Re: Ammeter Configurations

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike" <induction-at-comcast-dot-net> 

Hi Tristan,
                  About your amp meter, first I will assume you also own a 
digital multimeter as they are not expensive and most technical people have 
one.
Rather than do this with DC meters and diode bridges, plus opening the 
neutral (a safety no-no) I would offer this. Use a shunt. We want to keep 
the actual and heavy load off the multimeter so let a shunt do the work. It 
is very easy to do.
Take advantage of the fact that your power feeds have resistance. If your 
power system there in "Down Under" keeps the neutral near ground like it 
does here in the States, all the better.
If your target current is to be 35 amps, then simply measuring along your 
feed wire between two points on it will give you all the current reading 
you need. If you have a 35 amp load, read along the wire until you read 35 
milivolts, at which point you will also be reading a .001 ohm section of 
the wire. Want to calibrate it easy? Place a known 5 amp load (light bulb, 
anything steady) and move the meter along the bared wire until you read 5 
milivolts. Presto, you are at the .001 ohm point between the two meter leads.
You can always loan a calibrated meter to set this up then you know your 
.001 ohm sample points. Sure, you could add a 1 ohm resistor in series but 
at 35 amps it would have to be a 1.225 kilowatt resistor, though it would 
yield a 35 volt reading at 35 amps. With the tap points on the 
non-interrupted wire, you can read one milivolt per amp, direct conversion. 
You then have the option of direct reading on your multimeter or you can 
take the signal into an op-amp and amplify it, feed comparators to switch 
high or low limits, all sorts of things. Because you are reading the signal 
across such a low resistance, to the meter it is nearly a short and noise 
getting in across that short is not likely.
At this .001 shunt distance on the wire you get one milivolt per amp.  That 
is also typically the lowest your meter will read. If you wish to resolve 
to 1/10 amp steps or between, move the sample points further apart. If you 
wish 10 milivolt reading at 1 amp, that will be at the .01 ohm place on the 
wire.
I do this all the time in the system we have here. 4 digital meters with 
computer interface, each function has its own old computer so we have 4 
readings on 4 screens and logging. We can read them across the room with 
ease, use direct number display or a scope history pattern.
Shunts are really nice, we have several calibrated ones but you can make 
your own if you know the current.
Amps or hundreds of amps, the only limit is the wire size. When you find 
the proper points, simply solder small sample wires that will go to your meter.
Regards,
              Mike

----- Original Message ----- From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, October 04, 2004 9:45 AM
Subject: Ammeter Configurations


>Original poster: "Tristan Steele" <ozonejunkie-at-westnet-dot-com.au>
>G'day All,
>
>I am in the process of trying to work out a way to measure AC voltage and
>current flowing into my coil, and have run into a few problems in the
>availability (and price) of AC meters.  I have worked out a method to use
>the much cheaper DC versions that I can easily get.  Can someone please
>offer some advice on if this setup will work, or if I have missed some vital
>point. (Wouldn't surprise me!).
>
>The schematic can be found at:
>http://members.westnet-dot-com.au/gemabrown/meterschem.png
>(Only ~9.73kB)
>
>Any guidance would be appreciated,
>
>Thanks
>Tristan
>