#194: What is ammeter burden voltage, and why you should care.

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:

[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.

[gerhardgroenewald]

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!

[davidlisney]

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

[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.

[johntucker]

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

[jimadams]

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!!

[magnehaneberg]

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.

[nebicicek]

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.

[tonysfun]

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

[googletitsfost]

I have a collection of meters and always grab the one closest to hand, since I don't remember off-hand what all the parameters are of the various meters I have a 3 different power cables, each with a 1, 10 or 100 ohm resistor and just measure across it with the meter. It also means that I don't have to try and break into the circuit and attach my probes, which never ends up well when you want to use bulky equipment on small designs. 

[donaldfilbert]

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 !!  ;)

[TheBdd]

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

[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.

[fullwaverecked]

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

[maurosobreira]

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.

[GeorgeTsiros]

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

[billwilliams]

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.

[byronwatkins]

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.

[mk]

Good video, thanks.  So if we want to measure low currents, should we get a small (<1ohm) precision resistor insert it into our circuit and measure the voltage across it?