WHAT LIMITS CURRENT?

WHAT LIMITS CURRENT?

Post by Charles Sulliv » Sat, 17 May 1997 04:00:00





Quote:>Hi,

>I am using a 12V power supply to power ten identical circuits and need
>to build current limiting functions so that a short or a failure in one
>of the circuits will limit the current to that circuit to 2 Amps.
>Without current limiting one short will trip the power supply and then
>all ten circuits are down.  What is a good circuit to use for current
>limiting?

If a circuit has shorted or otherwise failed, you'd do better by shutting
it down entirely by using a circuit breaker or fuse.  Dissipating a
possible 24 watts where it's not necessarily meant to be dissipated (e.g.,  
in defective wire insulation) could lead to a fire.

Regards,
Charles Sullivan

 
 
 

WHAT LIMITS CURRENT?

Post by Megatro » Sun, 18 May 1997 04:00:00



> Hi,

> I am using a 12V power supply to power ten identical circuits and need
> to build current limiting functions so that a short or a failure in one
> of the circuits will limit the current to that circuit to 2 Amps.
> Without current limiting one short will trip the power supply and then
> all ten circuits are down.  What is a good circuit to use for current
> limiting?

> Talal Itani

>     --------------         2A limit
>     |            |------------------------ circuit 1
>     |   12V      |         2A limit
>     |   Power    |------------------------ circuit 2
>     |   Supply   |                                 .
>     |            |         2A limit            .
>     |            |------------------------ circuit 10

another thing to try is to install 12v light bulbs in series with the
circuits use ones with a little bit higher wattage rateing than your
circuit. then if the circuit shorts the light will come on indicating
the short and dissapaiting the current.

 
 
 

WHAT LIMITS CURRENT?

Post by Larry Gilstra » Mon, 19 May 1997 04:00:00



> Hi,

> I am using a 12V power supply to power ten identical circuits and need
> to build current limiting functions so that a short or a failure in one
> of the circuits will limit the current to that circuit to 2 Amps.
> Without current limiting one short will trip the power supply and then
> all ten circuits are down.  What is a good circuit to use for current
> limiting?

> Talal Itani

>     --------------         2A limit
>     |            |------------------------ circuit 1
>     |   12V      |         2A limit
>     |   Power    |------------------------ circuit 2
>     |   Supply   |                                 .
>     |            |         2A limit            .
>     |            |------------------------ circuit 10

Assuming that you are talking about a DC application,
you might consider putting a 2A constant current source in series
between the power supply and each load circuit.  But beware the
voltage drop (0.5vdc - 2.5 vdc)!
 
 
 

WHAT LIMITS CURRENT?

Post by Stephen M. Henni » Mon, 19 May 1997 04:00:00



> > Hi,

> > I am using a 12V power supply to power ten identical circuits and need
> > to build current limiting functions so that a short or a failure in one
> > of the circuits will limit the current to that circuit to 2 Amps.
> > Without current limiting one short will trip the power supply and then
> > all ten circuits are down.  What is a good circuit to use for current
> > limiting?

> > Talal Itani

> >     --------------         2A limit
> >     |            |------------------------ circuit 1
> >     |   12V      |         2A limit
> >     |   Power    |------------------------ circuit 2
> >     |   Supply   |                                 .
> >     |            |         2A limit            .
> >     |            |------------------------ circuit 10

The easiest way is to put a 2 amp circuit breaker or fuse on each leg.
That way only the bad leg(s) go(es) down.  Current limiting is a nightmare
and could lead to oscillations which would incapacitate the entire
system.  Current limiting is normally built internal to the supply and not
applied externally in parallel legs.

--
Cheers, Steve Henning, Reading, Pennsylvania, USA

Visit my home page at http://www.users.fast.net/~shenning
Contains Rhododendron, Boy Scout and Rotary information and more.

 
 
 

WHAT LIMITS CURRENT?

Post by Larry Gilstra » Mon, 19 May 1997 04:00:00



> Hi,

> I am using a 12V power supply to power ten identical circuits and need
> to build current limiting functions so that a short or a failure in one
> of the circuits will limit the current to that circuit to 2 Amps.
> Without current limiting one short will trip the power supply and then
> all ten circuits are down.  What is a good circuit to use for current
> limiting?

> Talal Itani

>     --------------         2A limit
>     |            |------------------------ circuit 1
>     |   12V      |         2A limit
>     |   Power    |------------------------ circuit 2
>     |   Supply   |                                 .
>     |            |         2A limit            .
>     |            |------------------------ circuit 10

IF you are refering to a 12 VDC application, and don't mind getting
a bit complicated, you can safely meet your requirements by putting
a low (voltage) drop, "constant current source" between the 12 V
power supply and each load circuit.  

The drawback is that there will be a voltage drop across the constant
current source, even when the load is not drawing all the current
available.  This drop can be anywhere from 1/2 volt for a really well
designed circuit, up to 2 volts or more for a more typically
constructed arrangement.  You may then need to consider changing your
12 volt supply to 13.5 or more.

 
 
 

WHAT LIMITS CURRENT?

Post by Larry Gilstra » Tue, 20 May 1997 04:00:00




> > > Hi,

> > > I am using a 12V power supply to power ten identical circuits and need
> > > to build current limiting functions so that a short or a failure in one
> > > of the circuits will limit the current to that circuit to 2 Amps.
> > > Without current limiting one short will trip the power supply and then
> > > all ten circuits are down.  What is a good circuit to use for current
> > > limiting?

> > > Talal Itani

> > >     --------------         2A limit
> > >     |            |------------------------ circuit 1
> > >     |   12V      |         2A limit
> > >     |   Power    |------------------------ circuit 2
> > >     |   Supply   |                                 .
> > >     |            |         2A limit            .
> > >     |            |------------------------ circuit 10

> The easiest way is to put a 2 amp circuit breaker or fuse on each leg.
> That way only the bad leg(s) go(es) down.  Current limiting is a nightmare
> and could lead to oscillations which would incapacitate the entire
> system.  Current limiting is normally built internal to the supply and not
> applied externally in parallel legs.

Certainly, fuses and circuit breakers are the "easiest" ways to
implement over-current protection.  But the question becomes whether
or not these load circuits, or their intended applications, can
withstand the voltage spikes and general brown-out conditions that
arise when one of these loads drags down or shorts out the entire
power supply output.  

The last reading I had on the subject, it takes a fast-acting fuse
several milliseconds to blow at twice rated current.  It takes a
circuit breaker dozens to hundreds of milliseconds to trip in an
over current state.  All the while, the voltage being delivered to the
other circuits will be dropping as the full current capacity of the
power supply is approached.

Does this matter to the intended application?  If not, go with fuses
or circuit breakers.  Otherwise, its time for some hearty engineering.

 
 
 

WHAT LIMITS CURRENT?

Post by Michael Skroc » Tue, 20 May 1997 04:00:00



> I am using a 12V power supply to power ten identical circuits and need
> to build current limiting functions so that a short or a failure in one
> of the circuits will limit the current to that circuit to 2 Amps.
> Without current limiting one short will trip the power supply and then
> all ten circuits are down.  What is a good circuit to use for current
> limiting?

Most home control/alarm systems these days use a "resettable fuse"
device.  Some of these work as heat-variable resistors.  If you get a
short, current heats them up and their resistance goes up quite a bit.
This limits current.  The concept is quite similar to the earlier
suggestion of using a 12 v light bulb.  While low current flows through,
its resistance is low.  As it heats up and glows, its resistance goes
up.  You can buy these from many parts suppliers.  Digi-Key has a bunch
of them.

You could use regular fuses on each leg, but of course they would not
reset automatically.  There are some "resettable fuses" sold as well.
Some may be as described above.

I would not use constant current sources devices because you can get
into some funny noise problems and heat dissipation problems if you're
not careful.  The simplest of these is a single transistor and resistor
connected to provide what is close to a constant current source (such as
NPN collector to +, resistor between base and emitter selected for
proper current based upon E-B voltage drop (about .5, .6 v, etc), and
base connected to load).

 
 
 

WHAT LIMITS CURRENT?

Post by Arthur L. Todes » Tue, 20 May 1997 04:00:00



>> Hi,

>> I am using a 12V power supply to power ten identical circuits and need
>> to build current limiting functions so that a short or a failure in one
>> of the circuits will limit the current to that circuit to 2 Amps.
>> Without current limiting one short will trip the power supply and then
>> all ten circuits are down.  What is a good circuit to use for current
>> limiting?

>> Talal Itani

>>     --------------         2A limit
>>     |            |------------------------ circuit 1
>>     |   12V      |         2A limit
>>     |   Power    |------------------------ circuit 2
>>     |   Supply   |                                 .
>>     |            |         2A limit            .
>>     |            |------------------------ circuit 10

>Assuming that you are talking about a DC application,
>you might consider putting a 2A constant current source in series
>between the power supply and each load circuit.  But beware the
>voltage drop (0.5vdc - 2.5 vdc)!

Well, in the telephone switch business we use large capacitors and fuses.
The caps store enough energy to clear the fuse element when a short circuit
is applied.  
 
 
 

WHAT LIMITS CURRENT?

Post by Stephen M. Henni » Tue, 20 May 1997 04:00:00




> > I am using a 12V power supply to power ten identical circuits and need
> > to build current limiting functions so that a short or a failure in one
> > of the circuits will limit the current to that circuit to 2 Amps.
> > Without current limiting one short will trip the power supply and then
> > all ten circuits are down.  What is a good circuit to use for current
> > limiting?

> >     --------------         2A limit
> >     |            |------------------------ circuit 1
> >     |   12V      |         2A limit
> >     |   Power    |------------------------ circuit 2
> >     |   Supply   |                                 .
> >     |            |         2A limit            .
> >     |            |------------------------ circuit 10

> another thing to try is to install 12v light bulbs in series with the
> circuits use ones with a little bit higher wattage rateing than your
> circuit. then if the circuit shorts the light will come on indicating
> the short and dissapaiting the current.

At first glance one says that this will obvioiusly not work.  One
intuitively thinks of the lamp as resistive and thinks it will have the
same resistance as the load and will divide the voltage in half.  That is
wrong however.  It will work.  Anyone who has studied the I-V
characteristic of a tungsten lamp knows that a tungsten lamp is nearly a
constant current element.  Hence, it can pass near the full current with
small voltage drop.  This is because its cold resistance is about 10% of
its hot resistance.  I would recommend using an 18-24 volt lamp with the
same power rating as the load or a 12 volt lamp with nearly a power
rating  1.5 to 2 times that of the load.

--
Cheers, Steve Henning, Reading, Pennsylvania, USA

Visit my home page at http://www.users.fast.net/~shenning
Contains Rhododendron, Boy Scout and Rotary information and more.

 
 
 

WHAT LIMITS CURRENT?

Post by Bill Boy » Wed, 21 May 1997 04:00:00


Multiple loads on one power supply?  Sounds like my car, house, etc.  Try
a fuse in each load-circuit.  Then if one load shorts out, the fuse blows,
and isolates that circuit!  

One second thought, maybe too simple.
 --
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