#194

This video describes what is meant by the "burden voltage" of your ammeter, and describes why you should be mindful of it. It also points out that it is not always crystal clear which DMM performs better with respect to burden voltage - it depends on the particular current range used as well as the particular meter used.
Video Notes Page:
www.dorkage.com...

[@xraytonyb]

Thanks Alan, I never really thought about how the shunt resistance affects the voltage in this way. At work, we have a piece of equipment (X-Ray machine) that has an internal 100mA current source. For calibration, you connect the current source to a test point through your amp meter and tell the machine what the current is (should be around100mA) This needs to be very accurate because the auto-calibration uses this information to calibrate the rest of the system. Some of our engineers have had problems getting this to work. Of course, we are each using a Fluke meter on the lowest scale. Some meters are 87's, some 179's and others non-Fluke. All with a current calibration tag. The service manual just says to use a digital multimeter on the mA scale. This solves the mystery! Great video!

[@AppliedScience]

That's a good point about auto-ranging features changing the sense resistance. It could cause oscillation. Thanks.

[@CKOD]

And if its in a bench meter that uses relays to switch ranges, you get a nice annoying rapid clicking of the relays as it toggles between the ranges, letting you know for sure that its unhappy. The relays also momentarily open the circuit, removing power, making the impact on the circuit that much worse.

[@gerhardgroenewald6560]

Damn, didn't think of it in that way. I've been binge-watching your channel all weekend. Quickly went for dinner with the lady and now back to business! Thanx Alan!

[@w2aew]

I hope you enjoy the rest of the videos!

[@davidlisney2059]

Yet another great video as always, many years ago I designed equipment that was then manufactured in house. Part of the process was that I had to write production test specifications, specifying procedures and measurements to be made and the test equipment to use. In one case I had specifically specified the multimeter to use for the measurements which happened to be a 20 amp type. The factory however probably flicked through the document and decided they could do it with a much lower rated meter. The resistance/burden voltage of the lower current meter significantly affected the measurements and of course none of the production units passed to be within spec. Your video is a helpful reminder, thank you! G0FVT

[@w2aew]

As you now, the devil is in the details!

[@johntucker8434]

Alan, I love this video. This is a topic we cover in detail in the Keithley Low Level Measurements Handbook. Well done!

[@w2aew]

Thanks John - the video would've been a lot shorter if I had an SMU to do the measurements! ;-)

[@bobfuller]

Really enjoying your videos, especially the Back to Basics. I've been in electronics for 35 years and still learning new things or reminded about stuff I'd forgotten or shedding light on theory that I hadn't quite grasped in the past.

[@jimadams2473]

W2AEW, you always seem to produce great topics and great quality videos. Just when I thought I knew what I was doing you show me something I should have known. Thanks man for the great video!!

[@donaldfilbert4832]

Great video !! Thanks for all the educational videos !! Very well organized and developed. Just the right amount of information on the topic !! I see that the less expensive meters don't provide the ammeter burden voltage spec info - so now I am off to measure and document my meters !! ;)

[@nebicicek7404]

Another thing worth mentioning is that the inline fuse in the multimeter can affect burden voltage. I've bought dirt cheap 400ma fuses on ebay that measured 5-6 ohms! They were not allowed in my meter.

[@TheBdd4]

And a nice thumbs up to you who have been so helpful to my design understanding!

[@tonysfun]

Great explanation again! You are the man that makes it clear what you are explaining! Thanks!

[@magnehaneberg8605]

Great video as always! I'd like to add that if you have a power supply that has voltage sense inputs, you can often ignore the burden voltage. Though, the burden voltage will have to be less than the allowed voltage difference maximum at the sense inputs. As an example, one power supply I've used recently has a maximum allowable voltage difference between "force" output and "sense" input of 0.5V per lead.

[@fullwaverecked]

Great video! Thank you for the time and effort it takes to make them!

[@robertcalkjr.8325]

Another great lesson. Thanks Alan.

[@maurosobreira8695]

Excellent video, as usual! I wonder why, for devices that are around for so long, the manufacturers did not work around and implement a solution, like Dave did.

[@w2aew]

I suspect it all comes down to money...

[@cny02253]

Well, if you are an engineer you know what this is about already and may work a lot with small currents and voltages. But I bet most meters are used in ways where burden voltage does not make much or any practical difference. In days gone by where high voltages and currents were the rule -it surely did not. But now with lower and shrinking voltage and current requirements -it becomes more of an issue. A different but similar can of worms becomes visible when trying to measure AC voltages of high frequency. Meter input capacitance, impedance -all sorts of things -can get in the way! Burden voltage is only the tip of the iceburg if you want to mostly live beyond 2 decimals to the right.

[@GeorgeTsiros]

A third of a volt? I did NOT expect that!

[@setitthen]

Another good demonstration. Could you also do a demonstration on the effects of loading A.C resonance circuits. Such as what happens when you probe or load a Resonant circuit. Limitations of the multimeter when measuring A.C circuits etc.

[@w2aew]

setitthen I'll add that to my list. Basically any measurement by any instrument can (and will) affect the operation of the circuit, even if it is in some very small way. Resonant circuits are particularly tricky, because the reactance of the probes can alter the resonant frequency, etc.

[@billwilliams6338]

240VAC times 3 amps is "720 watts" power dissipation of the Fuse filament, 120VAC times 3 amps is "360 watts" power dissipation of the fuse filament. The fuse rated for 240Vac at 3 amps can dissipate more heat without blowing.

[@dennisqwertyuiop]

last week I found this by using my 87 and a bench dvm and a cheap dvm on 10 amp scale I thought my bench dvm on ma had a problem because I have no specs for it Thank you very much

[@byronwatkins2565]

Knowing the meter resistance is 11 Ohm, you can supply your own 1.2 Ohm shunt (preferably 1%) to reduce the equivalent to 1.1 Ohm. This will reduce your measured current by 10X so you will need to multiply your reading by 10. This allows an intermediate option.

[@tamyboy1]

Thank you great explanation.

[@mk6595]

Good video, thanks. So if we want to measure low currents, should we get a small (

[@w2aew]

That's one way. Or, if your circuit doesn't mind the voltage drop across the meter, then you're OK. Or, you can use something like +EEVBlog uCurrent adapter.

[@SeAfasia]

Thanks Alan..

[@tomokolo3787]

Msr Alan is it possible if it is not much too ask to make video about small fet signal transistor .I ask about it because i have case where 3 of them act strange , they are 2SK161GR what i observe is there are drain source current leakage with gate pull too ground .In diode test drain too source is .070 volt drop in both direction So i suspect they are all bad.

[@CH_Pechiar]

Alan, maybe you could comment on EEVblog's uCurrent device made to help solve this problem.

[@w2aew]

Dave's +EEVBlog uCurrent has an extremely low burden voltage, and addresses this issue very well.

[@robertcalkjr.8325]

That would be good. I've been thinking about getting one.

[@tonysfun]

I have a question, I'm thinking to make an a adjustable AC bench power supply. I was looking for some schematics but I have not find much. My concern is to have the AC stable, when something using it draws more than 1A (or more) of current. When I needed for my project AC, I just used 2 resistors, wired like voltage divider and also tried to used 2 potentiometers - wire-wound style. It did the trick, but since I'm gonna play with more AC projects, I would like to make a nicer power supply that is stable. I do not need anything more than 10A (that would be at the most I can imagine for now). I have several transformers 120V to 30V, 16V, 12V outputs. One has both 12V and 30V in one. They came from large UPS (Uninterruptible Power Supplies from 1200W and 1500W respectively). I'm not very experienced in electronics, just a new hobby after a 50+ years of other hobbies, I'm trying to rekindle something that started when I was a teenager. I do have 100Mgz Oscilloscope, 50Mgz Function Generator, several meters, 3D printer and lots of parts I salvaged from printers, UPSes ... I love to recycle and fix everything. So I'm not looking to make something that would require lots of knowledge that I lack of right now. I do play with Arduino's and simple projects..... I have several pictures of the transformers - I could not find much about them on the Internet, not even how to connect the mains, but I figured it out - I think. You, or anyone else reading this, can contact me ob my Gmail: [e-mail] (Yes, I was born in Czech Republic, but I live here in the best country in the world! since 1981. Thank you and thanks everyone else who would like to help me out!

[@w2aew]

For a variable AC power source, it is difficult to do better than a simple Variac. Unfortunately, this is not something that you build yourself...

[@tonysfun]

Thank you very much for your suggestion. I do have a nice 2Kv Variac, but I demo many of my "projects" to people/kids... and I am afraid that someone will make an accident and turn it way to high.... If I was to use 2 wire-wound potentiometers to dial in the AC, do I need any other protection and or something to stabilize the simple voltage regulator? Or am I just worrying about nothing? This power supply will run some simple projects where no more than 1A will be used - again, just a simple projects that are mainly a demo, rather than dependable product for someone else. For that I would use a transformer with the correct output and properly wired, grounded and fuse....

[@w2aew]

Potentiometers can be a problem because they will make the voltage dependent upon the load current. If you want to lower the maximum output of the variac, simply put a step down transformer before it.

[@tonysfun]

Since I am not able to find any schema and or help from users out here, I purchased a small VARIAC and I'll use a transformer with it. Thanks for your help again!

[@billwilliams6338]

If you have a fuse that is 120VAC at 3 amps, can you replace it with a fuse at 240VAC at 3amps? Does the fuse filament change from the Voltage rating or just the current rating? What i mean is I often see this from USA equipment to UK equipment they have interchangable fuses. The USA equipment will have 240VAC fuses in them even tho we are useing 120VAC outlets. So I'm not sure if the fuse will blow if the fuse filament is at 240VAC and we are using 120VAC outlets. If the device under test has a short circuit the 240VAC at 3 Amps will blow "later" compared to 120VAC at 3 Amps when using an AC outlet at 120VAC?

[@w2aew]

You can generally use a higher voltage rating on the fuse. A 3A fuse rated for 120VAC or 240VAC will blow when current exceed 3A. The voltage rating is there to indicate what voltage it will block (and not arc over) after it is blown.

[@billwilliams6338]

If a battery is rated at 6 vdc at 3AH, if i put my current clamp around the RED battery wire i should measure 3 amps? If the circuit board is only drawing .5 amps from the 6vdc/3AH battery, how would you know if this 3AH battery is good or bad because the battery voltage is measuring 6 vdc and the battery current is measuring . 5 amps so how do you know if the battery is good or bad? Since the battery is rated for 3amphours this means that if a circuit is drawing 1 amp per hour the battery will only last for 3 hours?

[@billwilliams6338]

Ammeters current meters impedance can be 1meg, 2K or 1K. Why would you want to use a 2K or 1K impedance on the current meter? Ammeter current meters have a spec called frequency weighted impedance. What does it mean by frequency weighted impedance? weighted by what are they referring to

[@jaythompson1502]

Every time I watch one of these I realize how much stuff I've forgotten since I left school. Just a minor note about this video, about 4:25 min in you change to the 10 amp scale and mention you've lost some resolution because you're on "the 4 milliamp scale".

[@w2aew]

yeah - I have an annotation in the video that notes the error in my statement...

[@jaythompson1502]

***** I don't see any annotations in Roku, just watched it on my PC to see the note you added.

[@w2aew]

Interesting that the annotations don't show up when watching through the roku. Many of my videos have annotations.

[@davecc0000]

+w2aew Don't see this annotation on my Sam smartphone. FYI...

[@jpalm32]

Very interesting

[@leppie]

I have a Fluke 867B. It only lists max burden voltage per range. For example: 300mA range is 0.3V max. (30mA is 0.03V) So that would be 1mV/mA burden voltage? For uA range it is listed as 3000uA with 0.3V max. So that is 100mV/mA?

[@w2aew]

Yes.

[@leppie]

***** Thanks. She is an oldy and heavy, but specwise, it matches a 287-9 :)

[@billwilliams6338]

oh ok thanks, I thought the voltage X the current will be the wattage rating of the fuse filament like a resistors wattage rating. PTC fuses are often used for protecting output ports so if you short circuit the output ports of a circuit it will open the PTC fuse. The specifications to the device under test will say the PTC fuses at 1amp for an example. The PTC fuse doesn't have a filament or isn't like a solid state circuit breaker. I'm not really sure how PTC fuses work, it seems its based on the heating factor of the current, the higher the current more dissipation in heat which will open theh PTC fuse?

[@alanwolke6253]

Voltage times current is the power dissipation of the filament. Voltage divided by the current is the operating resistance.

[@yoramstein]

A beginner (electricians) question: Why companies (famous for quality) do not reduce the serie resistor (tapper) and build a higher gain amplifier to drive the display circuitry?

[@w2aew]

+Yoram Stein Usually it is a matter of noise and accuracy - higher gain = higher amplified noise and amplification of small offsets, etc.

[@KX36]

I blindly apply my meter to anything without considering burden voltage OH YEAH, WHO'S THE BADASS NOW! :D

[@richardgray8593]

@KX36 Respect!

[@billwilliams6338]

ok thanks

[@billwilliams6338]

How sensitive can a Clamped Current meter measure. To measure microamps of current EE engineers say that the Current Clamp meters can't measure microamps of current because of the magnetic field is so small it can't measure the small microamps of current. Even a fluke 87 or 89 can't measure microamps of AC and DC current because its very small current that the fluke meter can't pick up?

[@w2aew]

Clamped current meters are not very sensitive, partly because they also pick up noise (the clamp/loop is like an antenna).

[@101ful]

Why havent they called this shunt resistance? That burden voltage parameter is confusing IMO.

[@geoden1645]

I agree. Too much of this kind of obfuscating 'nonsense-speak' going on these days.

[@superdau]

GeoDen This isn't "speak of these days". It's called like that for decades. But I too think it would be helpful if it just would be called "...something... resistance". On the other hand, you are reading Amps on the meter, so with a value of "volts per amps" there's no way for someone using it to balls up Ohm's law. In this case it would amount to a multiplication with exactly the same value. But someone could trip and divide instead of multiply for example when there's a resistance given.

[@w2aew]

101ful It *used to* be called shunt resistance, back in the days when the meters were analog. An analog meter movement is a low-resistance device that has a full scale sensitivity in the 10s to 100s of microamps typically. To measure larger currents, a very low value resistor was connected in parallel with the meter (shunting most of the current away from the meter's coil), so that the meter would only see a small portion of the current. This is why it was called a shunt resistance. With digital meters, the "meter" is a very HIGH impedance, so the resistor isn't a "shunt" resistor anymore, it is more a of a "sense" resistor.

[@jonsanford0]

***** I remember "Ohms per Volt" as the term for analog meters. Much more critical to keep it in mind in the old days.

[@w2aew]

jon sanford The Ohms per Volt refers to the loading of the Voltmeter, not the ammeter.

[@billwilliams6338]

Thanks, But why do they call it parasitic? i thought parasitic ment coupling like parasitic capacitance means coupling i thought, its an unwanted capacitance you want in your circuit it parasitic capacitance its not something you want. But in cars parasitic drain doesn't mean coupling it means something else

[@w2aew]

Parasitic can apply to any unwanted/unintended thing or effect.

[@billwilliams6338]

When cars have a short they call it a "parasitic drain", What do they mean by this parasitic drain? What is parasitic in a car do they mean the wiring resistance harness or the load that is parasitic? When the car is turned off they measure the DC current of how much parasitic drain is drawing from the car battery with the car turned off. I'm not sure why there is any parasitic drain or current draw when the car is turned off it should measure in my opinion zero current, but in cars it measure normally 50mA of parasitic drain current from the car even with the car turned off. I'm not sure what this parasitic drain current is coming from or why its 50mA when it should be zero.

[@w2aew]

It is called parasitic drain because it is an untended drain on the battery caused by the standby currents of things connected, or by some faulty circuit or wiring. Most cars have some parasitic drain because even when they are off, there are circuits that are still powered up - security systems, keyless entry receivers, etc.