SEEK "shim" to measure battery current in appliance [OT]

Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Below are some details of what I mean.

Thanks
Zak

===============================================

I want to measure the current from the battery when something like
a radio or walkman is used. The battery might be anything from
AAA to D and could be any type(alkaline, zinc-carbon,
rechargeable, etc).

----

I once made a "shim" out of a small thin piece of plastic which
was sandwiched by thin metal conductors on either side. A wire
was attached to each conductor and the two wires went a
multimeter.

The idea is to insert this "shim" between the cells. Then run the
appliance and measure the current you settings like volume, fast
forwards, band select, etc.

----

Can I get something like this "shim" to insert between batteries
and measure current? My own "shim" was always a bit too thick and
wan't durable.

"Zak" wrote in message
...
Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Below are some details of what I mean.

Thanks
Zak

===============================================

I want to measure the current from the battery when something like
a radio or walkman is used. The battery might be anything from
AAA to D and could be any type(alkaline, zinc-carbon,
rechargeable, etc).

----

I once made a "shim" out of a small thin piece of plastic which
was sandwiched by thin metal conductors on either side. A wire
was attached to each conductor and the two wires went a
multimeter.

The idea is to insert this "shim" between the cells. Then run the
appliance and measure the current you settings like volume, fast
forwards, band select, etc.

----

Can I get something like this "shim" to insert between batteries
and measure current? My own "shim" was always a bit too thick and
wan't durable.


I think the easiest place to find some is in the K.S.Metal rack in your
nearest Model shop.

Then again coke cans are very thin metal too

Trev wrote:
"Zak" wrote in message
...

Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Below are some details of what I mean.

Thanks
Zak

============================================== =

I want to measure the current from the battery when something like
a radio or walkman is used. The battery might be anything from
AAA to D and could be any type(alkaline, zinc-carbon,
rechargeable, etc).

----

I once made a "shim" out of a small thin piece of plastic which
was sandwiched by thin metal conductors on either side. A wire
was attached to each conductor and the two wires went a
multimeter.

The idea is to insert this "shim" between the cells. Then run the
appliance and measure the current you settings like volume, fast
forwards, band select, etc.

----

Can I get something like this "shim" to insert between batteries
and measure current? My own "shim" was always a bit too thick and
wan't durable.



I think the easiest place to find some is in the K.S.Metal rack in your
nearest Model shop.

Then again coke cans are very thin metal too


And double sided sticky tape is fine as the insulator - after all, if it
does get shorted out, it isn't going to do any damage and will be easily
detected.

I would go for two layers of dolls' house lighting tape (copper tape,
easy to solder to), back to back. The tape is self-adhesive, about the
right size and rigidity and the backing is meant to be used as an
insulator (eg crossing or overlapping tape is quite acceptable.

It also works for making simple "printed circuit boards".

--
Sue

In article , Zak
wrote:

Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Below are some details of what I mean.

Thanks
Zak

===============================================

I want to measure the current from the battery when something like
a radio or walkman is used. The battery might be anything from
AAA to D and could be any type(alkaline, zinc-carbon,
rechargeable, etc).

----

I once made a "shim" out of a small thin piece of plastic which
was sandwiched by thin metal conductors on either side. A wire
was attached to each conductor and the two wires went a
multimeter.

The idea is to insert this "shim" between the cells. Then run the
appliance and measure the current you settings like volume, fast
forwards, band select, etc.

----

Can I get something like this "shim" to insert between batteries
and measure current? My own "shim" was always a bit too thick and
wan't durable.

The material you use, determines the accuracy of the measurement.
An alloy called "manganin" has a low temperature coefficient of
resistance, which means the resistance of the shunt doesn't change
with temperature. This material is shaped into convenient form
by a number of companies.

http://www.rhopointcomponents.com/result_resistor_1.asp?st1=Resistors&st2=Low%20Ohmi c%20(Through%20Hole)

In terms of properties, the shim or current shunt, should be
selected so that it doesn't adversely affect the voltage
delivered to the load. The voltage drop across the shim,
means there is less voltage to run the Walkman, and on some
devices, the appliance may become more unstable with the shim
in place (supply impedance increased).

Supply_voltage X-----shim----X--------walkman------X
|
Ground X-----------------------------------X

The lower the resistance value of the shim, the cooler it
runs. But if the shim has an extremely low value, there
may not be enough voltage developed across it, to measure
with a voltmeter. Amplfication of the voltage across the
shim is one answer to this problem, but that makes the
measurement less accurate, as the amplifier can drift more
with temperature, than the piece of manganin.

Some people buy raw manganin wire themselves, and put that
in their products. That eliminates the "middleman", such as
the rhopoint company above. Shunts can be expensive to buy
as finished products, but are a lot easier to work with.

To work through an example, let's say we want to measure
up to 100mA of current. We have a voltmeter with a 200mV
full scale readout.

The desired resistance value is: 0.2V/0.1A = 2 ohms
The power dissipated in shunt: 0.2V*0.1A = 0.02W

A shunt with a power rating more than 0.02W would work OK.
Like maybe a 1/2W or a 1W device, would give you plenty
of room to work.

The fun begins when you are trying to measure extremely
large currents. Then the resistance required can drop
to 0.001 ohms and the power dissipated becomes large.
But for your Walkman test case, that should not be a
problem. The only unknown, is whether the Walkman will
appreciate the 2 ohms in series with the supply - you
may notice a change in the audio properties of the
Walkman - I have no way to predict what will happen
for any given device. Some amplifiers, for example,
become unstable when the supply impedance increases.

Also, in the above diagram, the shim or shunt is shown
on the "high side" of the supply. The shim or shunt can
also be placed on the "low side", like this:

Supply_voltage X----------------------walkman------X
|
Ground X-----shim-----X--------------------X

Placing the shim on the low side can be an issue, as it
"lifts" the ground of the device being measured, and that
can be an issue if a device is AC powered or the ground
is referenced to another instrument. The "high side"
position of the shim, eliminates any problems with the
ground, but would be more of a problem if you were to
build an amplifier, to amplify the small voltage across
the shim.

There is some background he

http://www.maxim-ic.com/appnotes.cfm/appnote_number/746

There are also small integrated circuits that make
measuring current easy. I think I have seen a device
like this, that can measure up to about 7 amps or so.
The devices in this example are for an ampere or two.

http://www.national.com/ds.cgi/LM/LM3824.pdf

What that device does, is measure current, and put out
a pulse width modulated signal. The time average of the
signal, is a voltage that corresponds to the current
being drawn. For example, if the LM3824 runs from a
3V supply, and the current drawn is the maximum value,
the PWM output is on 100% of the time, so the output is
3.0V. If the current drops to zero, the PWM output is
turned on 50% of the time, and the average value is 1.5V.
If the current flows negative (back into the battery),
the PWM output is turned on less than 50% of the time,
and the time averaged voltage is below 1.5V. That is
a voltage range which is easy to measure with a multimeter.
The time average might be done with a simple RC
low pass filter. The filter has to have a time constant
which is much larger than the 166Hz update rate of the
PWM output, to smooth the pulses and make a steady DC
voltage for measurement.

The LM3824 is also a "high side" device, which means
it won't foul up the ground connection of the Walkman
to other audio devices. And it has bipolar measurement
capability, and is able to measure current flowing out
or current flowing into the battery. There are also
devices that are unipolar, and only measure current
flowing out of the battery. I suppose they would be
a bit less confusing to use.

So the LM3824 is functioning as an amplifier, and
making the tiny voltage across its internal shunt,
into a voltage that is easy to measure with a
multimeter.

Other companies also make stuff like this, and
"high side" or "current sensor" might make good search
terms.

Paul

In article
Zak wrote:
Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Wouldn't it be better to get some cell holders and make up some dummy
cells with contacts on the end so you can insert the dummy into the
appliance, the cells into the holder and measure current without having
to wedge anything extra in the already tight bay in the appliance?

On 22 Feb 2006, Rob Morley wrote:

Zak wrote:
Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Wouldn't it be better to get some cell holders and make up some
dummy cells with contacts on the end so you can insert the dummy
into the appliance, the cells into the holder and measure
current without having to wedge anything extra in the already
tight bay in the appliance?


It migth be a bit of extra work to do it this way as I would need to
make up dummy cells for several battery sizes (AAA, AA, C, D).

-------------
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In article , Rob Morley
wrote:

In article
Zak wrote:
Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Wouldn't it be better to get some cell holders and make up some dummy
cells with contacts on the end so you can insert the dummy into the
appliance, the cells into the holder and measure current without having
to wedge anything extra in the already tight bay in the appliance?

OK. Now I understand what he wants. Something very thin, that
functions as a current shunt. Perhaps a flat strip of a resistive
material, folded into a V and shoved between the battery and the
battery holder. With a piece of insulation between the two halves
of the V, so when the V is squashed flat, the full length of the
material is used.

Maybe bend a thing like the item at the top of this page, into
a V shape, and place the two legs of the V, between battery
terminal and cell holder terminal. The manganin ribbon would
have to be very thin, to raise the resistance enough to make
it useful for measuring a Walkman. (Also, bending Manganin
spoils the calibration, so a known current would have to be
run through it, and the voltage measured, to get an idea of
the resistance after the ribbon is formed.)

http://www.mesaproducts.com/catalog/...s/shunts1.html

Generally, you don't find products like this at the hardware
store, and even searching on the web, the companies that
make the stuff do a poor job of selling it.

Building a measurement solution that lives outside the
Walkman, does sound a lot simpler.

You can measure current with a Hall Probe, a device with
clamp jaws that fit around the wire to be measured. The
device measures the magnetic field created by the current
flowing though the wire. In the case of the Walkman, you'd
need to bring a wire out of the unit, that had all of the
load current flowing through it. Then the clamp-on meter
could be used to measure the current. To increase the
sensitivity of a clamp-on meter, just wind the wire around
the jaws N times, to multiply the readout by N.

Paul

On Wed, 22 Feb 2006 17:51:50 GMT, (Paul)
wrote:


You can measure current with a Hall Probe, a device with
clamp jaws that fit around the wire to be measured. The
device measures the magnetic field created by the current
flowing though the wire. In the case of the Walkman, you'd
need to bring a wire out of the unit, that had all of the
load current flowing through it. Then the clamp-on meter
could be used to measure the current. To increase the
sensitivity of a clamp-on meter, just wind the wire around
the jaws N times, to multiply the readout by N.


I wouldn't expedt that to be very accurate with low voltage
and current DC power leads.

I suppose it could depend on the level of accuracy needed
though, I often don't need much and just put a piece of
(forget exact thickness, maybe...) 0.16" DS copper clad
board in with a couple leads to a DMM. I suppose it can
then matter quite a bit what DMM is used too, my $3 bargain
bin special doesn't come remotely close to some low current
readings as a Fluke. This is still the issue you mentioned
previously, that the minor voltage drop can effect current,
it works best for several-cell battery packs so the drop is
least % of total.

"Zak" wrote in message
...
Can I buy something here in the UK like a conducting "shim" to
insert between batteries and measure current?

Below are some details of what I mean.


If the current is under 20 amps...
many DVM have a 20 amp meter in them...and no shunt is needed.

you can make a shunt out of virtually any wire...
but the problem is you need to calibrate it.

a few feet of #16 wire should do the trick...
to calibrate it you'll need a know resistance and voltage source

On 22 Feb 2006, Johnny B Good wrote:

The message
from (Paul) contains these words:

In article ,
Rob Morley wrote:

In article
Zak wrote:
Can I buy something here in the UK like a conducting "shim"
to insert between batteries and measure current?

Wouldn't it be better to get some cell holders and make up
some dummy cells with contacts on the end so you can insert
the dummy into the appliance, the cells into the holder and
measure current without having to wedge anything extra in the
already tight bay in the appliance?

OK. Now I understand what he wants. Something very thin, that
functions as a current shunt. Perhaps a flat strip of a
resistive material, folded into a V and shoved between the
battery and the battery holder. With a piece of insulation
between the two halves of the V, so when the V is squashed
flat, the full length of the material is used.

====snip====

FFS !!!

Zak simply wants to open circuit the battery so he can connect
a meter across the break to measure the current.


You got it. I am a bit surprised that there have been some very
complicated ideas in some replies. I did my best to make my needs
clear!