Electronics Forums > Measuring mA (debate solution needed)

# Measuring mA (debate solution needed)

James Lerch
Guest
Posts: n/a

 08-04-2007, 07:17 PM
So,

A friend and I are having a polite debate.

He's calculating the current in a circuit my measuring the voltage
drop across a 20ohm resistor. (Current is ~30mA range for refernce)

I think best approach is to put a mA meter in series with circuit and
read the meter. He disagrees and said his method of measuring the
voltage drop across the resistor is more accurate.

His claim is the mA meter adds resistance to the circuit and under
reports the results. I agree in theory with his statement, but
disagree on the magnitude of the error.

I claim the amount of error in both the ability to measure 0.6vdc and
the error in the certaitnty of his "known" resistance is of a larger
concern.

For the record, were just simple tinkerers, using simple (but not
junk) tools, working remotetly from each other. No one's life or
livelyhood depend on the results.

If a little background will help, here's the circuit.

5vdc -> led -> resistor -> led -> pin on shift register -> gnd

Leds drop ~2v each (rated at max of 30mA current)
Resistor is 1/4 watt carbon film 20ohm +/- 5% variety
Shift register on resistance is rated at 6.5ohm at 50ma at temp of 25c

I agree with him that he should use a 33ohm resistor for safety sake,
but would like debate resolution over using a mA meter

So, what's the collective opinion on this?
--
Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "

James Lerch
Guest
Posts: n/a

 08-04-2007, 08:07 PM
On Sat, 04 Aug 2007 15:50:34 -0500, (E-Mail Removed) wrote:

>On Sat, 04 Aug 2007 15:17:31 -0400, James Lerch
><(E-Mail Removed)-spam-seeds.rr.com> wrote:
>
>>So,
>>
>>A friend and I are having a polite debate.
>>
>>He's calculating the current in a circuit my measuring the voltage
>>drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>>
>>I think best approach is to put a mA meter in series with circuit and
>>read the meter. He disagrees and said his method of measuring the
>>voltage drop across the resistor is more accurate.
>>

>The 20 ohm resistor is MUCH more than a current meter will put in the
>circuit. Is the 20- ohm already there, or is he inserting it?

The 20ohm resistor is In circuit,

5vdc -> led -> resistor -> led -> pin on shift register -> gnd

Leds drop ~2v each, spec sheet below, these are LY T776 S2 leds
http://catalog.osram-os.com/catalogu...02c54b0003003a

Resistor is 20ohm

Shift register drain on-resistance is rated at 6.5 ohms @ 50ma and
25c.

Asides from suggesting he measures with DVM on mA scale, also suggest
he goes to 33ohm resistor as he's right on the edge of max current for
the Osram Leds. The is for a christmas decoration display panel with
approx 4k leds on it Would suck for him if he ran it one night and
the next night half the leds were dead or substantially dimmer than
before..

--
Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "

RST Engineering \(jw\)
Guest
Posts: n/a

 08-04-2007, 08:27 PM
Check the specs on the multimeter. SOme of the cheapies from China drop
half a volt right off the crack of the bat, no matter the current scale.
Some drop voltage as a function of current.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

sdeyoreo@hotmail.com
Guest
Posts: n/a

 08-04-2007, 08:50 PM
On Sat, 04 Aug 2007 15:17:31 -0400, James Lerch
<(E-Mail Removed)-spam-seeds.rr.com> wrote:

>So,
>
>A friend and I are having a polite debate.
>
>He's calculating the current in a circuit my measuring the voltage
>drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>
>I think best approach is to put a mA meter in series with circuit and
>read the meter. He disagrees and said his method of measuring the
>voltage drop across the resistor is more accurate.
>

The 20 ohm resistor is MUCH more than a current meter will put in the
circuit. Is the 20- ohm already there, or is he inserting it?
>His claim is the mA meter adds resistance to the circuit and under
>reports the results. I agree in theory with his statement, but
>disagree on the magnitude of the error.
>
>I claim the amount of error in both the ability to measure 0.6vdc and
>the error in the certaitnty of his "known" resistance is of a larger
>concern.
>
>For the record, were just simple tinkerers, using simple (but not
>junk) tools, working remotetly from each other. No one's life or
>livelyhood depend on the results.
>
>If a little background will help, here's the circuit.
>
>5vdc -> led -> resistor -> led -> pin on shift register -> gnd
>
>Leds drop ~2v each (rated at max of 30mA current)
>Resistor is 1/4 watt carbon film 20ohm +/- 5% variety
>Shift register on resistance is rated at 6.5ohm at 50ma at temp of 25c
>
>I agree with him that he should use a 33ohm resistor for safety sake,
>but would like debate resolution over using a mA meter
>
>So, what's the collective opinion on this?

Spehro Pefhany
Guest
Posts: n/a

 08-04-2007, 09:08 PM
On Sat, 04 Aug 2007 15:17:31 -0400, the renowned James Lerch
<(E-Mail Removed)-spam-seeds.rr.com> wrote:

>So,
>
>A friend and I are having a polite debate.
>
>He's calculating the current in a circuit my measuring the voltage
>drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>
>I think best approach is to put a mA meter in series with circuit and
>read the meter. He disagrees and said his method of measuring the
>voltage drop across the resistor is more accurate.
>
>His claim is the mA meter adds resistance to the circuit and under
>reports the results. I agree in theory with his statement, but
>disagree on the magnitude of the error.
>
>I claim the amount of error in both the ability to measure 0.6vdc and
>the error in the certaitnty of his "known" resistance is of a larger
>concern.
>
>For the record, were just simple tinkerers, using simple (but not
>junk) tools, working remotetly from each other. No one's life or
>livelyhood depend on the results.
>
>If a little background will help, here's the circuit.
>
>5vdc -> led -> resistor -> led -> pin on shift register -> gnd
>
>Leds drop ~2v each (rated at max of 30mA current)
>Resistor is 1/4 watt carbon film 20ohm +/- 5% variety
>Shift register on resistance is rated at 6.5ohm at 50ma at temp of 25c
>
>I agree with him that he should use a 33ohm resistor for safety sake,
>but would like debate resolution over using a mA meter
>
>So, what's the collective opinion on this?

Measuring the voltage across an existing resistor of known value is
generally more accurate, provided your voltmeter does not shunt the
resistor too much (figure 10M for a modern cheap multimeter on a 2V
range and virtually infinity on a 200mV range, neither of which will
affect a 20 ohm resistor much. You can also measure the resistor and
get closer if the value is not known. Chances are the mA meter will
insert a resistance of 1 ohm on the 200mA scale, which could affect
the current a bit. With 2 LEDs in series, depending on type, you
probably don't have enough voltage to get a well controlled current
from 5V.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
(E-Mail Removed) Info for manufacturers: http://www.trexon.com

Gareth
Guest
Posts: n/a

 08-04-2007, 09:25 PM
James Lerch wrote:
> So,
>
> A friend and I are having a polite debate.
>
> He's calculating the current in a circuit my measuring the voltage
> drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>
> I think best approach is to put a mA meter in series with circuit and
> read the meter. He disagrees and said his method of measuring the
> voltage drop across the resistor is more accurate.
>
> His claim is the mA meter adds resistance to the circuit and under
> reports the results. I agree in theory with his statement, but
> disagree on the magnitude of the error.
>
>
> I claim the amount of error in both the ability to measure 0.6vdc and
> the error in the certaitnty of his "known" resistance is of a larger
> concern.
>

Understanding measurement errors can get quite complicated, and the fact
that you understand that there are errors rather than just writing down
the number on the meter is a good start.

If it is a digital meter it will probably measure current by measuring
the voltage across a small, known resistance, so the accuracy of the
meter when measuring a small voltage is probably not an issue. On the
other hand, if it is a simple analog moving coil meter the meter
actually responds to current and the voltage has to be converted to
current internally (e.g. by a series resistor).

As you rightly say, both methods will have errors, and if you really
want to know which is best you will probably have to do some math.

For the voltage across the resistor you have:

1) Error in the resistor value - probably quite significant if you just
go be the nominal value. Lower if you measure it, but there will be
some error in this measurement too.

2) Current flowing through the meter - when you put the meter across the
resistor you will have the resistance of the meter in parallel with the
resistor which will reduce the total resistance. In this case I would
expect this to be negligible unless you have a very poor meter.

3) Voltage measurement accuracy of the meter.

For the direct current measurement you have:

1) Change in resistance due to series resistance of the meter

2) Current measurement accuracy of the meter.

If you have the specifications for the meter you should be able to find
its accuracy and series resistance and work out which sources of error
are the most significant.

Personally, I would probably measure the current across the resistor
because it is easier than breaking the circuit and inserting the current
meter.

--
-----------------------------------------------------------------------

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

Chuck
Guest
Posts: n/a

 08-04-2007, 09:51 PM
Gareth wrote:

>
> Personally, I would probably measure the current

voltage, of course intended

across the resistor
> because it is easier than breaking the circuit and inserting the current
> meter.
>

With one DMM doing a series current
measurement, you can use a second DMM to
measure the voltage dropped across the
first one and calculate the actual
series resistance the first one
introduces. Might be interesting.

Chuck

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Tom Bruhns
Guest
Posts: n/a

 08-04-2007, 10:05 PM
On Aug 4, 1:07 pm, James Lerch <(E-Mail Removed)-spam-seeds.rr.com>
wrote:
> On Sat, 04 Aug 2007 15:50:34 -0500, (E-Mail Removed) wrote:
> >On Sat, 04 Aug 2007 15:17:31 -0400, James Lerch
> ><(E-Mail Removed)-spam-seeds.rr.com> wrote:

>
> >>So,

>
> >>A friend and I are having a polite debate.

>
> >>He's calculating the current in a circuit my measuring the voltage
> >>drop across a 20ohm resistor. (Current is ~30mA range for refernce)

>
> >>I think best approach is to put a mA meter in series with circuit and
> >>read the meter. He disagrees and said his method of measuring the
> >>voltage drop across the resistor is more accurate.

>
> >The 20 ohm resistor is MUCH more than a current meter will put in the
> >circuit. Is the 20- ohm already there, or is he inserting it?

>
> The 20ohm resistor is In circuit,
>
> 5vdc -> led -> resistor -> led -> pin on shift register -> gnd
>
> Leds drop ~2v each, spec sheet below, these are LY T776 S2 ledshttp://catalog.osram-os.com/catalogue/catalogue.do?act=showBookmark&f...
>
> Resistor is 20ohm
>
> Shift register drain on-resistance is rated at 6.5 ohms @ 50ma and
> 25c.
>
> Asides from suggesting he measures with DVM on mA scale, also suggest
> he goes to 33ohm resistor as he's right on the edge of max current for
> the Osram Leds. The is for a christmas decoration display panel with
> approx 4k leds on it Would suck for him if he ran it one night and
> the next night half the leds were dead or substantially dimmer than
> before..
>
> --
> Take Care,
> James Lerchhttp://lerch.no-ip.com/atm(My telescope construction,testing, and coating site)http://lerch.no-ip.com/ChangFa_Gen(My 15KW generator project)
>
> "Anything that can happen, will happen" -Stephen Pollock from:
> "Particle Physics for Non-Physicists: A Tour of the Microcosmos"
>
> " Press on: nothing in the world can take the place of perseverance.
> Talent will not; nothing is more common than unsuccessful men with talent.
> Genius will not; unrewarded genius is almost a proverb.
> Education will not; the world is full of educated derelicts.
> Persistence and determination alone are omnipotent. "

Given that the resistor is already in the circuit, so long as you know
its resistance accurately, that will be the better way. I'm assuming
a volt meter that has something like a 10 megohm input resistance,
common in DVMs. If you insert a meter, you'll disturb things by
whatever the meter resistance is. Say it's 0.1 ohms. That's 0.5% of
the total of 20 ohms plus the meter. On the other hand, 10 megohms in
parallel with 20 ohms disturbs it by about .0002%.

For current in an LED, neither matters a whit. But measuring the
voltage across a resistor is almost always trivial compared with
inserting a meter in series with an existing circuit.

How accurately you know the value of the 20 ohm resistor would be an
issue, along with how stable it is with self-heating and stuff like
that, but again, measuring current in an LED really doesn't require
extreme accuracy. The max current rating of the LED is for rated
life; it's not going to die in an hour if you go 1% over. You can
measure the value of the resistor with the power turned off in that
particular circuit; if the LED is reverse biased by less than a volt
by the meter, the reading should be quite accurate.

I've never met a DC current meter that drops half a volt at
(practically) no current, like Jim mentioned, and hope I never do.
Even my cheapies don't do that. On AC, yeah, quite possibly, but on
DC???

Cheers,
Tom

Phil Allison
Guest
Posts: n/a

 08-05-2007, 01:30 AM

"James Lerch"
>
> A friend and I are having a polite debate.
>
> He's calculating the current in a circuit my measuring the voltage
> drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>
> I think best approach is to put a mA meter in series with circuit and
> read the meter. He disagrees and said his method of measuring the
> voltage drop across the resistor is more accurate.
>
> His claim is the mA meter adds resistance to the circuit and under
> reports the results. I agree in theory with his statement, but
> disagree on the magnitude of the error.
>
> I claim the amount of error in both the ability to measure 0.6vdc and
> the error in the certaitnty of his "known" resistance is of a larger
> concern.
>

** Time you measured the internal resistance of a DMM when set to the 200
/ 300 mA range.

A typical 3.75 digit meter I have here exhibits a whooping 5.5 ohms !!!

The voltage drop at full reading = 1.8 volts !!

Most of it is due to the 500mA glass fuse employed on that range.

Your friend is right & you are 100% wrong.

........ Phil

Eeyore
Guest
Posts: n/a

 08-05-2007, 05:05 AM

James Lerch wrote:

> So,
>
> A friend and I are having a polite debate.
>
> He's calculating the current in a circuit my measuring the voltage
> drop across a 20ohm resistor. (Current is ~30mA range for refernce)
>
> I think best approach is to put a mA meter in series with circuit and
> read the meter. He disagrees and said his method of measuring the
> voltage drop across the resistor is more accurate.

You mean a moving coil meter ? They are inherently inaccurate (subject to modest
calibration, linearity and measurement errors due to parallax).

Graham

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