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'[EE]: Measuring automotive current....'
2002\05\09@193639 by

Hi all,

Back from a "break" which has me running on 9 fingers and 9 toes for
a while. :-(  But fortunately, I'll be back to 10 and 10 in some weeks.

Trying to figure out the best way to measure current in a car (12V neg gnd
system).  The current path to be measured is that going into/out of the
battery.  For now, I'll use a "safe" figure of 100A, although the current
drain/alternator charge current should not be anywhere near that.

First thought is to use a very small resistance with high current capacity.
I believe that is what a shunt is, so a quick web search found me some
shunts rated at a certain millivolt value and max current.  For example,
100mV and 100A.  My guess is that this means that the voltage drop at
100A will be 100mV, so the resistance is 0.001 ohms.  That's small !
So I'll use these figures for now.

However, with this small of a resistance, max power = 100A x 0.1V = 10W.
Not bad.  And 100mV of voltage drop will not be a problem.  Now all I have
to do is measure the voltage drop across the 2 resistor terminals.

The questions are:
- I have an ammeter that is "shuntless".  Does this mean that it has an
internal resistor for voltage drop?  Or some other method of current
measurement?  If internal resistance, how does it deal with resistance
of the (long) connecting wires?
- How do I get a PIC to measure -100mV to 100mV accurately?  A small
voltage reference would be necessary, but would the PIC be accurate
given noise, etc?  Or does the 100mV now need to be amplified?
- How about getting an accurate resistor -- if I use an actual resistor
instead of a shunt, can I get a resistor that small and that is accurate
to within a couple percent?

Cheers,
-Neil.

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For "engineering" purposes while testing charging
systems on autos I have simply used a digital
millivoltmeter and two points tapped in either
the negative or positive battery leads -

- choosing points with enough separated to provide
a suitable 'scale factor' while drawing a known
'calibration current' (like 10 Amps) ...

Jim

{Original Message removed}
Hi Neil:

Rather than insert a shunt (that should be used way below it's max spec) you might
consider:
(1) measuring the voltage drop across the longer heavy wire connected to the battery,

(2) using a Hall effect probe, these are built into snap on amp meters that can read
DC currents

Some design factors are cost, bandwidth, accuracy, reliability, ease of installation,
etc.

Have Fun,

Brooke Clarke
whttp://www.prc68.com

{Quote hidden}

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You might want to check out Fr. Tom Mcgahee's circuit

http://www/pic101.com/mcgahee

Rick

Pic Dude wrote:

{Quote hidden}

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>You might want to check out Fr. Tom Mcgahee's circuit
>
>http://www/pic101.com/mcgahee

you tried to type it :)

http://www.pic101.com/mcgahee/

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How about measuring the voltage drop over the existing wire ? Try to connect
at
two points on the wire as far as possible from each other and if possibly,
use
some kind of a differential amplifier.

You can also find special chips for current sensing. Maxim has for an
example a bunch
of them with internal or external shunt and of various gains (e.g. 25, 50,
100).

You may also want to filter the output voltage to get rid of all kind of
noise (which
is probably very common when you get under the hood).

There are other way to measure current based on the magnetic field created
by
the current. Take a look for example at Tektronix site, they may have one
though I am sure it will not be cheap.

Tal

> {Original Message removed}
>Trying to figure out the best way to measure current
>in a car (12V neg gnd system).  The current path to
>be measured is that going into/out of the battery.
>For now, I'll use a "safe" figure of 100A, although
>the current drain/alternator charge current should
>not be anywhere near that.

Try using a "DC Current transformer" in the shape of a hall effect sensor. I
am currently about to try some from LEM http://www.lem.com/ and go to
sensors. These are a ferrite core with a hall effect sensor in a gap, and an
op amp to give a voltage output. Some of them go to the sort of current
level you are looking for. Worst problem may be supplying +/-15V for the
sensor output, although some versions will accept single supply 5V, but not
sure if they are available in the rating you want.

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> Trying to figure out the best way to measure current in a car (12V neg gnd
> system).  The current path to be measured is that going into/out of the
> battery.  For now, I'll use a "safe" figure of 100A, although the current
> drain/alternator charge current should not be anywhere near that.

Peak starting current could even be higher than 100A.

> The questions are:
> - I have an ammeter that is "shuntless".  Does this mean that it has an
> internal resistor for voltage drop?

Most likely.

> If internal resistance, how does it deal with resistance
> of the (long) connecting wires?

It doesn't.  It's measuring current, so any extra voltage drop in the wires
won't effect the accuracy of the reading, although it might effect the
circuit under test.

> - How do I get a PIC to measure -100mV to 100mV accurately?

You amplify and level shift it.  This can be done with an opamp circuit.

> - How about getting an accurate resistor -- if I use an actual resistor
> instead of a shunt, can I get a resistor that small and that is accurate
> to within a couple percent?

At resistances this small, any "resistor" is going to be a measured length
of wire.  Plain old copper wire would be the simplest, but other metals have
different temperature sensitivity.

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(978) 742-9014, http://www.embedinc.com

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Ahhh... so that's just using the resistance of the
wire as the very-low-ohms resistor.  Good thought.

Perhaps I could even use a PCB trace for this --
hmmm....  Need to figure out the resistance of a
PCB trace while ensuring its wide enough to handle
the current.

{Original Message removed}
The hall effect probe is a good idea, except from a
cost perspective.  The wire/resistance and a small amp
seem to be more cost-effective.

Thanks.

{Original Message removed}
Nice -- the same amp design, but the work's already been
done for me.  Should be a dot instead of slash after the
"www".

Thanks.

{Original Message removed}
Okay, this is even better than the op-amp design -- I just
looked at the Maxim datasheets real quickly and it seems
like a winner so far.  They even show the possibility of
using a PCB trace for the sense resistor.  Sweet.

The new question then is how to figure out the resistance
of a PCB trace.  From AWG/resistance tables for copper wire,
I can calculate the resistance of the copper layer with the
known thickness/width, but the solder coating adds an unknown
factor, as I haven't seen any PCB house specs on that.

Thanks,
-Neil.

{Original Message removed}
"Peak starting current could even be higher than 100A."

I assume you mean the current for the starter, in which case
this is true.  However, the vehicle starter should be wired to
the battery directly, and not thru the ammeter.  At least, this
is the way to do it correctly.

Thanks,
-Neil.

{Original Message removed}
How about measuring an existing piece of known
trace from your favorite board house?

Flow x milliamps (perhaps 100's of ma) across
a piece of foil and measure the drop right
across the length of foil with your DMM in
the most sensitive DCV setting.

In the end, you're going to have to have a means
percent anyway - so a means to calibrate (adjust
a 'tap' point or potentiometer across the trace,
etc) is going to be needed ...

Jim

----- Original Message -----
From: "Pic Dude" <picdudeAVN-TECH.COM>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Friday, May 10, 2002 12:51 PM
Subject: Re: [EE]: Measuring automotive current....

{Quote hidden}

> {Original Message removed}
It appears that this circuit is designed to measure the amperage in the power return (ground) lead of your supply. Connecting this to your positive lead will cause a nice 0.001 Ohm short across the power supply :)
>>> picdudeAVN-TECH.COM 05/10/02 01:30PM >>>
Nice -- the same amp design, but the work's already been
done for me.  Should be a dot instead of slash after the
"www".

Thanks.

-----Original Message-----
From: pic microcontroller discussion list
[PICLISTMITVMA.MIT.EDU]On Behalf Of Rick C.
Sent: Thursday, May 09, 2002 8:32 PM
To: PICLISTMITVMA.MIT.EDU Subject: Re: [EE]: Measuring automotive current....

You might want to check out Fr. Tom Mcgahee's circuit

http://www/pic101.com/mcgahee
Rick

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Do NOT attempt to run 100A +  thru a PCB trace. This is just trouble waiting
to happen. If not with the trace itself (which is going to have to be quite
wide to avoid becoming a fuse), then with whatever interconnect (some kind
of lugs?) you use to get the current on to and back off the PCB.

Far better to use an external shunt of some sort: either a commercial shunt,
or just a piece of (heavy) wire.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Pic Dude" <picdudeAVN-TECH.COM>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Friday, May 10, 2002 1:51 PM
Subject: Re: [EE]: Measuring automotive current....

{Quote hidden}

> {Original Message removed}
How about using a hall effect type of system? I've never tried it but I
think it should work. TTYL

> {Original Message removed}
On Fri, 10 May 2002, Pic Dude wrote:

>Okay, this is even better than the op-amp design -- I just
>looked at the Maxim datasheets real quickly and it seems
>like a winner so far.  They even show the possibility of
>using a PCB trace for the sense resistor.  Sweet.
>
>The new question then is how to figure out the resistance
>of a PCB trace.  From AWG/resistance tables for copper wire,
>I can calculate the resistance of the copper layer with the
>known thickness/width, but the solder coating adds an unknown
>factor, as I haven't seen any PCB house specs on that.

Search the archives for a thread on how to adjust shunts when you can't
measure them. The thread is 'Re: [OT]: Job Interviews' and the technically
correct response was given by Ray Gardiner. You'd use it backwards, i.e.
fabricate a trace using your process, apply a load and thus a known
current (car headlight comes to mind here) and measure the voltage. Then
figure the gain of the opamp to accomodate this. If you plan for a trimmer
for the gain you don't even need to make another board. At your current a
PCB shunt will be about 1/2 inches to 1 inch long (and very wide) and
have an extra bit of thick wire soldered on top of it with lots of solder.
If temperature (accuracy) matters then do look into Constantan and other
temperature compensated shunt materials.

hope this helps,

Peter

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Well that of course would depend on whether the starter current is part of
the measurement or not, doesn't it? I don't think you can call one way
versus the other "correct" without knowing the intended purpose. TTYL

> {Original Message removed}
On Fri, 10 May 2002, Herbert Graf wrote:

>How about using a hall effect type of system? I've never tried it but I
>think it should work. TTYL

I tried and it works fine if you use bare linear Hall sensor bridges that
have a non-iron chip grid support, because if they do then you have
hysteresis problems and you will misread a lot at low current. It appears
that all inexpensive bare bridge Hall sensors made for motor and sensor
applications have a magnetisable chip grid. The remanent magnetisation of
the best is 2.5 times Earth field. WHY??!! FYI a straight wire with about
3A through it makes a field comparable to the Earth field at 1 inch
distance (very roughly) so your lower measuring limit with such a simple
arrangement and considering what I wrote above (hysteresis) will be 6 to
10A. Of course you can use a specialised Hall current probe which comes

Peter

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Yes, it helps.  I'll go search.
Thanks.

-----Original Message-----
From: pic microcontroller discussion list
[PICLISTMITVMA.MIT.EDU]On Behalf Of Peter L. Peres
Sent: Friday, May 10, 2002 1:54 PM
To: PICLISTMITVMA.MIT.EDU
Subject: Re: [EE]: Measuring automotive current....

On Fri, 10 May 2002, Pic Dude wrote:

{Quote hidden}

Search the archives for a thread on how to adjust shunts when you can't
measure them. The thread is 'Re: [OT]: Job Interviews' and the technically
correct response was given by Ray Gardiner. You'd use it backwards, i.e.
fabricate a trace using your process, apply a load and thus a known
current (car headlight comes to mind here) and measure the voltage. Then
figure the gain of the opamp to accomodate this. If you plan for a trimmer
for the gain you don't even need to make another board. At your current a
PCB shunt will be about 1/2 inches to 1 inch long (and very wide) and
have an extra bit of thick wire soldered on top of it with lots of solder.
If temperature (accuracy) matters then do look into Constantan and other
temperature compensated shunt materials.

hope this helps,

Peter

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True.  But rather than have a physical tap point, I
could just get as close as possible with the calcs
and adjust for error in the code.  Not sure why, but
I tend to believe in minimal external hardware.

Cheers,
-Neil.

{Original Message removed}
Well, perhaps I should've said "better" rather than
"correct".  The "accepted" way to wire an ammeter in a
car is to bypass the starter for a few reasons -- it
draws a lot of current that the ammeter would need to
handle; mesurement of it's current draw is usually not
important; and during starting, you really want as
direct a connection as possible from the battery to
to date that require the starter to be connected on the
non-battery side of the ammeter.

{Original Message removed}
> The new question then is how to figure out the resistance
> of a PCB trace.  From AWG/resistance tables for copper wire,
> I can calculate the resistance of the copper layer with the
> known thickness/width, but the solder coating adds an unknown
> factor, as I haven't seen any PCB house specs on that.

The board house should be able to spec the copper thickness, usually in
"ounces", which can be translated to ohm/square after some figuring.
However, they will usually spec only the minimum thickness, and I would
worry about differences between boards and production batches.  This could
be overcome by doing calibration at production time and storing values in
EEPROM or whatever.  It's going to be tough to get a PCB trace to handle
100A though.  I still think that measuring the drop accross the existing
wire or inserting a calibrated wire is a better solution.

*****************************************************************
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(978) 742-9014, http://www.embedinc.com

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> "Peak starting current could even be higher than 100A."
>
> I assume you mean the current for the starter, in which case
> this is true.  However, the vehicle starter should be wired to
> the battery directly, and not thru the ammeter.  At least, this
> is the way to do it correctly.

It all depends what you are trying to measure.  If you don't want to measure
the starter current then you are right.

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(978) 742-9014, http://www.embedinc.com

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> True.  But rather than have a physical tap point, I
> could just get as close as possible with the calcs
> and adjust for error in the code.  Not sure why, but
> I tend to believe in minimal external hardware.

I agree with you.  If you are going to have a computing element anyway, that
is usually the easiest place to perform calibration.

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(978) 742-9014, http://www.embedinc.com

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pdfserv.maxim-ic.com/arpdf/MAX471-MAX472.pdf on Maxim site has a
short
discussion on using PCB traces as Rsense. Take a look at the end of
page 9 ('Cost') and the top of page 10 (Fig 6).

Tal

> {Original Message removed}
On Sat, 11 May 2002, Olin Lathrop wrote:

>> The new question then is how to figure out the resistance
>> of a PCB trace.  From AWG/resistance tables for copper wire,
>> I can calculate the resistance of the copper layer with the
>> known thickness/width, but the solder coating adds an unknown
>> factor, as I haven't seen any PCB house specs on that.
>
>The board house should be able to spec the copper thickness, usually in
>"ounces", which can be translated to ohm/square after some figuring.
>However, they will usually spec only the minimum thickness, and I would
>worry about differences between boards and production batches.  This could
>be overcome by doing calibration at production time and storing values in
>EEPROM or whatever.  It's going to be tough to get a PCB trace to handle
>100A though.  I still think that measuring the drop accross the existing
>wire or inserting a calibrated wire is a better solution.

I had PCB traces handle 50A pulsing current in a SMPSU. They were less
than 1" wide and very short. I made sure the board gets havily tinned.

Also if my advice will be followed (soldering a piece of suitable
conductor on top of the shunt trace) then the resistance will depend more
on the soldered wire than on the trace and tin, on account of the cross
section ratios. The board only serves as support, the conduction is done
by the soldered wire shunt.

As Bob Ammerman has said there are problems with attaching connectors and
wires to the board at this current. Directly soldering a 0.3" stranded
cable onto a large pad where the extra wire also starts will work.

Just remember that these things run hot (shunts, wires, boards,
everything). Add to this ambient heat in a car and you get a very short
liived project. Use automotive rated parts (125C etc).

Peter

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Olin Lathrop wrote:
>
> > The new question then is how to figure out the resistance
> > of a PCB trace.

> The board house should be able to spec the copper thickness, usually in
> "ounces", which can be translated to ohm/square after some figuring.

I picked up a nice little freeware program
called "PCBtemp" which calculates voltage drop
and temp rise of a PCB trace once you input
current, length, trace thickness etc. You will
also need to know the copper thickness and
whether it is tinned or not too.
-Roman

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Pic Dude wrote:
>
> Okay, this is even better than the op-amp design -- I just
> looked at the Maxim datasheets real quickly and it seems
> like a winner so far.  They even show the possibility of
> using a PCB trace for the sense resistor.  Sweet.

Not so sweet once you try putting 100 amps
through the PCB trace. And 100A 100mV, let's see
that's about 10 watts dissipated, even with the
"length of wire" trick it's going to melt the
insulation etc. :o)

> > The questions are:
> > - I have an ammeter that is "shuntless".  Does this mean that it has an
> > internal resistor for voltage drop?

You can make a cheap hall effect current sensor
using a linear hall sensor which gives 2.5v
on no field and varies up to 5v or down to 0v
depending on field direction, these are about \$2
or less, and glue it onto a coil made from a
few turns of heavy wire (air core). I wouldn't
worry about residual magnetism in the sensor
compared to the strong fields it will experience,
and they will be bi-directional (sort-of) anyway.
:o)
-Roman

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Roman,
About thes \$2 linear hall devices. The ones I usually see are more than
that, got any pointers?

{Quote hidden}

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I am doing 100..150Amax, smps current pulses on heavily tinned double sided
parallelled PCB copper planes of 2.5x2.5". To connect the 25 sqrmm supply
cables to the PCB, I use soldered M5 brass spacers.

Daniel...

{Quote hidden}

could
> >be overcome by doing calibration at production time and storing values in
> >EEPROM or whatever.  It's going to be tough to get a PCB trace to handle
> >100A though.  I still think that measuring the drop accross the existing
> >wire or inserting a calibrated wire is a better solution.

>
>

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>I am doing 100..150Amax, smps current pulses on heavily tinned double sided
>parallelled PCB copper planes of 2.5x2.5". To connect the 25 sqrmm supply
>cables to the PCB, I use soldered M5 brass spacers.
>
>Daniel...

I don't see how one can know the resistance of a trace if you
have "heavily tinned it", unless it's done by a machine to
a known thickness.

If I needed to measure starter current I likely would use a
separate shunt (or other pickup) on that lead.

Barry

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Hi Alan, try:
http://www.jaycar.com.au
for

UGN3503U (cat number ZD-1902)
3-pin
5v 9mA in
output typ 2.5v (50 ohm)
high sensitivity
applications include notch sensor, current
monitor etc.
\$4.95 AUD (to \$3.45 AUD 25 q)

Jaycar will take orders via the net.
:o)
-Roman

Alan Shinn wrote:
{Quote hidden}

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> >I am doing 100..150Amax, smps current pulses on heavily tinned double
sided
> >parallelled PCB copper planes of 2.5x2.5". To connect the 25 sqrmm supply
> >cables to the PCB, I use soldered M5 brass spacers.
> >
> >Daniel...
>
> I don't see how one can know the resistance of a trace if you
> have "heavily tinned it", unless it's done by a machine to
> a known thickness.
>
> If I needed to measure starter current I likely would use a
> separate shunt (or other pickup) on that lead.

It's not that I use the tinned PCB traces as current shunts. I was just
commenting on the discussion about routing high currents over PCB's.
Obviously, if you heavily tin a PCB trace, the final copper thinkness is not
known exactly.

Daniel...
>
> Barry
>
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>

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