Zak said:
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
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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 Ohmic (Through Hole)
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 here:
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
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