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'[EE]: Measuring instantaneous current'
2002\09\16@193324 by Jinx

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What's the best /easiest ? way to measure instantaneous
current into components such as motors (starting current)
and solenoids (initial energising current). If the current were
to be converted to a voltage pulse could this be accurately
and meaningfully read on an oscilloscope ? It would be
helpful to know how to do this so drivers can be specced
properly rather than simply over-engineering

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2002\09\16@194202 by Welch, Ken

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Simpson (and others)make a series of current shunts up to about 500 amps

http://www.simpsonelectric.com/pdf/webpdfg/Shunts.pdf

check with a solar supply distributor -- they usually have these (or
equivalent) in stock...

Ken
Honolulu

{Original Message removed}

2002\09\16@195303 by hard Prosser

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A digital storage scope is the best - as long as you can convert the
current into a useful voltage. For small/medium currents this is not such a
problem - a resistive shunt is normally adequate. For High currents (esp
fault currents) this can be more difficult although the duration is
hopefully short enough to minimise the dissipation.

For AC current, a CT is hard to beat although you have to watch out for
core saturation & burden dissipation - and that the burden (load resistor)
is ALWAYS connected.

For HV DC - things can get akward. Measuring the current in the return leg
is normally the safest way rather than trying to measurre at full voltage.

Watch out for capacitively coulpled spikes/transients - they can give
misleading results.

Hope this helps

Richard P



What's the best /easiest ? way to measure instantaneous
current into components such as motors (starting current)
and solenoids (initial energising current). If the current were
to be converted to a voltage pulse could this be accurately
and meaningfully read on an oscilloscope ? It would be
helpful to know how to do this so drivers can be specced
properly rather than simply over-engineering

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ways.  See http://www.piclist.com/#archives for details.

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2002\09\17@075049 by Olin Lathrop

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> What's the best /easiest ? way to measure instantaneous
> current into components such as motors (starting current)
> and solenoids (initial energising current).

A small current sense resistor in series with the coil.

> If the current were
> to be converted to a voltage pulse could this be accurately
> and meaningfully read on an oscilloscope ?

Yes, but watch out for the ground path thru the scope.


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2002\09\17@110526 by Mike Mansheim

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Jinx wrote:
> What's the best /easiest ? way to measure instantaneous
> current into components such as motors (starting current)
> and solenoids (initial energising current).

best and easiest, in my opinion, but certainly not the
cheapest, is a current probe attached to your scope.  You
also need a wire to get around, but you would with motors
and solenoids.  This also makes it completely non-invasive.
We use an Agilent 1146A, which costs about $500, and isn't
very fast (100kHz bandwidth), but is fast enough for
measuring in-rush into motors around here.
We also have an extremely fast Tektronix probe (100 MHz?)
that requires a separate amplifier, but I suspect it was
frightfully expensive.

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2002\09\17@112326 by Alan B. Pearce

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> What's the best /easiest ? way to measure instantaneous
> current into components such as motors (starting current)
> and solenoids (initial energising current).

Check out the "current transformer" modules made by LEM. They are sold by RS
Components (just do a word search for LEM if you have their CD catalogue).
You will need an external +/-15V supply to run these though. They include a
hall effect device, so go down to DC.

http://www.lem.com/

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2002\09\17@114140 by 4HAZ

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----- From: "Jinx" <joecolquitt@

> What's the best /easiest ? way to measure instantaneous
> current into components such as motors (starting current)
> and solenoids (initial energising current). If the current were
> to be converted to a voltage pulse could this be accurately
> and meaningfully read on an oscilloscope ? It would be
> helpful to know how to do this so drivers can be specced
> properly rather than simply over-engineering

Personally I have a handful of low resistance power resistors for this
purpose, just place a the resistor in series with the load and scope the
voltage across it, if your scope has a single triggered sweep mode you can
dim the room lights and see from the afterglow the peak and duration.
A 1 ohm resistor will drop 1 volt per amp, a 0.1 ohm will drop 100mv/a.
The resistor need only be capable of a little more than the average full
load current, 1 ohm 1 watt is good for 1 amp, 1 ohm 10W is good for ~3A, 1
ohm 100W is good for 10 amps average.

Lonnie - KF4HAZ -

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2002\09\17@145021 by Peter L. Peres

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On Tue, 17 Sep 2002, Jinx wrote:

*>What's the best /easiest ? way to measure instantaneous
*>current into components such as motors (starting current)
*>and solenoids (initial energising current). If the current were
*>to be converted to a voltage pulse could this be accurately
*>and meaningfully read on an oscilloscope ? It would be
*>helpful to know how to do this so drivers can be specced
*>properly rather than simply over-engineering

There exists no 'instantaneous' anything, except in physics books. In
reality everything has a finite duration. Once you know that duration in
your relevant context you can choose a method that works. The initial
current into inductor-based devices is relatively easy to measure after
you determine Rdc. Use an integrator.

Peter

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2002\09\17@151516 by Mike Singer

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Peter L. Peres wrote:
> There exists no 'instantaneous' anything, except in
> physics books.

You didn't hear about physicist Einstein? This guy
wrote a lot of physics stuff on non-'instantaneous'
things.

-
- Mike.

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2002\09\17@180911 by Jinx

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> best and easiest, in my opinion, but certainly not the
> cheapest, is a current probe attached to your scope.  You
> also need a wire to get around, but you would with motors
> and solenoids.  This also makes it completely non-invasive

So, a Hall Effect sensor would work ? The data for Allegro
sensors quotes rise-time figures of a few 10s of ns

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2002\09\18@140646 by Peter L. Peres

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On Wed, 18 Sep 2002, Jinx wrote:

*>So, a Hall Effect sensor would work ? The data for Allegro
*>sensors quotes rise-time figures of a few 10s of ns

The rist time of the output of a bare hall sensor is limited by the
capacitive load on the output and by its Zo. Usually you have something
like 1kOhm/5pF for a bare bridge connected directly to a nearby amplifier.

Peter

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