- Win

whill_at_picovolt-dot-com (use hill_at_rowland-dot-org for now)
 
"Rileyesi" <rileyesi@aol.com.gov> schreef in bericht
news:20040427120456.19729.00000411@mb-m10.aol.com...
I have a product that I want to add short circuit protection to. The
system
measures analog voltages from several sensors. Some of the sensors are
self
powered and some require external power. There is a PCB that provides the
power to the sensors that need it and reads the signals from both styles
of
sensors. I am using ADC on a micro to read the signals.

The power supply scheme I am using has one source to power the sensors,
micro,
and it also acts as the reference for the ADC. I did this to be sure that
there was never a time when the signal is a greater value than the
reference
for the ADC.

My world collapsed when one customer shorted the power supply driving the
sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection without
sacrificing the accuracy of the system. Ideally, I would like this to be
external to the PCB so I can have all my customers retrofit the systems
that
are already in operation.

I hope this is clear!
Click to expand...
What do you mean 'cooked', blown traces? The simplest form of
protection might be a glass fuse, or a more sophisticated
polyfuse.

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)
 
"Winfield Hill" <Winfield_member@newsguy.com> schreef in bericht
news:c6m4qd02lch@drn.newsguy.com...
Any time a power-supply voltage is made available externally, there
should be adequate short-circuit protection. For example, with an
ordinary +5V power line, an easy way to obtain this is by adding a
low-dropout-voltage (LDO) regulator IC in series, like an LM2940,
etc. This will add a small extra voltage drop, e.g., 50mV for 50mA
load, or 200mV for 300mA load, etc., which will acceptable because
it's less than the poor tolerance of the 5V supply to begin with,
but it'll provide you with a protective short-circuit current limit.
Click to expand...
Nice trick, to use a LDO regulator for that.

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)
 
Rileyesi wrote:
I have a product that I want to add short circuit protection to. The system
measures analog voltages from several sensors. Some of the sensors are self
powered and some require external power. There is a PCB that provides the
power to the sensors that need it and reads the signals from both styles of
sensors. I am using ADC on a micro to read the signals.

The power supply scheme I am using has one source to power the sensors, micro,
and it also acts as the reference for the ADC. I did this to be sure that
there was never a time when the signal is a greater value than the reference
for the ADC.

My world collapsed when one customer shorted the power supply driving the
sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection without
sacrificing the accuracy of the system. Ideally, I would like this to be
external to the PCB so I can have all my customers retrofit the systems that
are already in operation.

I hope this is clear!

Any ideas would be appreciated.

Thanks.
Click to expand...
FUSE

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
"Rileyesi" <rileyesi@aol.com.gov> wrote in message
news:20040427132815.21319.00000349@mb-m04.aol.com...
There are some very "application specific" requirements and I could not
find an
off the shelf solution.

Sometimes life works out that way!
Click to expand...
Agreed. Just checking. Too often I start out thinking "oh, I can make that
for a lot cheaper, I don't need all the bells and whistles" - but then
people start asking for the bells and whistles, and I end up reengineering
something that I could have bought off the shelf. But if what's available
off the shelf doesn't meet the needs (as opposed to being overkill), then
that's a different problem.

In addition to the suggestions others have posted, you might be able to get
some ideas from the published specs/schematics/block diagrams of
commercially available data acquisition cards.
 
In article <20040427120456.19729.00000411@mb-m10.aol.com>,
Rileyesi <rileyesi@aol.com.gov> wrote:
[snip]
My world collapsed when one customer shorted the power supply
driving the sensors. This cooked part of the PCB.

I am looking for a scheme to provide short circuit protection
without sacrificing the accuracy of the system. Ideally, I would
like this to be external to the PCB so I can have all my
customers retrofit the systems that are already in operation.
Click to expand...
I've seen 3-terminal regulators used as current
limiters on a multi-output system. ie, They
had a 24V regulated supply powering a set of
24V regulators. In normal operation the regs
ran bottomed, with just a small voltage drop
across them, but with a fault current the reg's
overcurrent/overtemperature mechanism did the
req'd limiting.

--
Tony Williams.
 
On 27 Apr 2004 10:21:17 -0700, Winfield Hill <Winfield_member@newsguy.com>
wrote:


Any time a power-supply voltage is made available externally, there
should be adequate short-circuit protection. For example, with an
ordinary +5V power line, an easy way to obtain this is by adding a
low-dropout-voltage (LDO) regulator IC in series, like an LM2940,
etc. This will add a small extra voltage drop, e.g., 50mV for 50mA
load, or 200mV for 300mA load, etc., which will acceptable because
it's less than the poor tolerance of the 5V supply to begin with,
but it'll provide you with a protective short-circuit current limit.

If the sensor power provided must be precise, like for a ratiometric
sensor such as a strain gauge, the LM2940-5.0 is accurate to 5%, but
other parts are better, such as an LP2950A at 0.5%. However, to be
safe you'll need to provide this with a higher input voltage (the 5V
source might actually be lower than 5V, creating a problem), even as
much as 12 or 15V if that's all that you have available. In this
case, to keep the precision-regulator dissipation low, you may want
to pre-regulate to say 6V with another IC like a 7806.
Click to expand...
Good scheme, but why not put the limiting reg UPSTREAM of the "regulating" reg.
That way, you still get protection but without droop.
 
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