Measuring your multimeter while attempting to measure your circuit
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
Don't forget the tolerance of your resistors. Look at the last color band. Tolerances of 10% or worse were not uncommon, but today resistors with low tolerances are not so expensive anymore.
I have lived with several Zen masters - all of them were cats. His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
It's no big deal, it's just a case of knowing the error bars for your measurement. Every measurement in science is meaningless without knowing the precision of the measuring instrument.
“That which can be asserted without evidence, can be dismissed without evidence.”
― Christopher Hitchens
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I'm weighing 500 gr of rice. I put the scale to 0, and put the bowl on the scale. What's that, it weighs 153 grams? It's called calibrating, and your measuring-device should know how to compensate for its own interference.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
You also need to know the sensitivity of measurement. So while you may have zeroed your scale you will also need to know that the scale is, for example, accurate to within 1 gram.
“That which can be asserted without evidence, can be dismissed without evidence.”
― Christopher Hitchens
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You also need to know the sensitivity of measurement. So while you may have zeroed your scale you will also need to know that the scale is, for example, accurate to within 1 gram.
“That which can be asserted without evidence, can be dismissed without evidence.”
― Christopher Hitchens
-
It's no big deal, it's just a case of knowing the error bars for your measurement. Every measurement in science is meaningless without knowing the precision of the measuring instrument.
“That which can be asserted without evidence, can be dismissed without evidence.”
― Christopher Hitchens
AND the tolerances of the parts you are measuring, in this case the resistor.
I have lived with several Zen masters - all of them were cats. His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
There are several factors affecting the measurement; the meter most certainly does not have an internal impedance anything close to 10 ohms. Most likely is that you're far overloading the battery - 300 mA is a lot for a button cell or even a small alkaline battery. They're designed for a discharge rate much lower, and their internal resistance increases when you try to pull more current than they can deliver. Even a cheap multimeter is expected to measure 10A without disturbing the circuit - perhaps a 100 mV drop = .01 ohm. Try again using a larger resistor.
Will Rogers never met me.
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
Quote:
So it looks like my meter has around 9-10 Ohms
That's difficult to believe. A quick and dirty test with my cheap multimeter gave a internal resistance of about 0.2 Ohms while measuring about 0.1 A (2.2 V lab power supply on 22 Ohm resistor). By the way, is the resistor properly sized (common 1/4W ones cannot stand the 0.9 W of your test)?
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Don't forget the internal resistance of the battery or power source. 9-10 Ohms sounds like a bit much for the multimeter.
Wrong is evil and must be defeated. - Jeff Ello
I watched a YouTube that explains it very well #194: What is ammeter burden voltage, and why you should care. - YouTube[^] And someone else pointed out a special port on my meter (marked 20A) which has a smaller burden and I was was able to read 270mA on the circuit. Interesting that it was the meter and not the battery.
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And how is about to replace your 10 Ohm resistor by e.g. a 100 Ohm? Can you confirm with 100 Ohm your Observation?
It does not solve my Problem, but it answers my question
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Don't forget the tolerance of your resistors. Look at the last color band. Tolerances of 10% or worse were not uncommon, but today resistors with low tolerances are not so expensive anymore.
I have lived with several Zen masters - all of them were cats. His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
-
I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
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There are several factors affecting the measurement; the meter most certainly does not have an internal impedance anything close to 10 ohms. Most likely is that you're far overloading the battery - 300 mA is a lot for a button cell or even a small alkaline battery. They're designed for a discharge rate much lower, and their internal resistance increases when you try to pull more current than they can deliver. Even a cheap multimeter is expected to measure 10A without disturbing the circuit - perhaps a 100 mV drop = .01 ohm. Try again using a larger resistor.
Will Rogers never met me.
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Quote:
So it looks like my meter has around 9-10 Ohms
That's difficult to believe. A quick and dirty test with my cheap multimeter gave a internal resistance of about 0.2 Ohms while measuring about 0.1 A (2.2 V lab power supply on 22 Ohm resistor). By the way, is the resistor properly sized (common 1/4W ones cannot stand the 0.9 W of your test)?
Here's a very good short YouTube which I was guided to that explains it and the one meter the presenter uses does indeed have 10 ohms of resistance. #194: What is ammeter burden voltage, and why you should care. - YouTube[^] Also I was guided to use another port on my meter (marked 20A) which has more like 1 ohm resistance and I was able to read 270mA.
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By the way, did you check the battery voltage when it was actually connected to the resistor?
I measure approx 2.2 volts across the 10 Ohm resistor in my circuit. I'm assuming that would mean something like:
2.2 / 10 Ohm = 220mA
However, I know when I measure that I only see 157mA of current. So...
220mA - 157mA = 63mA (which are missing somewhere).
I plug in the missing 63 like so:
.8V (voltage drop) / .063 = 12.69 Ohms
So I'm still seeing a extra ~12 Ohms that I can't account for. That is obviously higher than my 9-10 Ohm extra I was calculating but could be due to rounding. Does that seem at least somewhat correct? I'm just noodling my way through this to learn more. Thanks,
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That's a great point and helps me to learn. However I am using 2 AA in series to obtain 3v. I still thought 300mA might be a bit too much to pull but in the spirit of experimentation I rolled on. :)
Carry on! :-D Electronics remains my first love, despite nearly 40 years since school! It's fascinating, always...
Will Rogers never met me.
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I measure approx 2.2 volts across the 10 Ohm resistor in my circuit. I'm assuming that would mean something like:
2.2 / 10 Ohm = 220mA
However, I know when I measure that I only see 157mA of current. So...
220mA - 157mA = 63mA (which are missing somewhere).
I plug in the missing 63 like so:
.8V (voltage drop) / .063 = 12.69 Ohms
So I'm still seeing a extra ~12 Ohms that I can't account for. That is obviously higher than my 9-10 Ohm extra I was calculating but could be due to rounding. Does that seem at least somewhat correct? I'm just noodling my way through this to learn more. Thanks,
Combining a cheap multimeter and an overloaded battery may lead to all kinds of silly conclusions. If you want to comment on a multimeter, make sure you use a lab-grade power supply and resistor both capable to act nominally in your circuit; and likewise when checking a battery, make sure to use a lab-grade multimeter and resistor. And make sure none of the components involved heats up (which indicates operation outside the nominal operating range), as currently your resistor probably and your battery most likely does. :)
Luc Pattyn [My Articles] Nil Volentibus Arduum
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I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
In addition to the other sources of error (I'd wager battery internal resistance is the biggest), you've run into ammeter burden[^]. In most modern digital multimeters, the maximum burden is approximately the full scale on your lowest voltage range, probably something like 200mV. (It measures current by the voltage drop across a shunt.) Welcome to real-world electronic engineering! Cheers, Peter
Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012
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I watched a YouTube that explains it very well #194: What is ammeter burden voltage, and why you should care. - YouTube[^] And someone else pointed out a special port on my meter (marked 20A) which has a smaller burden and I was was able to read 270mA on the circuit. Interesting that it was the meter and not the battery.
Next time you'll measure the voltage over the resistor. :)
Wrong is evil and must be defeated. - Jeff Ello
-
I'm working on Chapter 4 of Practical Electronics For Makers and I'm working the reader through some very simple circuits so we can see how resistors affect (lower) current. So I set up the simplest circuit that is simply a 3V battery supply and one 10 Ohm resistor. Fist I calculate the expected current value I will see using Ohm's law (E = IR).
3V / 10 Ohm = 0.3A (300mA)
I hook up the circuit to run through my multimeter so I can measure (and confirm) the value. My meter displays:
157.7 mA
What? Google... Apparently meters have their own internal resistance. I decide to turn Ohm's law around and calculate internal resistance.
3V / 0.157 = 19.10 (Ohms)
So it looks like my meter has around 9-10 Ohms of its own resistance. Hmmm... Measuring the measurements of a measuring device.:~ Of, course if I had higher resistance in my circuit I'd probably not notice this, because it is only 10 Ohms. I posted a question about this on electronics stack exchange where you can see pictures and the circuit. Multimeter, measuring current calculates less. Can I calculate meter's internal resistance this way? - Electrical Engineering Stack Exchange[^]
Years ago, I had a weird problem with a prototype board and slapped a Tektronix scope on to start looking for spurious signals and the like. And the problem went away. Finally worked out that the scope probe was smoothing out a high frequency nasty so I contacted Tektronix to find out the probe capacitance - and their suggestion was "ship one of our scopes with each board" ... funny guys, very funny. (They did tell us, and a capacitor added to the PCB cured the problem).
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay... AntiTwitter: @DalekDave is now a follower!
