Current Sensing using copper trace

[Andy Pichotta]

Hi all,

I have a control board that runs a small DC motor (about 1 amp nominal) and
I sense
current using a .02 ohm resistor in series with armature. Works fine but I
was wondering
if it would be possible to eliminate the resistor and just use a trace on
the board for the IR
drop. Has anyone ever tried this? If I was to size a PCB trace to be .02
ohms how bad
could I expect the tolerance to be? Any comments welcome.

Thanks,
Andy

 

[Louis Bybee]

Hi all,

I have a control board that runs a small DC motor (about 1 amp nominal) and
I sense
current using a .02 ohm resistor in series with armature. Works fine but I
was wondering
if it would be possible to eliminate the resistor and just use a trace on
the board for the IR
drop. Has anyone ever tried this? If I was to size a PCB trace to be .02
ohms how bad
could I expect the tolerance to be? Any comments welcome.

Thanks,
Andy

I observed a scenario a while back where a field service tech was installing
a new circuit board with an onboard current sense shunt. It appeared to be a
trace that fanned out from a large single one to multiple smaller traces. He
was severing a number of the smaller traces while applying a constant
current, and monitoring the output across the shunt. He indicated the reason
for not using a resister was one of cost, and one board being able to
accommodate multiple applications. I suspect even with very small traces
being severed it would result in large stepped outputs. Possibly close was
good enough?

Louis--
*********************************************
Remove the two fish in address to respond

 

[Garrett Mace]

Hi all,

I have a control board that runs a small DC motor (about 1 amp nominal) and
I sense
current using a .02 ohm resistor in series with armature. Works fine but I
was wondering
if it would be possible to eliminate the resistor and just use a trace on
the board for the IR
drop. Has anyone ever tried this? If I was to size a PCB trace to be .02
ohms how bad
could I expect the tolerance to be? Any comments welcome.

On 1-oz copper, a stretch of 14 mil trace .5454 inches long has .020000 ohms
resistance. If you miss the length by a mil, the resistance will change by
about 0.2%. It would be difficult to predict exactly how the resistance
would actually work out, once you consider the resistance of the conductors
where you are tapping into the shunt, component leads, and so on. You might
get close, and then be able to adjust the circuit around that.

Of course your resistance could change from board to board depending on
varying copper thickness, if it was plated through, edge geometry for
different etch solutions, etc. It's something you'll very likely want to be
able to adjust for. Perhaps build an adjustable gain into the amp you're
using to measure the voltage across your resistance.

 

[Spehro Pefhany]

On 1-oz copper, a stretch of 14 mil trace .5454 inches long has .020000 ohms
resistance. If you miss the length by a mil, the resistance will change by
about 0.2%. It would be difficult to predict exactly how the resistance
would actually work out, once you consider the resistance of the conductors
where you are tapping into the shunt, component leads, and so on. You might
get close, and then be able to adjust the circuit around that.

Of course your resistance could change from board to board depending on
varying copper thickness, if it was plated through, edge geometry for
different etch solutions, etc. It's something you'll very likely want to be
able to adjust for. Perhaps build an adjustable gain into the amp you're
using to measure the voltage across your resistance.

Or just spend the lousy $0.50 and get a 1% guaranteed resistor that
doesn't have a +4000ppm/K tempco and vary with the production controls
of your PCB supplier. That's my advice.

Best regards,
Spehro Pefhany

 

[Garrett Mace]

Or just spend the lousy $0.50 and get a 1% guaranteed resistor that
doesn't have a +4000ppm/K tempco and vary with the production controls
of your PCB supplier. That's my advice.

Best regards,
Spehro Pefhany

Which was kind of where I was trying to nudge him, while allowing him the
dignity of choosing between two options instead of choosing between idiocy
and shame. ;-)

 

[Spehro Pefhany]

Which was kind of where I was trying to nudge him, while allowing him the
dignity of choosing between two options instead of choosing between idiocy
and shame. ;-)

Yes, I know. I thought it was a bit too subtle for his good.

Best regards,
Spehro Pefhany

 

[bushbadee]

tHE traces on circuit boards can be used to check out boards
..When I was warrentee repair for a computer company, I often had to check
for shorted IC's on circuit boards to replace them.
NOw multiple chips would be on the same ground line.
It was not practical to cut the traces on a multi layer board.
I would take a low voltage, high current current source and connect it to
the board under test.
I would connect one lead of a sensitive voltmeter, with scales down to about
1 millivolt. to the negitive pin on the card.
I would then with the probe check all the lines that fed into the ground
line.
At each point in the circuit where there was a branch I would measure the
voltage.
I would then check further up the trace on each of the feeds into the
common point.
If the voltage was the same as at the node, that would mean there was no
current flow in that line. and the IC was ok.
When I got to the bad IC I would find that there was a further voltage drop
across this ground line, and I had my short circuit and bad IC.
I used this method to locate hundreds of shorted IC's.
The same method worked when trying to locate shorted components in other
electronic equipment..

The same can be done on the high side.

 

[DarkMatter]

Or just spend the lousy $0.50 and get a 1% guaranteed resistor that
doesn't have a +4000ppm/K tempco and vary with the production controls
of your PCB supplier. That's my advice.

Jeez... even our cheap 1/4 w resistors are 25 ppm. Are you
referring to carbon comp, or carbon film maybe?

Metal film resistors are a lot more stable than that. At least ours
are.

 

[bushbadee]

Dark matter is still shooting off his mouth.

25ppm is 25 parts per million.
I believe that is 0.0025 %
I do not know if you can even buy a resistor that is .0025% as even lead
length and soldiering job could change the resistance more than that. Let
alone buy it cheap.
I think DM is still being stupid.
I have seldom see him post any thing but curses and foul language.

 

[Spehro Pefhany]

Jeez... even our cheap 1/4 w resistors are 25 ppm. Are you
referring to carbon comp, or carbon film maybe?

+4000ppm, give or take, is the tempco of copper @ room temperature,
depending on purity and annealing. That is a decent output for a
temperature sensor, and they are used commercially as such in some
low-accuracy applications (stator RTDs). Copper corrodes so it's not
as good as, say, Pt for long-term stability.

Most (if not all) pure metals have about this tempco at room
temperature (~PTAT), you need alloys to get a good resistor.

Metal film resistors are a lot more stable than that. At least ours
are.

Depends on the metal, eh?

Best regards,
Spehro Pefhany

 

[DarkMatter]

Dark matter is still shooting off his mouth.

No. your retarded ass is showing how little you know about the
industry AGAIN.

25ppm is 25 parts per million.

YEP... Sure is.

I believe that is 0.0025 %

Nope. Thermal tolerance is NOT the same as accuracy in value, you
retard!

I do not know if you can even buy a resistor that is .0025% as even lead
length and soldiering job could change the resistance more than that. Let
alone buy it cheap.

You are an idiot. Try Mouser, or Digi-Key. Both sell standard
metal film resistors at 25 ppm 1%.

For you to shoot off your mouth (as usual) before even researching
these facts that you obviously know not a goddamned thing about, is
par for your alzheimer's course.

I think DM is still being stupid.

I think you are a riot!

I have seldom see him post any thing but curses and foul language.

You have seldom comprehended anything you have seen in your entire
pathetic life, guess as you go boy!

Try again, you idiot! Get rid of the "B" baby bullshit, retard!

 

[DarkMatter]

Thanks everyone for all the feedback. I kind of had an feeling it was not a
good idea.
I realy need to be within about +-10% or so on current measurement. I
suppose if it
was that easy no one would bother making these low ohm resistors would they?

Andy

Two 0.01 presicion resistors in series, or the exact 0.02 value is
readily available.

It is a common thing in the industry... shunts.