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Resistance measurement in the 0.5 - 1.0 Ohm range

E

Ed

Jan 1, 1970
0
I have two digital multimeters of the type typically used by auto
techs. One has
a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What accuracy
can be expected
if I measure a resistance in the 0.5-1.0 range and subtract the meter
0 value?

TIA

Ed
 
P

Phil Allison

Jan 1, 1970
0
"Ed"
I have two digital multimeters of the type typically used by auto techs.
One has
a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What accuracy can
be expected
if I measure a resistance in the 0.5-1.0 range and subtract the meter 0
value?


** The basic reading error of a DMM is always +/- 1 digit

As you are no involving two readings to arrive at a value, the errors might
compound.

So the answer is, the number shown, minus the residual reading, +/- 2
digits.



........... Phil
 
R

redbelly

Jan 1, 1970
0
Ed said:
I have two digital multimeters of the type typically used by auto
techs. One has
a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What accuracy
can be expected
if I measure a resistance in the 0.5-1.0 range and subtract the meter
0 value?

TIA

Ed

With two meters, you could simultaneously measure the current and
voltage of the resistor and get an answer accurate to 3 digits.

Mark
 
P

Phil Allison

Jan 1, 1970
0
"redbelly"
With two meters, you could simultaneously measure the current and
voltage of the resistor and get an answer accurate to 3 digits.


** What if Ed's 0.5 ohm resistance is a lamp filament ??




........ Phil
 
C

Chris

Jan 1, 1970
0
Ed said:
I have two digital multimeters of the type typically used by auto
techs. One has
a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What accuracy
can be expected
if I measure a resistance in the 0.5-1.0 range and subtract the meter
0 value?

TIA

Ed

Hi, Ed. If you have to put a number on it NOW, and you don't have any
other options, try this:

* Flip open the battery cover of meter 1. Measure the OFF voltage of
the battery with meter 2 set for DCV. Then turn on meter 1, and set it
for resistance. Short the leads together, then measure the battery
voltage with meter 2. If it doesn't bog down, you've got a good
battery. Reverse, and check the battery of meter 2 also. If batteries
check OK, go to step 2.

* If your meter test leads are removable, try twisting the lead banana
plugs around in the socket, and see if you can get your 0.6 ohms or 0.3
ohms short reading to change. Also, try gently tugging and twisting
the leads (particularly where they meet the banana plug and the test
probe) for intermittents. If they don't change (good wires and
contacts), you can try to measure your low ohm resistor.

The reading you get, as mentioned in other posts, should be good for
+/- 2 counts. That is, if you're using the 0.6 ohm short meter, and
you're measuring an 0.7 ohm resistor, your reading will probably be
somewhere between 1.2 and 1.4 ohms, and could be between 1.1 and 1.5
ohms. Not very accurate, but it will give you some kind of indication
of resistance better than a WAG.

Actually, even though this isn't an accurate measurement, it's done all
the time. Since you can periodically check your meter on the bench,
you don't have to go through this test every time you take a reading.
And if you can find or scrounge a precision 1 ohm 1% resitor in your
travels, hold on to it. This can be another part of your periodic
check.

If you make sure to check your meter operation every couple of weeks,
you can have this as an option for getting a rough measurement of low
ohm resistance in the field.

Good luck
Chris
 
R

redbelly

Jan 1, 1970
0
Phil said:
** What if Ed's 0.5 ohm resistance is a lamp filament ??

Then (A) he should have told us that, and (B) if he wants the "hot"
resistance then he can't simply use an ohmmeter, however the two-meter
method I suggested will work.

Mark
 
P

Phil Allison

Jan 1, 1970
0
"redbelly"
Then (A) he should have told us that,


** What else has 0.5 to 1 ohms when cold in an automotive context ??


and (B) if he wants the "hot"
resistance then he can't simply use an ohmmeter, however the two-meter
method I suggested will work.


** But not for a 12 volt lamp & not without a steady current source of
circa 1 amp OR not without a voltage source of 1 volt.

You did cheekily say 3 digits - now didn't you ?



........ Phil
 
E

Ed

Jan 1, 1970
0
redbelly said:
With two meters, you could simultaneously measure the current and
voltage of the resistor and get an answer accurate to 3 digits.

Mark

Mark,

Thanks for the suggestion. As I mention in a clarification to my
original post,
the resistance is an ignition coil primary. What voltage would you
recommend
for your test? I don't know what the ignition module energizes the
coil with,
and in service the on-time is very short relative to what might be
required for
the testing. Don't want to fry the thing.

Ed
 
E

Ed

Jan 1, 1970
0
Phil Allison said:
** The basic reading error of a DMM is always +/- 1 digit

As you are no involving two readings to arrive at a value, the
errors might compound.

So the answer is, the number shown, minus the residual reading, +/-
2 digits.

Thanks, Phil. So, if I arrive at 0.4 volts after the subtraction that
means the actual resistance
might be between 0.2 and 0.6 Ohms?

Ed
 
E

Ed

Jan 1, 1970
0
Ed said:
I have two digital multimeters of the type typically used by auto
techs. One has
a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What
accuracy can be expected
if I measure a resistance in the 0.5-1.0 range and subtract the
meter 0 value?

TIA

Ed

Thanks to all for their responses.

I guess I should have given the specific context of my question. The
resistance in question
is that of the primary winding of an ignition coil, more specifically
a 2-coil pack as used on
the GM 3.8L V6 engine. The spec is 0.5- 0.8 Ohms. By subtracting the
short resistance of
the meter(s) I get about 0.4 for each of the 3 coil packs. The dealer
replaced them because
they were "out of spec," although the tech didn't record the actual
readings he got. The
charge was close to $300, parts and labor. Unfortunately, it didn't
fix the problem, which
is an intermittent engine miss. Right now I'm trying to get my facts
together to approach the
dealer for possible refund or credit towards a second try at fixing
the car. At the same time
I'm under pressure at home to get on over to the Toyota dealer.



Ed
 
E

Ed

Jan 1, 1970
0
Chris said:
* Flip open the battery cover of meter 1. Measure the OFF voltage
of
the battery with meter 2 set for DCV. Then turn on meter 1, and set
it
for resistance. Short the leads together, then measure the battery
voltage with meter 2. If it doesn't bog down, you've got a good
battery. Reverse, and check the battery of meter 2 also. If
batteries
check OK, go to step 2.

* If your meter test leads are removable, try twisting the lead
banana
plugs around in the socket, and see if you can get your 0.6 ohms or
0.3
ohms short reading to change. Also, try gently tugging and twisting
the leads (particularly where they meet the banana plug and the test
probe) for intermittents. If they don't change (good wires and
contacts), you can try to measure your low ohm resistor.

The reading you get, as mentioned in other posts, should be good for
+/- 2 counts. That is, if you're using the 0.6 ohm short meter, and
you're measuring an 0.7 ohm resistor, your reading will probably be
somewhere between 1.2 and 1.4 ohms, and could be between 1.1 and 1.5
ohms. Not very accurate, but it will give you some kind of
indication
of resistance better than a WAG.

Actually, even though this isn't an accurate measurement, it's done
all
the time. Since you can periodically check your meter on the bench,
you don't have to go through this test every time you take a
reading.
And if you can find or scrounge a precision 1 ohm 1% resitor in your
travels, hold on to it. This can be another part of your periodic
check.

If you make sure to check your meter operation every couple of
weeks,
you can have this as an option for getting a rough measurement of
low
ohm resistance in the field.

Thanks, Chris. I really appreciate the completeness of your reply.
BTW, if
a person wanted to get better accuracy, say +/- 5% what would he have
use? A Wheatstone
bridge and a precision decade box?

Ed
 
A

Anthony Fremont

Jan 1, 1970
0
Phil Allison said:
** What else has 0.5 to 1 ohms when cold in an automotive context ??

A few things immediately come to mind:

1) The resistor inline with the coil of any older vehicle using points.
2) The resistors for the blower motor.
3) Wiring running to the tail lamps, for example.
4) The resistance from the ground terminal of the battery and the rear
of the body or metal bumper.
5) A starter solenoid.
 
R

Ralph Mowery

Jan 1, 1970
0
Ed said:
Thanks to all for their responses.

I guess I should have given the specific context of my question. The
resistance in question
is that of the primary winding of an ignition coil, more specifically
a 2-coil pack as used on
the GM 3.8L V6 engine. The spec is 0.5- 0.8 Ohms. By subtracting the
short resistance of
the meter(s) I get about 0.4 for each of the 3 coil packs. The dealer
replaced them because
they were "out of spec," although the tech didn't record the actual
readings he got. The
charge was close to $300, parts and labor. Unfortunately, it didn't
fix the problem, which
is an intermittent engine miss. Right now I'm trying to get my facts
together to approach the
dealer for possible refund or credit towards a second try at fixing
the car. At the same time
I'm under pressure at home to get on over to the Toyota dealer.

It is very difficult to tell if coils are bad just by going by the DC
resistance unless they are open. A few shorted turns will not show up.
Also there is a differance in the hot and cold resistance. The shop was
just doing what many do, relpacing parts and hopeing to hit the correct one.
It takes more than the usual VOM to check the low resistances. I have a
Fluke digital meter that costs several hundred dollars. I don't trust it
for the very low ohm readings. Just the resistance of the leads, the
pressure I put on the probes and a few other things will throw off the
readings a small ammount.
 
R

redbelly

Jan 1, 1970
0
Ed said:
Mark,

Thanks for the suggestion. As I mention in a clarification to my
original post,
the resistance is an ignition coil primary. What voltage would you
recommend
for your test? I don't know what the ignition module energizes the
coil with,
and in service the on-time is very short relative to what might be
required for
the testing. Don't want to fry the thing.

Ed

Good question. I'm not familiar enough with autos and ignition coils
to give the best answer. But if we go with a conservative figure that
it can handle a mere 1 mW, then 20 mV (at 0.5 ohms) should work. You
won't get 3-digit accuracy, if you can measure to +/- 1 mV, but you'll
improve on what you have.

It probably can handle a lot more than this, but I'm not the person to
answer that question. If you still have the old coil to experiment
with, you could try 70 mV (for 10 mW dissipation at 0.5 ohms), and get
your resistance accurate to 2-3 percent.

Also: you have two choices to set up this measurement; be sure to use
choice #1:

1. Connect the voltmeter directly in parallel with the coil, and
connect the ammeter in series with that parallel combination, or

2. Connect the ammeter directly in series with the coil, and connect
the voltmeter in parallel (i.e., across) that series combination.

You want to use setup #1. Setup #2 can be innacurate because you'll be
measuring the voltage of the coil + ammeter, not just of the coil. If
the ammeter impedance is comparable to the coil impendance, you'll be
way off.

Mark
 
C

Chris

Jan 1, 1970
0
Ed said:
Thanks, Chris. I really appreciate the completeness of your reply.
BTW, if
a person wanted to get better accuracy, say +/- 5% what would he have
use? A Wheatstone
bridge and a precision decade box?

Ed

Hi, Ed. The kelvin method of measuring resistance is very accurate for
measuring low ohms. Using the kelvin method, you put a known current
through the resistor using two leads, and then use two other leads to
measure the voltage across the resistor.

Even with a coil that is sensitive to too much power, you can make a
fairly accurate resistance measurement using a DMM with a 200mV range,
if you have a precision series resistor to give a known current.

You've got your 12V battery already. See if you can find a 100 ohm
resistor that's 3 watts or more. The better the precision and the
higher the wattage, the better. I like to scrounge power resistors
when I find them, so I've got a collection of 10 watt and up 1%
resistors to do this type of thing.

Now, hook up your battery in series with the 100 ohm resistor and the
coil, like this (view in fixed font or M$ Notepad):


| ___
| .---|___|-----.
| | 100 ohm |
| | o
| | |
| | o------.
| | | |
| +| | | |
| --- 120mA| C| / \
| - | C| (DMM)
| 12V| | C| \_/
| | | | |
| | V | |
| | o------'
| | |
| | o
| | |
| '-------------'
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Your coil resistance is very small in relation to the 100 ohm resistor,
so for the sake of calculations let's assume it's basically zero. Now
measure your battery voltage under load, then calculate the load
current using Ohm's Law:

I = V / R

Assuming your battery puts out 12.00V under load, and your resistor is
100.0 ohms, you'll have 120mA going through the circuit. Now use your
DMM on the 200mV range to measure voltage across the coil, and use
Ohm's law again to calculate coil resistance. Let's assume you read
60mV across the coil, then you can calculate:

R = V / I = .060V / .12A = 0.5 ohms

With even the cheapest DMM, you can get a resistance reading which is
pretty much within the tolerance of your series resistor. Even cheap
DMMs have 1% or better accuracy on the 200mVDC range.

Make sure you place the DMM leads on the device to be measured
carefully, so you don't measure anything but the coil. Post back if
you're unsure on this. It's the major source of error in kelvin
measurements. The rule of thumb is, current on the outside, potential
on the inside.

By the way, your question about power is a good one. But at this low
current, it shouldn't be an issue. Power = voltage * current, so

P = .12A * .06V = a little over 7 milliwatts for a half ohm coil, which
shouldn't even get it warm.

Good luck
Chris
 
B

Bob Monsen

Jan 1, 1970
0
Thanks to all for their responses.

I guess I should have given the specific context of my question. The
resistance in question
is that of the primary winding of an ignition coil, more specifically
a 2-coil pack as used on
the GM 3.8L V6 engine. The spec is 0.5- 0.8 Ohms. By subtracting the
short resistance of
the meter(s) I get about 0.4 for each of the 3 coil packs. The dealer
replaced them because
they were "out of spec," although the tech didn't record the actual
readings he got. The
charge was close to $300, parts and labor. Unfortunately, it didn't
fix the problem, which
is an intermittent engine miss. Right now I'm trying to get my facts
together to approach the
dealer for possible refund or credit towards a second try at fixing
the car. At the same time
I'm under pressure at home to get on over to the Toyota dealer.

Your wife is right. It doesn't matter what the reading on the coil is.
They have contracted to fix your car, and claimed that it was fixed, and
you paid them on that basis. They need to make good on that claim. You
don't have the equipment or knowledge to determine how they failed, but
you know that they did. Call them on it, nicely, preferably in front of
other service customers.

--
Regards,
Bob Monsen

The important thing is not to stop questioning. Curiosity has its own
reason for existing. One cannot help but be in awe when he
contemplates the mysteries of eternity, of life, of the marvelous
structure of reality. It is enough if one tries merely to comprehend a
little of this mystery every day. Never lose a holy curiosity.
Albert Einstein
 
J

Jamie

Jan 1, 1970
0
Ed said:
Mark,

Thanks for the suggestion. As I mention in a clarification to my
original post,
the resistance is an ignition coil primary. What voltage would you
recommend
for your test? I don't know what the ignition module energizes the
coil with,
and in service the on-time is very short relative to what might be
required for
the testing. Don't want to fry the thing.

Ed
hmm
normally you can get data on ignition coils how ever, if you
do your math with some common sense factors as to what the coil
was designed to be used for originally! you could then calculate the
duty on cycle..
assume that you have one of those engines that has a coil for each
cylinder, 800 RPM, so this coil would get energized 800 times a
minute. so this is around 13 times a second. thus being
13 hz. or 0.077 ms average on time! how ever, this isn't of course
a 50% duty operation but it gives you some info to work with.
 
J

Jamie

Jan 1, 1970
0
Ed said:
Thanks, Phil. So, if I arrive at 0.4 volts after the subtraction that
means the actual resistance
might be between 0.2 and 0.6 Ohms?

Ed
to get accurate readings of that low of level you
should have a Kelvin bridge meter. that is a meter
that has 2 leads that are connected to gether at the
test point on each lead output. so in total you have
4 leads .
it will have a balance knob of such etc.. or you can
get your self one of those DMM that are designed for
mohms. etc..
 
C

Chris

Jan 1, 1970
0
Ed said:
Thanks to all for their responses.

I guess I should have given the specific context of my question. The
resistance in question
is that of the primary winding of an ignition coil, more specifically
a 2-coil pack as used on
the GM 3.8L V6 engine. The spec is 0.5- 0.8 Ohms. By subtracting the
short resistance of
the meter(s) I get about 0.4 for each of the 3 coil packs. The dealer
replaced them because
they were "out of spec," although the tech didn't record the actual
readings he got. The
charge was close to $300, parts and labor. Unfortunately, it didn't
fix the problem, which
is an intermittent engine miss. Right now I'm trying to get my facts
together to approach the
dealer for possible refund or credit towards a second try at fixing
the car. At the same time
I'm under pressure at home to get on over to the Toyota dealer.



Ed

Hi, Ed. Seconding Mr. Monsen here. They're moving cars through the
service bays, and trying to fix things as quickly as possible. It
looks like they made a mistake in thinking that the coil was the cause
of the problem, even if it was out of spec.

One thing you might try that I've found helpful. It's kind of like
going to the doctor with a list of symptroms and things you want to
ask. Write down in your own words what you're experiencing with the
intermittent, and especially when the intermittent happens
(particularly during condensing humidity, only when the engine is warm
or cold, when it's below or above a certain temp out, when it's
raining, &c). Make two copies. Make sure the service rep writing your
job ticket attaches a copy of your observations to the job ticket.
Never miss a chance to communicate directly with the tech actually
troubleshooting your problem.

If you have a problem with any technical service, take it back as soon
as you discover the problem wasn't fixed, while the problem is still
fresh in the mind of the person who worked on it. Nothing spurs a
little extra effort in a conscientious tech faster than an immediate
"Wrong -- try again". And there's no easier time to negotiate about
the charges.

Oh, and by the way -- if your sparkplug wires are more than 3 years
old, and you're having intermittent misfiring, you should replace them
as a matter of principle. You can't really troubleshoot them. And get
the good ones -- the cheapies aren't worth it.

If you found this advice of help, Rich Grise keeps the tip jar for
s.e.b. ;-)

Good luck
Chris
 
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