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'Measuring Large (1A-7A) Currents ?'
1999\03\15@084101 by Jim Dolson

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Is there an easy way to measure currents in the range of 1A-7A.  The
circuit in question is for part of a constant-current battery charger.
Accuracy and precision isn't too important - measurements to 1/10 of an
amp are sufficient.

Thanks,
Jim

1999\03\15@091638 by Jochen Feldhaar

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Hello Jim,

the easy way to do this is to measure the small voltage drop along one of
the existing cables to or from the accumulator. If the rsistance is known,
or the current in one instance, it can be calculated from the obtained
voltage. The advantage of this procedure is the absence of high current in
the leads used for measuring

Jochen DH6FAZ
spam_OUTjfTakeThisOuTspamdetektor.de

1999\03\15@095150 by Tjaart van der Walt

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Jim Dolson wrote:
>
> Is there an easy way to measure currents in the range of 1A-7A.  The
> circuit in question is for part of a constant-current battery charger.
> Accuracy and precision isn't too important - measurements to 1/10 of an
> amp are sufficient.
>
> Thanks,
> Jim

If you use a FET for charging, you can use the drain/source
voltage drop. Measure the two sides with an over-the-top
opamp, and amplify enough for the A/D on your PIC.

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1999\03\15@110557 by Herbert Graf

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On Mon, 15 Mar 1999 08:34:57   Jim Dolson wrote:
>Is there an easy way to measure currents in the range of 1A-7A.  The
>circuit in question is for part of a constant-current battery charger.
>Accuracy and precision isn't too important - measurements to 1/10 of an
>amp are sufficient.

    Try the NS LM3812, 3813, 3814 and 3815, they have 1A and 7A models, output
is PWM (I think) which should be relativily easy to decode with a pic. TTYL



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1999\03\15@113127 by mwestfal

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Tjaart van der Walt wrote:
>
> If you use a FET for charging, you can use the drain/source
> voltage drop. Measure the two sides with an over-the-top
> opamp, and amplify enough for the A/D on your PIC.

Keeping in mind that Rds(on) varies with temperature...


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1999\03\15@120150 by mlsirton

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Hi,

On 15 Mar 99 at 8:34, Jim Dolson wrote:
> Is there an easy way to measure currents in the range of 1A-7A.  The
> circuit in question is for part of a constant-current battery charger.
> Accuracy and precision isn't too important - measurements to 1/10 of an
> amp are sufficient.

You could try a linear Hall-effect sensor, I don't remember a part
number off my head but I believe I saw something in the Digikey
catalog.  Basically you need some core shaped like a C, you
insert the sensor in the "hole" in the C and wrap your conductor
around the core a few turns (more turns = higher voltage).
The sensor I've used required 5V and GND and produced an analog
output proportional to the current (2.5V = 0 amp).

Hope this helps,
Guy - mlsirtonspamspam_OUTinter.net.il

1999\03\15@121026 by Wagner Lipnharski

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Jim Dolson wrote:
>
> Is there an easy way to measure currents in the range of 1A-7A.  The
> circuit in question is for part of a constant-current battery charger.
> Accuracy and precision isn't too important - measurements to 1/10 of an
> amp are sufficient.
>
> Thanks,
> Jim

With the availability of high gain op-amplifiers, you
can just measure the millivolts voltage drop along the
power cables.  A regular op-amp as the LT1413CN8, low
ppm drift, low VOS, hig voltage operation, by a low cost,
can be used to build a 1mA full scale amper-meter to
be used with a analog indicator, or to generate 0-5V
based on your 0-7A, easy and simple.  It will need
some calibration, that is not difficult to do.
If a 30 inches cable can generate a drop of 0.6Volts,
at 7 Amperes, it would be 20mVdc per inch, so the pick
up point in the cable can be just one inch apart, like
a "collar" with two needles to penetrate the cable
conductor and measure the voltage, amplify and display.
--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\03\15@122034 by Richard Martin

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I looked a bit at using automobile fuses, particularly the
"buck tooth" variety that seems to be used today. The idea was to
use them both as a current sensor and a circuit protector.
They seem to measure in the range of 40milliohms  (.040 ohms = 40mv/Amp)
with very high unit to unit consistency (with
my tools). I stopped (among other reasons) when I found
no easy source (as in my local auto parts) for the female connectors
but didn't look hard.

Dick

1999\03\15@153007 by Gerhard Fiedler

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At 09:16 03/15/99 -0800, Richard Martin wrote:
>I looked a bit at using automobile fuses, particularly the
>"buck tooth" variety that seems to be used today. The idea was to
>use them both as a current sensor and a circuit protector.
>They seem to measure in the range of 40milliohms  (.040 ohms = 40mv/Amp)
>with very high unit to unit consistency (with
>my tools).

how would that be with the contact resistance (2 contacts for the fuse)? it
might be in the same range as the fuse resistance, which would then
influence the measurements considerably.

ge

1999\03\15@153818 by paulb

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Richard Martin wrote:

> I looked a bit at using automobile fuses, particularly the
> "buck tooth" variety that seems to be used today.

 Generally called "blade" fuses.

> The idea was to use them both as a current sensor and a circuit
> protector.  They seem to measure in the range of 40milliohms  (.040
> ohms = 40mv/Amp) with very high unit to unit consistency (with my
> tools).  I stopped (among other reasons) when I found no easy source
> (as in my local auto parts) for the female connectors but didn't look
> hard.

 I think you'd need special fuseholders with four contacts to use the
fuses as a reference.  All low-resistance shunts are used in 4-terminal
mode as implied by the improbable suggestions made in this thread such
as using needles (in a corrosive environment?) to sample from a length
of cable.

 Otherwise your potentially unreliable contact resistance is added to
that of the fuse and you could see significant variations in the
controlled current, albeit all downward.
--
 Cheers,
       Paul B.

1999\03\15@163017 by Gerhard Fiedler

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At 07:33 03/16/99 +1000, Paul B. Webster VK2BZC wrote:
>  I think you'd need special fuseholders with four contacts to use the
>fuses as a reference.

ever seen any for standard fuses?

ge

1999\03\15@171946 by paulb

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Gerhard Fiedler wrote:

>>  I think you'd need special fuseholders with four contacts to use the
>> fuses as a reference.

> ever seen any for standard fuses?

 On a PCB, it's easy, you just use four clips, the two inner ones with
the end-stops bent out of the way.  Note that this partially defeats the
voltage rating, but...

 And you would really need a screwdriver to prise the fuse out.
--
 Cheers,
       Paul B.

1999\03\15@184608 by Jeff Jolie

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Dick,

Littlefuse makes the female receptacles for these fuses.
http://www.littlefuse.com/

Check out their info on "Autofuse".

Jeff Jolie
New England Automated
Machine Engineering, Inc.
@spam@jeffKILLspamspamneame.com


> {Original Message removed}

1999\03\15@210956 by Wagner Lipnharski

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Remember that you will not collecting the measurement voltage from the
fuse, but from the fuse holder, so, whenever you change
the fuse, or some temperature contracts or expands the fuse, the
resistance between the fuse and fuse holder contacts will change
considerabily, creating problems for you.  It would be wise to read the
voltage from the fuse itself, but it is a little bit difficult.

Also, I believe the fuse changes its resistance according to temperature
as any other conductor, so your reading will be a non linear voltage.
As the main function of the fuse is just create heat, even using a 20A
fuse into a 7A circuit, it will get a little warm. Changes in the
ambient temperature can create more troubles to you.

If you never did that, measure a resistance of a 10k Ohms common
resistor while touching its lead with a hot solder iron tip, and take a
look what happens with its resistance.  You can say it is an almost
perfect thermometer...

Measuring current is one of those nasty electronic jobs, because you
need to open the circuit and introduce lots of problems and connections,
read here "resistances and voltage drops".  If you can find a solution
that not involves opening the actual circuit, it would be nice, like for
example measuring the voltage drop in some cable alctually installed.

If you really need to introduce something in the circuit, there are
calibrated current shunts, a flat piece of thin metal, 1" by 1/4", one
hole in each side, generates 50mV for some specific currents, as 30A,
20A, 15A, and so on.  I can try to find where you can buy it.  They are
widelly used as Shunts in analog Amper-meters around.

Wagner

1999\03\15@223329 by rweaver8

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I have seen them in The Auto Zone store. They have an inline version
that can be used with the high power flood lights sometimes used on
offroad vehicles.Richard Martin wrote:
>
> I looked a bit at using automobile fuses, particularly the
> "buck tooth" variety that seems to be used today. The idea was to
> use them both as a current sensor and a circuit protector.
> They seem to measure in the range of 40milliohms  (.040 ohms = 40mv/Amp)
> with very high unit to unit consistency (with
> my tools). I stopped (among other reasons) when I found
> no easy source (as in my local auto parts) for the female connectors
> but didn't look hard.
>
> Dick

1999\03\16@030128 by Lee Jones

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> I looked a bit at using automobile fuses, particularly the
> "buck tooth" variety that seems to be used today. The idea was to
> use them both as a current sensor and a circuit protector.  40mOhms

> I stopped (among other reasons) when I found no easy source (as in
> my local auto parts) for the female connectors but didn't look hard.

It may not be a one-piece connector, but it works.  Use flexible
wire with female quick-disconnect connectors; insult the ends with
heat shrink tubing.  The QDs just push on to the tabs on the fuse.

                                               Lee Jones

1999\03\16@035145 by Gerhard Fiedler

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At 23:59 03/15/99 -0800, Lee Jones wrote:
>insult the ends with heat shrink tubing.

you might try to avoid insulting them, especially not with green heat
shrink tubing. they could get upset, and you can forget about correct
measurements... :)

ge

1999\03\19@022656 by Harold Hallikainen

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       Just today saw an ad for a new chip from National that will do this.  It
has a built-in 4 milliohm sense resistor and outputs a PWM proportional
to the current.  High side and low side versions available.  Check their
web site!

Harold


On Mon, 15 Mar 1999 08:34:57 -0500 Jim Dolson <KILLspamjdolsonKILLspamspamISERV.NET> writes:
{Quote hidden}

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'Measuring Large (1A-7A) Currents ?'
1999\04\19@134752 by Larry G. Nelson Sr.
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Take a look at Maxim. I just used a high side sense circuit from them to
detect charging levels in a medical equipment docking system.


At 08:34 AM 3/15/99 -0500, you wrote:
>Is there an easy way to measure currents in the range of 1A-7A.  The
>circuit in question is for part of a constant-current battery charger.
>Accuracy and precision isn't too important - measurements to 1/10 of an
>amp are sufficient.
>
>Thanks,
>Jim
>
>
Larry G. Nelson Sr.
RemoveMEL.NelsonTakeThisOuTspamieee.org
http://www.ultranet.com/~nr

1999\04\19@135753 by Norman Gillaspie

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Look at the LM3813 or LM3814. This does what you want.

If you don't have a large common mode voltage it is perfect.

This device has the sense resistor etc. Internal to the IC.
It gives you a Pulse Width out that corresponds to the current.

They have one that will measure 10 amps peak and 7 amps continuos

Norman


PCS Engineering
Norman Gillaspie
325M Sharon Park Dr. #210
Menlo park, Ca. 94025
Tel 650-854-5263
Fax 650-854-5445
Email spamBeGonenormanspamBeGonespampcseng.com


> {Original Message removed}

1999\04\21@231150 by Harold Hallikainen

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       Also have a look at National.  They have a new current sense chip (high
side and low side available) that measures 7 A.  It has an internal sense
resistor in the milliohms.  It outputs a PWM signal that is proportional
to the current (with 50% representing zero current and deviation from 50%
representing negative and positive current).  I'm looking at using it as
part of a "pwer tracker" for charging batteries from solar panels.

Harold

On Sun, 18 Apr 1999 10:47:15 -0400 "Larry G. Nelson Sr."
<TakeThisOuTnrEraseMEspamspam_OUTMA.ULTRANET.COM> writes:
{Quote hidden}

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1999\04\21@233435 by David Covick

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Harold,

What is the National part number?  (low side)
Does it handle AC?

Thanks in advance.

David


{Original Message removed}

1999\04\24@061205 by Dr. Imre Bartfai

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Hi,
we have similar project to measure high current (up to 20A). We do it in
traditional way, using an 0.01 ohm resistor and LM324 set to 20 amplifying
factor. Then we read it with PIC. The project is not ready, though.
Imre


On Sun, 18 Apr 1999, Larry G. Nelson Sr. wrote:

{Quote hidden}

1999\04\24@073035 by Bob Drzyzgula

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FWIW, here are two more things to look at:

* Unitrode sells a chip that can measure up to 20A:

 http://www.unitrode.com/products/powsup/ucc3926.htm

* The July 1998 Circuit Cellar INK had a design for
 a PIC-based AC Power meter. Rick May modified a
 design from an old issue of Electronic Design
 that used a sense resistor and optoisolators to
 sense the power use; I haven't thought it through,
 but you'd probably have to to modify the circuit
 to read current instead of power. It is one of the
 few CCI articles that is available online:

 www.circuitcellar.com/pastissues/articles/Rick96/rick-96.pdf
 http://www.circuitcellar.com/pastissues/articles/Rick96/article.htm

 The html doesn't have the schematics and other
 drawings, but the PDF is exact as from the magazine.

 BTW, I did manage to get a reprint of the original
 ED article by W.S. Woodward, but it was a huge pain
 to get it and I wouldn't say it added much; it was
 just a design note.

--Bob

On Sat, Apr 24, 1999 at 11:14:03AM +0200, Dr. Imre Bartfai wrote:
> Hi,
> we have similar project to measure high current (up to 20A). We do it in
> traditional way, using an 0.01 ohm resistor and LM324 set to 20 amplifying
> factor. Then we read it with PIC. The project is not ready, though.
> Imre

--
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EraseMEbobspamdrzyzgula.org                until something bad happens
============================================================

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