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

In article , Zak
wrote:

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!

You can solder a wire to the end cap of one of your batteries.
Place tape over the end, or use your plastic disk idea to
insulate. Either solder the second wire somewhere to the battery
compartment wiring, or just jam the wire between the plastic disk
and the contact. A battery compartment is just a lousy place to
have to work in.

I actually tried to make a battery look-alike once, to allow
an adapter to connect to a device that was battery only (no
jack on it and no room to install a jack), and the idea was
a flop. My substitute batteries wouldn't stay in place. The
dimensions would have to be just right for that to work, and
I didn't get close enough to the exact dimensions needed. So
soldering would be one way to make a connection to things,
that won't immediately fall off or dislodge.

You might also look through your collection of alligator
clips. The contacts on some battery compartments have
enough metal present, to allow an alligator clip to grip.
You can then connect an external power source, with your
meter in series, to make the measurements.

Yeah, I guess I stopped reading the original post after
the first few lines :-)

Paul

On Thu, 23 Feb 2006 01:10:32 GMT, Zak
wrote:


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!


It can depend on what you're measuring, how much versatility
you need, the voltage range, the precision. A simple
open-circuit multimeter-shunt type measurement won't even
work in some cases... for example I've an MP3 player that
can't power on with a shunt in place, presumably the initial
startup current to power the backlighting is too low with
the shunt in place. If I were really anxious to get a
reading I could short across the shunt and then break short
after the backlight had come on, but once again the simple
idea of breaking the circuit is not as simple as it
initially appears.

If you just want a suggestion, grab a couple pieces of thin
insulated wire, tin both ends, and cut out a piece of this
stuff,
http://www.goldmine-elec-products.co...m=9&mitem =16
double sided copper clad board comes thinner, more expensive
at the various electronics supply houses but unless you need
to reduce the thickness an extra 0.01" or more, that should
fit inbetween the batteries or battery and contact in most
devices that have the spring-action battery contacts. So
you have a piece of wire soldered onto each side, the piece
of board cut long enough to extend out so it clears the
batteries.... perhaps cutting it in a long thin strip like a
popscicle stick shape. You can either tin the contact area
or periodically clean it off since corroded copper isn't a
good conductor. Solder the wires far enough out of the way
to each side and use your DMM's appropriate current range,
starting out with the high current range to be sure you
don't have a high-startup-current device that blows the
low-range fuse (typically is between 200-500mA in many
handheld meters).

Zak wrote:
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!


Well, one of the beauties of usenet is that you will get a range of
ideas, in this case ranging from a couple of bits of doll's house wiring
back to back, to something far more complicated but with advantages:

The "doll's house" puts the unknown current through the test meter -
which could be pretty terminal (no pun intended. ) for the meter if
there is a fault and much more current flows than is expected.

The "series shunt" basically only puts the shunt at risk - you aren't
/as/ likely to damage the meter.

(I would suspect that those more used to industrial instrumentation went
for the latter, whereas those more used to battery powered kit went for
the former. )

A dummy battery, such as:
http://www.maplin.co.uk/Module.aspx?...ery& doy=23m2
is no bad idea. The connecting wire passing through it can be broken and
wired out to an external battery holder and meter. Several of these,
plus one wired out mains adapter can be an easy way of mains powering
something without soldering wires (which tends to cause sparks with
warranty claims) or using croc clips (which just tends to produce sparks)..

HTH
--
Sue

In message , Zak
writes
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.

OK. You could probably do this with a small piece of double-sided
printed circuit board, solder a wire on each side. Maplins sell them.
Use the thickest copper wire you can find, speaker cable would do it.
make sure your solder joints are perfect.

That's the easy part.

The difficult part is measuring the current. Multimeters work by
measuring the voltage drop across a resistor. They usually aim for a
200mV drop at full scale. If your supply is 1.2V then a 200mV drop will
significantly affect the circuit and so the current measured with the
shunt will be lower than the current without it.

When I worked for Fluke we sold a shunt that dropped 200mV at 20A. It
had standard 4mm plugs and screw-terminal connectors. If you used
something like that you would get an output of 200mcV at 20ma, which
would probably be acceptable if you have a meter that can measure that.
If you only have a toy multimeter that only has a 200mV range then
forget it, you aren't going to get meaningful results.



--
Bernard Peek
London, UK. DBA, Manager, Trainer & Author.

20 AMPS!!!!!!!!!!!!!!

It's a walkman not a welding machine!!!!!!!!!!!!!!!



"philo" wrote in message
...

"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

"don" wrote in message
...
20 AMPS!!!!!!!!!!!!!!

It's a walkman not a welding machine!!!!!!!!!!!!!!!




a welder is several hundred amps...

a standard 20 am multimeter should be plenty good for a walkman though

philo wrote:


a welder is several hundred amps...

For some uses of the term "welder", yes.

:-)

Chris

--
Remove prejudice to reply.

I have a Mig welder that only draws 20 amps.

It delivers up to 88 amps at the electrode (wire fed) but only draws 20 amps
Max.

A typical walkman would not be in a range much higher that a few
milliamps.(just a guess)


"philo" wrote in message
. ..

"don" wrote in message
...
20 AMPS!!!!!!!!!!!!!!

It's a walkman not a welding machine!!!!!!!!!!!!!!!




a welder is several hundred amps...

a standard 20 am multimeter should be plenty good for a walkman though