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'[EE] Ammeter for 2A-20A range?'
2007\02\13@030149 by Jesse Lackey

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Hello all, I will soon have the need for relatively simple measurements
in this range (DC).  I don't need a "true RMS" (I don't think) but some
decent averaging of what will undoubtedly be somewhat peaky pulses would
be good.  I could mock up something with 0.1/0.01R resistors and look at
on scope with two probes set to differential and eyeball it but ... I
also like inexpensive, decent test gear.  :)

Suggestions?

I'd certainly consider spending more to get something that can go to 50A
or more for those occasional really big LED displays.  :)

Thanks!
J

2007\02\13@033902 by Jinx

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> Suggestions?

http://www.siliconchip.com.au/cms/A_30551/article.html

0-80A, PIC-based

2007\02\13@034749 by Rich Satterlee

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Hi-

Think shunt resistor. Voltmeter with ranging factor and if you do
need to hook up a scope to it no problem.  One (or two that might
be of interest are at:

http://mpja.com/directview.asp?product=15320+ME

http://mpja.com/directview.asp?product=15431+ME

Not affliated with mpja of course, just have eyed these low cost
shunts for awhile.  You did mention inexpensive after all.

Hope this helps.

 Cheers,

  Rich S.



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> --

2007\02\13@125720 by peter green

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> Hello all, I will soon have the need for relatively simple measurements
> in this range (DC).  I don't need a "true RMS" (I don't think) but some
> decent averaging of what will undoubtedly be somewhat peaky pulses would
> be good.  I could mock up something with 0.1/0.01R resistors and look at
> on scope with two probes set to differential and eyeball it but ... I
> also like inexpensive, decent test gear.  :)
just get an appropriate resistor and stick an ordinary multimeter set up for voltage measurement accross it.




2007\02\13@221552 by Rich

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   If you are designing something for production you need to be sure that
you source a suitable shunt.  Ideally, you can find a 30Amp @ 30mV out full
scale.  It is better to have a little headroom.  If you use an op amp you
should use a precision low drift op amp like the OPA177, for example.  There
are many choices.  Bipolar op amps are inherently more stable than FET op
amps because the leakage versus temperature is less for bipolar than it is
for FETs.
   A 3-1/2 digit LCD or LED panel meter is cheap and typically set at 200
mV equals 1999 counts.  You can scale an op amp driver for 200 mV full scale
output.  The digital PMs typically use a CAZ (commutating auto zero) to
insure accuracy.
   All can be packaged in a relatively small case because the heat will not
be significant.  The Heat from the shunt in calories should be I squared RT
divided by 4.18 for the conversion.  The shunt resistance is going to be
quite small.  I would not recommend using ordinary resistors for a shunt.
   Also you could include a high speed analog comparator for over-current
detection, if you want to add some bells and whistles.  At a 210 mV
reference, for example, you could light an LED and drop out a relay to
disconnect the load.  You could also use a comparator for under-current
detect. This could be a lot of fun to design or build.  If you add a RMS to
DC converter (LT has a great one) and lay it out on a four layer board you
can have a nice marketable product.


{Original Message removed}

2007\02\14@085954 by Martin Klingensmith

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If you use a shunt you really should use a precision chopper-stabilized
op-amp with the lowest DC offset error that you can afford to get the
best output. The TLC2652A is an example of a pretty good op-amp with 1uV
max. DC offset. If you amplify by 25x your error will be only 25uV max.
or 25mA on a 30A/30mV shunt. Drift is very important too, like Rich
mentioned.
--
Martin K

Rich wrote:
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> {Original Message removed}

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