Help with Basic Circuits

dl324

Joined Mar 30, 2015
16,722
Is there another term for that parameter?
It can also be specified as a shunt resistance. This is for my Simpson 467:
1693272709962.png

This is how meter perturbation is corrected for my Fluke 27:
fluke27CurrentCorrection.jpg

The bottom line is that no meter is ideal and you can avoid hassle by calculating current instead of measuring it. I assure you that can trust Ohm's Law.

There are times when the non-ideal nature of voltmeters can also affect readings, but in your experiment, your voltmeter isn't introducing any meaningful error.
 
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BobTPH

Joined Jun 5, 2013
8,677
To validate Ohms law, you need to measure two things at the same time.

The voltage across the resistor and
The current through the resistor.

It takes two meters to do this. It cannot be done with 1 meter.

You are using the unburdened voltage at the terminals of the battery. That is not the voltage across the resistor. There are three resistors in the circuit: The internal resistance of the battery, the resistance of the ammeter, and the resistor under test. Actually, all the wires also have resistance. The point is that the voltage measurement must be done with the circuit operating and must be across the resistor under test only. And this must be done with the ammeter in the circuit since it will introduce its own resistance.
 

Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
To validate Ohms law, you need to measure two things at the same time.

The voltage across the resistor and
The current through the resistor.

It takes two meters to do this. It cannot be done with 1 meter.

You are using the unburdened voltage at the terminals of the battery. That is not the voltage across the resistor. There are three resistors in the circuit: The internal resistance of the battery, the resistance of the ammeter, and the resistor under test. Actually, all the wires also have resistance. The point is that the voltage measurement must be done with the circuit operating and must be across the resistor under test only. And this must be done with the ammeter in the circuit since it will introduce its own resistance.
Good points. That is what I am trying to do just for experimental and research inquiry in a simple circuit. All the parameters
involved.. I did have the resistor in the circuit when I measured the current, and I measured the voltage at the cell with and
without the load.

Also how do you measure the internal resistance of the cell or battery?

Would this apply also to power supplies, even little cheap ones you get on amazon?

Thanks
 
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Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
It can also be specified as a shunt resistance. This is for my Simpson 467:
View attachment 301520

This is how meter perturbation is corrected for my Fluke 27:
View attachment 301521

The bottom line is that no meter is ideal and you can avoid hassle by calculating current instead of measuring it. I assure you that can trust Ohm's Law.

There are times when the non-ideal nature of voltmeters can also affect readings, but in your experiment, your voltmeter isn't introducing any meaningful error.
Are you saying that current measurement (with a meter) is the most unreliable parameter and that measuring Voltage and Resistance is more reliable with a meter? In other words, measuring current is somewhat innacurate?

Thanks
 

MrChips

Joined Oct 2, 2009
30,504
Are you saying that current measurement (with a meter) is the most unreliable parameter and that measuring Voltage and Resistance is more reliable with a meter? In other words, measuring current is somewhat innacurate?

Thanks
No.

Every meter has resistance. An ideal voltmeter has infinite resistance. Real voltmeters have finite resistance that must be taken into consideration. You want the voltmeter’s resistance to be greater than 10 times the resistance of the circuit being measured for less than 10% error.

An ideal ammeter has zero resistance. You want the real ammeter to have a resistance that is 10 times lower than that of your circuit.
 

boostbuck

Joined Oct 5, 2017
492
measuring current is somewhat inaccurate?
Any measurement has a variance from an absolutely correct value.

Generally a multimeter will have lower accuracy on amps compared to volts, but measuring current per se is not 'less accurate' than measuring volts. It depends on the tool you use and its accuracy. You can, with sufficient care (and money) measure any parameter to almost any accuracy required.

Your resistor is given a value to 1% accuracy. Your multimeter can measure voltage across it to a defined accuracy. From that you can calculate the current in your circuit AND calculate your confidence in that measurement (the accuracy). You can never achieve an accuracy of measurement of 0.0%.
 

BobTPH

Joined Jun 5, 2013
8,677
Also how do you measure the internal resistance of the cell or battery?
You don’t need to. Just measure the voltage across your 10 Ohm resistor instead of across the the battery terminals.

Do you have two meters? If not, I think you are out of luck.

The internal resistance is not a simple resistance at all, it might vary considerably with different currents. To measure it, you can just measure the voltage at the terminals with no current flowing, then measure it again with a known current flowing. The the resistance is calculated from Ohm’s law as:

Rinternal = (Vunloaded - Vloaded) / I

Again you need two meters. The measurements must be at the same time.
 

MrChips

Joined Oct 2, 2009
30,504
The burden voltage of a Fluke 87 DMM is 1.8mV/mA.
Your data (if measured correctly) is showing a burden voltage of 2.8mV/mA for your meter.

If you have a second DDM, measure the voltage across the 10Ω resistor, the ammeter, and across the resistor and ammeter together (i.e. across the power source).

Here is how to measure the current and voltage simultaneously.

1693283011779.png
 

dl324

Joined Mar 30, 2015
16,722
Are you saying that current measurement (with a meter) is the most unreliable parameter and that measuring Voltage and Resistance is more reliable with a meter? In other words, measuring current is somewhat innacurate?
Not exactly. I didn't say anything about the reliability of readings.

I said that on amps or voltage (or ohms, which I didn't mention), the meter affects the circuit, but the effect on volts (or ohms) usually has less of an effect because the input impedance of most DVMs is in the 1-10GΩ range.

If you put 1GΩ in parallel with 10Ω, the difference is so small that it isn't meaningful (10Ω || 1GΩ = 9.999999Ω). However, if you put that meter across a 1GΩ resistor, the resistance with the meter in the circuit becomes 500MΩ.

Your 10.1Ω resistor would become 10.0999999Ω.

In this circuit, if you measured voltage from the drain to source, the meter would introduce significant error when the MOSFET was off.
1693318088227.png

Measuring current is more problematic because the burden voltage/shunt resistance is more likely to introduce a significant error in the reading. Because of that, I very rarely measure current with a meter. If I did, I'd be more inclined to use a clamp on meter that measures current by inductive coupling.

Old timers will be very cognizant of error introduced by meters because analog meters were more likely to perturb circuits and we took that into consideration.
 
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MrChips

Joined Oct 2, 2009
30,504
If you want to measure the current in a 10Ω circuit you need a burden resistor of 0.1Ω or lower for 1% accuracy.
Measure the voltage across a 0.1Ω resistor in series.
With 1.5V source, you would be measuring about 15mV in a 10Ω circuit.
 

Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
Not exactly. I didn't say anything about the reliability of readings.

I said that on amps or voltage (or ohms, which I didn't mention), the meter affects the circuit, but the effect on volts (or ohms) usually has less of an effect because the input impedance of most DVMs is in the 1-10GΩ range.

If you put 1GΩ in parallel with 10Ω, the difference is so small that it isn't meaningful (10Ω || 1GΩ = 9.999999Ω). However, if you put that meter across a 1GΩ resistor, the resistance with the meter in the circuit becomes 500MΩ.

Your 10.1Ω resistor would become 10.0999999Ω.

In this circuit, if you measured voltage from the drain to source, the meter would introduce significant error when the MOSFET was off.
View attachment 301569

Measuring current is more problematic because the burden voltage/shunt resistance is more likely to introduce a significant error in the reading. Because of that, I very rarely measure current with a meter. If I did, I'd be more inclined to use a clamp on meter that measures current by inductive coupling.

Old timers will be very cognizant of error introduced by meters because analog meters were more likely to perturb circuits and we took that into consideration.
What's a good clamp on meter to get?
Are any of these adequate?
https://www.amazon.com/s?k=clamp+on+meters&crid=2FTTSL97Z13QR&sprefix=clamp+on+meters,aps,431&ref=nb_sb_noss_1

Note: right off I would buy the least expensive one.

This one looks like a good deal. What do you think?
https://www.amazon.com/Multimeter-Auto-ranging-Temperature-Capacitance-Plusivo/dp/B08MTY51R4/ref=sr_1_8?crid=2FTTSL97Z13QR&keywords=clamp+on+meters&qid=1693341032&sprefix=clamp+on+meters,aps,431&sr=8-8&th=1

What are other advantages of a clamp on meter?

If Old-timers don't like to use meters, then what do they use?


I'm on a tight budget, resurrecting electronics interest I started many years ago,
when I took some AC-DC courses. I also took some digital courses, and Boolean algebra.
including an intro course of microprocessors using a Heathkit breadboard microprocessor.
It might have been this one, not sure: https://www.ebay.com/itm/266386084678?hash=item3e05d91346:g:DFcAAOSwPXZk59zx&amdata=enc:AQAIAAAA0MBHJxvsU7IFcg7QRc1Z+pjicNMJs8icBY/b5Tl0sPx6v5KIP08iqEtNLNIG3do7vtwdAOjEJK2vaVMh8eyk3kZt60hLHyqOcIXro86Hmeog0ho1XxjKllnTMUCNT31A4BDEDl5B6SAiL+wTZtTlwac3LbgfV7C88O9KOB6P3zxv++OnQ7rhyN2jqLiWfXn3JmS/iryILMYVarFPSCljnMe4SNfnZHUCPrJeJhpzPKvc60Sl/iLiy3EPwSuVmxJvdhdzzQCsWQ/6yUSbFpYytWc=|tkp:Bk9SR_SjjbDIYg

I don't know what good this resurrecting will do, it's probably not enough to get a job,
but maybe I will be able to fix some things eventually or do something productive after
getting the cobwebs out and getting the self taught train rolling.


Thanks
 
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Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
That does not tell us much. Tell us the make and model of the meter and perhaps we can look up the specifications.
No Simpsons or Flukes here although I have used those.

My main meter is: Radio shack 22-811
I paid $100 about that much 20-25 years ago. Now I seen them on ebay for $18
I also have a cheap Chinese DMM I got in a kit off of Amazon
Additionally I have a very cheap DMM I got at Harbor Freight. It even has a transistor tester.
 
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dl324

Joined Mar 30, 2015
16,722
What's a good clamp on meter to get?
I have one that I use for currents in the 2-10A range from Harbor Freight when I'm working on my cars or house wiring. For electronics, I use this Fluke:
1693349509821.png
What are other advantages of a clamp on meter?
Not having to have it in-circuit is the main one.
If Old-timers don't like to use meters, then what do they use?
I didn't say old-timers didn't like to use meters. I said we understand when a meter is likely to affect the circuit enough to affect the accuracy of readings.
I'm on a tight budget, resurrecting electronics interest I started many years ago,
when I took some AC-DC courses.
Then you won't want the Fluke clamp on meter I use.

Don't spend a lot of money on equipment until you're sure you're going to need it. I did electronics as a hobby for 20-30 years without having scopes, function generators, frequency counters, etc. I had a couple power supplies (one was a project), a clock generator (also a project) that was just a crystal oscillator with some counters and a rate multiplier, a DVM that was also a project (just a panel meter with some dividers and a voltage reference for calibration). My first meter was an RCA analog.

It wasn't until 15-20 years ago that I started buying test equipment from the 1970-90's because I learned on that equipment and I didn't need anything better.
 

WBahn

Joined Mar 31, 2012
29,878
The 100 ohm resistor is reading 154 ma good but trying to locate the missing 6 ma is interesting for learning.

No doubt its taken up somewhere in the hardware, but where? No big deal except for learning.
That 6 mA amounts to <4% difference. You have the actual value of the resistance. The actual voltage of the supply under load. The error in the meter measurement. The perturbation of the test setup on the results. The changes in one or more parameters under load (such as due to heating). It doesn't take much to rack up a 5% total error.

The basic accuracy of the meter is +/-1% and +/-3 in the last digit (and that's when it was new).

Also I would like to know the burden voltage for my meter for general purposes if anyone know how to determine that?
You can measure it if you have another meter.

Put a source (battery) and a resistor in series with your meter and then use the other meter to measure the voltage across the terminals of the first meter. The burden voltage is the voltage divided by the current. Check it at a few different current levels to see if you get the same result (or at least pretty close). Each range will have it's own burden voltage.

Keep in mind that another name for "burden voltage" is simply the meter's shunt resistance on that range. You can most likely just use one meter to measure the resistance of the other meter on a current range setting -- which will incidentally also tell you how much current the other meter is trying to use in it's resistance measurements. But there is a bit of a warning here -- the two meters may not play well with each other and, in some cases, you could actually damage one or both meters, but this is pretty unlikely.

Additionally, knowing the internal resistance of the 1.5 (1.6) V cell.
The notion of internal resistance is more conceptual -- there is not actual resistor in there. But the terminal voltage tends to drop as more current is drawn from the cell and this can be reasonably modeled as effectively having an internal resistor in series with an ideal voltage source within the cell. But what's important is to keep in mind that there isn't a single number that describes a cell's internal resistance -- it will vary with current, with temperature, and with state of charge.

To come up with a value that is useful, you want to measure the cell voltage with no load, the cell voltage with a load of approximately what you will be drawing from the cell, and to get a better model, the cell voltage with a load that is about 10% different from the first one. But whatever you come up with, it will change (generally go up) as the battery discharges.
 

Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
I have one that I use for currents in the 2-10A range from Harbor Freight when I'm working on my cars or house wiring. For electronics, I use this Fluke:
View attachment 301604
Not having to have it in-circuit is the main one.
I didn't say old-timers didn't like to use meters. I said we understand when a meter is likely to affect the circuit enough to affect the accuracy of readings.
Then you won't want the Fluke clamp on meter I use.

Don't spend a lot of money on equipment until you're sure you're going to need it. I did electronics as a hobby for 20-30 years without having scopes, function generators, frequency counters, etc. I had a couple power supplies (one was a project), a clock generator (also a project) that was just a crystal oscillator with some counters and a rate multiplier, a DVM that was also a project (just a panel meter with some dividers and a voltage reference for calibration). My first meter was an RCA analog.


It wasn't until 15-20 years ago that I started buying test equipment from the 1970-90's because I learned on that equipment and I didn't need anything better.
You say you bought 70s to 90s test equipment. You don't see any value in the newer digital oscilloscopes ? (for your electronics hobby).
 

Thread Starter

RUSTYWIRE

Joined Aug 28, 2023
61
That 6 mA amounts to <4% difference. You have the actual value of the resistance. The actual voltage of the supply under load. The error in the meter measurement. The perturbation of the test setup on the results. The changes in one or more parameters under load (such as due to heating). It doesn't take much to rack up a 5% total error.

The basic accuracy of the meter is +/-1% and +/-3 in the last digit (and that's when it was new).


What is the accepted procedure or initial setup and adjustment regarding these tolerance parameters you cite above
for multimeters when working in industry? If you had a test requirement spec for the proprietary circuit being tested
(made by design & QC engineers), would you have to calculate all those foresaid test equipment tolerance parameters
and inefficencies?
Are these math considerations to be calculated before beginning addressing and manufactured circuits at hand, or
do you just jump right in to analyzing the circuits relying on and trusting the accuracy of the test equipment?

The reason I ask is if in a few months down the line if I would get an entry level, even minimum wage job as a tech
would I be expected to go over and grasp the specs of the test equipment first, before even looking at the product to be tested
or is that someone else takes care of?




You can measure it if you have another meter.

Put a source (battery) and a resistor in series with your meter and then use the other meter to measure the voltage across the terminals of the first meter. The burden voltage is the voltage divided by the current. Check it at a few different current levels to see if you get the same result (or at least pretty close). Each range will have it's own burden voltage.

Keep in mind that another name for "burden voltage" is simply the meter's shunt resistance on that range. You can most likely just use one meter to measure the resistance of the other meter on a current range setting -- which will incidentally also tell you how much current the other meter is trying to use in it's resistance measurements. But there is a bit of a warning here -- the two meters may not play well with each other and, in some cases, you could actually damage one or both meters, but this is pretty unlikely.



The notion of internal resistance is more conceptual -- there is not actual resistor in there. But the terminal voltage tends to drop as more current is drawn from the cell and this can be reasonably modeled as effectively having an internal resistor in series with an ideal voltage source within the cell. But what's important is to keep in mind that there isn't a single number that describes a cell's internal resistance -- it will vary with current, with temperature, and with state of charge.

To come up with a value that is useful, you want to measure the cell voltage with no load, the cell voltage with a load of approximately what you will be drawing from the cell, and to get a better model, the cell voltage with a load that is about 10% different from the first one. But whatever you come up with, it will change (generally go up) as the battery discharges.
"See Underlined & bold text above ..

Thanks
 
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WBahn

Joined Mar 31, 2012
29,878
The steps that you would take depend on the requirements of the task at hand.

In most situations, a result within 5% is probably good enough. Good designs take into account component tolerances and are intended to make it so that these variations do not prevent the system from working properly. If, for some reason, the problem being solved requires tighter tolerances, then measurement procedures that are consistent with those tolerances need to be used. What that entails depends on the specifics of the measurement. Often it is nothing more than using higher quality test equipment that has been properly calibrated. But the steps can get extreme. When I was working at NIST, we made measurements were the alligator clips used to connect to the test system not only had to be connected in the proper orientation because the thermal voltages in the hinge going from one side of the clip to the other introduced unacceptable noise, but we also had to pack the connection with lead-filled shot bags (some of which my landlady made for us) and let it sit and equalize because the air currents in the room would also result in excessive noise. But we were measuring nanovolt-level signals, so extreme measures where needed.

The first step in this process is determining how good the measurement needs to be to be good enough. That guides your efforts from there as you strive to achieve a measurement that is good enough.
 

dl324

Joined Mar 30, 2015
16,722
You don't see any value in the newer digital oscilloscopes ? (for your electronics hobby).
Not really. I have several 7D20 70MHz digital scope plug-ins for Tektronix 7xxx series oscilloscope mainframes. I have a 2 channel HP 15xxx series 2GSa/s scope that's not even worth the bother of using. I have a 20MHz Hantek USB scope that someone gave me. He never used it and neither have I. I also have a few of the 100KHz-1MHz DSO toys. I bought them just to see what all of the fuss was about.

The scope I use the most is a Tek 7704A 200MHz mainframe with 2 7A26 vertical amplifiers (4 channels total at 200MHz) with a 7B53A time base (that limits the system to 100MHz). A 7D20 plug-in converts that mainframe to a 70MHz DSO.

From 7704A - TekWiki (w140.com) :
1693431478429.png
7D20 - TekWiki (w140.com)
1693431588771.png
 
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