Re: Amp Meter

From: Rich Grise (richgrise_at_example.net)
Date: 02/19/05 

Date: Sat, 19 Feb 2005 03:06:54 GMT

On Fri, 18 Feb 2005 21:37:55 +0000, Roger Johansson wrote:

> Rich Grise <richgrise@example.net> wrote:
>
>> I've also seen a circuit where a microammeter is turned into a
>> voltmeter - in this case, 10.4K gives 1.5V full-scale. A shunt resistor
>> that drops 1.5V at 60A would be .025 ohm, which is much more manageable
>> of a value than 0.0083 ohm, and the calibration isn't anywhere near as
>> persnickety.
>>
>
> In this case you are wasting a major part of the signal. That is not a
> good idea, seen from an engineering point of view.
>
> We want a shunt which gives as low voltage drop as possible. Your
> idea would make the voltage drop many times bigger, from 50mV to 1500mV.
>
> The 0.0083 Ohm shunt is easy to construct from heavy gauge copper wire.
> Or a thinner wire laid double, in quadruple, etc..
>
> Adjustment can be made by moving the probe measuring point a little, or
> by connecting an adjustment pot in parallell with the shunt. A wirewound
> pot will be suitable for this.

Well, in my defense, in a different post, I did mention that this
technique depends on the headroom of the supply. And you can certainly
decrease the ranging resistors, to decrease the shunt drop. It's just
a seat-of-the-pants sort of thing - you decrease the pickiness of the
shunt by adding _some_ resistance in series with the meter.

I've calibrated ammeter shunts, and it's a PITA. They wouldn't let
me grind on the brass strip with the Dremel while power was on. ;-)

I've also seen ammeters that have no leads at all - they have a clip
on the back that clips over the cable, and the meter itself is nothing
but a spring-loaded compass. You calibrate them by moving the cable
around until the meter reads right, then hot-glue it.

Cheers!
Rich

Relevant Pages

  • Re: what size shunt do I need?
    ... I have a 70A alternator and 220AH house battery capacity. ... "you need a 50mv, 70A shunt", where do I get one? ... If you have a meter calibrated for full scale at 50A to use with a 50A, ... Navy, can do this calibration, and does, even on matched 50mv units. ...
    (rec.boats.electronics)
  • Re: what size shunt do I need?
    ... The 100A shunt can be filed down between the meter contacts to increase its ... one for a circuit breaker tester a long time ago. ... failed calibration. ...
    (rec.boats.electronics)
  • Re: Current measurement
    ... troubleshooting I would say the roll your own shunt and a cheap meter ... A copper shunt looks like a good idea until you look at copper's ... used to make inexpensive RTDs (resistance temperature devices). ... I keep a 100 amp and a 500 amp shunt in my tool box all the time. ...
    (rec.outdoors.rv-travel)
  • Re: tubes Vs fets, ohmmeter schematics
    ... voltage of say 9V across the DUT, ... 100MegOhm FS = 100K Ohms shunt ... The 1M shunt would make the meter have about 800k of input resistance, ...
    (rec.audio.tubes)
  • Re: Current measurement
    ... A copper shunt looks like a good idea until you look at copper's ... used to make inexpensive RTDs (resistance temperature devices). ... I keep a 100 amp and a 500 amp shunt in my tool box all the time. ... also have a DC clamp-on meter or two but when I really need the right ...
    (rec.outdoors.rv-travel)