Alternative Power sources

Author

Shunt??

Jack Russell

2005-08-10, 10:21 pm

I have been looking at meters for my solar system. It is very simple and
small at this point but it will expand.

QUESTION: What is a shunt and why do I need or not need one in a meter.

Thanks, Paul...

Bill Kaszeta / Photovoltaic Resources

2005-08-10, 11:21 pm

Scott Willing

2005-08-10, 11:21 pm

On Wed, 10 Aug 2005 17:32:23 -0700, "Jack Russell"
<jack_russell@cox.net> wrote:

>I have been looking at meters for my solar system. It is very simple and
>small at this point but it will expand.
>
>QUESTION: What is a shunt and why do I need or not need one in a meter.
>
>Thanks, Paul...

A shunt is a resistor having a very low but precise resistance.

A meter - analog or digital - doesn't generally measure current in any
direct sense. Rather, it measures the voltage across a resistance that
is placed in series with the current of interest.

You want that resistance to be very small so as not to interfere with
the circuit and draw any significant power from it.

When currents are large - e.g. when you're making toast with a 12V
system and drawing 150A from the batteries - the resistor you're
measuring current with has to be a very low value or it would
dissipate a LOT of power.

In this example the system would most likely have a "500A/50mV" shunt.
This means that only 50mV would be dropped across the shunt at 500A.
From this we can calculate that the shunt has a resistance of:

R = E/I = 50mV/500A = 0.05V/500A = 0.0001 Ohm = 100microOhms

....a very small value indeed.

The voltage across the shunt at 150A would be

E = IR = 150A * 0.0001Ohm = 0.015V = 15mV

And the power dissipated by the shunt...

P = EI = 15mV * 150A = 2.25W

A resistor dissipating even a few Watts isn't something you want to
touch.

With the exception of magnetically-coupled ones, any
current-measurement meter needs a shunt. It's just that if you're not
measuring a lot of current, the shunt may be small enough to fit
inside the meter.

-=s

>

Jack Russell

2005-08-10, 11:21 pm

The meters that I have been looking at have the shunt integrated into the
meter (I assume). So what you are saying is that I would be better with a
"Shunted" meter...

"Jack Russell" <jack_russell@cox.net> wrote in message
news:u0xKe.145$sw6.78@fed1read05...
>I have been looking at meters for my solar system. It is very simple and
>small at this point but it will expand.
>
> QUESTION: What is a shunt and why do I need or not need one in a meter.
>
> Thanks, Paul...
>

barry@sme-online.com

2005-08-11, 6:21 pm

Plain-old passive (d'Arsonval) have a sensitivity characteristic which
is usually given in ohms/volt of the actual voltmeter (in parallel
w/shunt in ammeter). The higher, the better, translating into lower
shunt resistance for a given range of current (when used as an
ammeter.)

See how this works? The less effective resistance a current-measuring
device puts in series w/circuit, the better; for voltage measurement,
the opposite for parallelled resistance. You'd like zero series
resistance, and the higher the sensitivity (above) of the actual meter,
the closer you can come to it. Or the mfg. of the meter.

Then, there are meters with active electronic devices- I still have on
old VTVM around somewhere. :')

If you don't have significant energy loss at shunt, and can accurately
measure current, you're done. When not measuring current, you might
want to investigate something like a bar of copper to bypass the shunt,
to avoid losses and protect meter.

HTH,
J

daestrom

2005-08-11, 6:21 pm

"Jack Russell" <jack_russell@cox.net> wrote in message
news:moyKe.277$sw6.200@fed1read05...
> The meters that I have been looking at have the shunt integrated into the
> meter (I assume). So what you are saying is that I would be better with a
> "Shunted" meter...
>

Some intermediate sized ammeters have the shunt 'built-in'. This sounds
like what you're seeing. These are called 'internal-shunt ammeters'. The
'good' is that you don't have to worry about matching the correct mV meter
with the correct shunt. The bad news is you have to wire the meter into the
main current path. So you have to put the meter somewhere not too far out
of the way of the circuit path.

External-shunt meters, have the others have said require two parts, the
shunt and a precious mV meter. Depending on the type of mV meter, the total
resistance of the leads to and from the mV meter can be important. Most
such meters also include a factor of 'total lead resistance'. Too much or
too little resistance and the accuracy is affected. But the nice thing
about these is the shunt is the only part that needs to be in the power
circuit. So a shunt between your batteries and charger (perhaps in the
garage), can have small-gauge meter leads (properly fused) run to some
central monitoring panel. Meter leads running over 100 ft are not uncommon.

For really high current applications, 'internally shunted' meters are rare.
Almost always see external shunts when you get up into the hundreds of amps
range.

daestrom

Ecnerwal

2005-08-11, 11:21 pm

In article <u0xKe.145$sw6.78@fed1read05>,
"Jack Russell" <jack_russell@cox.net> wrote:

> I have been looking at meters for my solar system. It is very simple and
> small at this point but it will expand.
>
> QUESTION: What is a shunt and why do I need or not need one in a meter.
>
> Thanks, Paul...

A shunt is a very small resistance, which is used to generate a small
voltage from a large current, so that a small voltmeter (marked in Amps)
can indicate the amperage passing through the shunt.

In practice, with today's electronic voltmeters, you can do without one
by using one of the cables you'll have in there anyway as a shunt, but
not adding any _extra_ resistance in the form of a shunt and its
associated connections. You will need to convert a voltage measurement
(from each end of the cable) into a current measurement (using the
resistance of the cable), and this may not be _quite_ as precise across
wide temperature ranges as a "real" shunt, but with a modern digital
voltmeter (more or less infinite input resistance) it's Pretty Darn
Good; especially if the operating temperature is reasonably close to the
temperature at which you measured the cable resistance (or take
resistance measurements at various temperatures if you want to be picky,
and use the closest one).

--
Cats, coffee, chocolate...vices to live by

Jack Russell

2005-08-11, 11:21 pm

THANKS FOLKS!! for all your help

"Jack Russell" <jack_russell@cox.net> wrote in message
news:u0xKe.145$sw6.78@fed1read05...
>I have been looking at meters for my solar system. It is very simple and
>small at this point but it will expand.
>
> QUESTION: What is a shunt and why do I need or not need one in a meter.
>
> Thanks, Paul...
>

Duane C. Johnson

2005-08-11, 11:21 pm

Hi Jack;

Jack Russell <jack_russell@cox.net> wrote:

> THANKS FOLKS!! for all your help

Do an ebay search:
http://search.ebay.com/current-shunt

> "Jack Russell" <jack_russell@cox.net> wrote:
[color=darkred]
[color=darkred]
[color=darkred]

Duane

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