How to measure the Burden Voltage of your multimeter - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 03:07:26 pm
content: Hi,I have the UNI-T UT61E multimeter and many people complain about its high burden voltageof around 0.85 Volts.When measuring current the multimeter adds a small resistance in the circuit under measurementthat will affect significantly all low current (with low voltage) measurements.If I measure the mA + COM terminals of my UT61E, I get 11.84 Ohms.How can I calculate the burden voltage of the multimeter?Thank you.

author: Balaur
author_rank: Unknown
timestamp: May 14, 2013, 03:48:45 pm
content: You can measure it directly using a second multimeter on the voltage range, in parallel with the one that's measuring the current.Otherwise, assuming a current measuring method based on measuring the voltage drop (i.e. the burden voltage) on the internal constant resistance, then, on a given scale:Burden voltage = Internal resistance * measured currentIn your case, for the mA scale:Burden voltage (mv) = 11.84 * Indication in mACheers

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 04:12:51 pm
content: Actually that's what I did and got a very small number for the burden voltage.I had a 325? resistor and a small 1.5V battery connected to the mA terminalsof the multimeter that was reading 4.455 mA. So If I multiply the multimetermeasured resistance (11.84 ?) times 4.455mA I will get 52.7 mV, correct?Something is wrong...You said "based on measuring the voltage drop (i.e. the burden voltage)"Is the burden voltage the same as the voltage drop?Burden voltage is measured in mV per mA.

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2013, 04:26:21 pm
content: Quote from: hgg on May 14, 2013, 03:07:26 pmThe burden voltage is just Ohm's law.So if you place your meter on the mA range and measure a current of 5 mA, then the burden voltage for that measurement will be:Voltage burden = 0.005 A x 11.84 ohms = 0.06 V, or 60 mVNote that the burden voltage is not a constant, it is a function of the current you are measuring and the range selection on the meter.There is no magic to it. If you take any multimeter and place it in a current range you will measure a fixed resistance between the current measuring terminals. This resistance will be higher on a "mA" range and lower on an "A" range.

author: Balaur
author_rank: Unknown
timestamp: May 14, 2013, 04:34:52 pm
content: Sorry for my previous message, got things mixed up in my head. Please ignore it.Let's start over:IfI. Burden voltage = Voltage drop/CurrentandII. You measured an internal resistance of 11.84 ohmsthen:Burden voltage = 11.84 mV/mAMaybe there is a 10 Ohm shunt on this scale + fuse + leads

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 04:49:03 pm
content: IanB, what are the units of measurement of Burden Voltage?They are not mV. They are mV/mA, correct?Balaur, I am confused...How did you calculate the 11.84mV/mA burden voltage?11.84 Ohms is the resistance I measure when I place the probes on the mA & COM.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 14, 2013, 04:57:08 pm
content: Quote from: hgg on May 14, 2013, 04:49:03 pmMr. Ohm's said so1 Ohm = 1V/1A = 1mV/1mASo, the units of "Burden Voltage" could be expressed as mV/mA with IS THE SAME as Ohms.

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2013, 05:16:19 pm
content: Quote from: hgg on May 14, 2013, 04:49:03 pmAs kfitch42 said.If you measure a current of 500 mA with your meter, and the voltage drop across the meter terminals is 1 V, then the voltage burdenfor that measurementis 1 V.You could also say that the meter has a characteristic burden voltage on that range of 2 V/A, or 2 mV/mA.This is exactly the same as saying the meter has an internal resistance on that range of 2 ohms.

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 05:33:17 pm
content: kfitch42, I see what you are saying, so maybe the term Burden Voltage is a misleading term.It should have been called theBurden Resistance!, correct?Is the internal resistance of the multimeter in a specific scale, constant?If it is, wouldn't it make more sense to talk about theresistanceand not the voltage drop,since the voltage drop is variable?So theburden resistanceacross the multimeter leads (in mA&COM) in this circuit should be:I x R => 4.455mA x 11.84 Ohms = 52.7mV/mA which I suppose is high compared for examplewith the 1.8mV/mA of the Fluke 87V...Can anybody with a Fluke 87V measure the resistance of the mA scale?

author: Unknown
author_rank: Unknown
timestamp: May 14, 2013, 05:53:51 pm
content: So many worries

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 06:00:25 pm
content: Ok, it turns out that in my little collection of crappy multimetersthe tiny UT120Cwill make the most accurate low current measurements!Its internal resistance in the mA scale is 3.84 Omhs and the current that it measuredin the same circuit was 4.55mA. Burden Resistance = 17.5mV/mA 3 times better thanthe UT61E. I actually measured the voltage drop of the UT120C with the UT61E andit gave me a reading of 16.5mV/mA. Still 9 times worse than the Fluke 87V. Oh, well...

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2013, 06:19:37 pm
content: Quote from: hgg on May 14, 2013, 05:33:17 pmYou seem to be confused.mV/mA is exactly the same as ohms.Therefore 11.84 ohms is 11.84 mV/mA is 11.84 V/A...

author: Balaur
author_rank: Unknown
timestamp: May 14, 2013, 06:21:12 pm
content: Quote from: hgg on May 14, 2013, 05:33:17 pmSorry, but no. In this context, the burden voltage value is identical to the internal resistance of the multimeter on that scale.In your case, that means:- UNI-T UT61E: 11.84 mV/mA on the mA scale- UT120C: 3.84 mV/mAHere are a few examples of my own: (measurements include short, good leads)- Fluke 187 mA - 1.873 mV/mA- Fluke 187 A - 0.1 mV/mA- Fluke 112 A - 0.1 mV/mA- Fluke 117 A - 0.086 mV/mA- Velleman DVM890 mA - 2.835 mV/mA- Velleman DVM890 A - 0.075 mV/mA

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 06:45:19 pm
content: So,(a) if you actually measure only the internal resistance of the multimeter,that's also its burden voltage in mV/mA, Yes? and(b) it looks like that Burden Voltage is a constant value for each scale of the multimeter (?)Don't you agree that it should be called Burden Resistance or Burden Impedance or somethingother than Voltage?(sorry for my many questions...)

author: darrylp
author_rank: Regular Contributor
timestamp: May 14, 2013, 07:13:58 pm
content: The actual circuit diagram for the ut61e shows a 0.01 resistor for the amp range.A resistor divider of 1ohm to 9ohm for the mA range. And a combined divider of an extra 990 ohms on the microamp range. Ie. 1ohm to 999ohm diviider.Amps = 0.01 ohms.mA = 10 ohmsuA = 1000 ohms.--Darryl

author: ivan747
author_rank: Super Contributor
timestamp: May 14, 2013, 07:22:10 pm
content: Quote from: darrylp on May 14, 2013, 07:13:58 pmDon't forget to add the resistance of the test leads, fuses and PCB tracks connecting these resistors to the outer world.

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 07:37:32 pm
content: Darryl, indeed that was what I actually measured on the mA & uA scale.

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 14, 2013, 07:40:21 pm
content: The UT61E is an odd bird when it comes to burden voltage.ItThe resistance changes based on the amount of current. The clue is in the circuit diagram. For the mA range the voltage is sensed across R29 which measures 1.0 ohm on my unit despite being labeled 2.0 on the diagram. The 9 ohm portion of the shunt is bridged with antiparallel diodes. Once the diodes start to conduct, the burden voltage is reduced.All the tests pictured below are with the UT61E locked in the 220.00mA range.1. Measured resistance: 10.3 ohms2. 106mV / 10.22mA = 10.37 ohms3. 520mV / 50.46mA = 10.31 ohms4. 862mV / 100.32mA = 8.59 ohms5. 1072mV / 200.79mA = 5.34 ohms

author: darrylp
author_rank: Regular Contributor
timestamp: May 14, 2013, 07:52:41 pm
content: The 2ohm resistor is actually two in parallel. Thus 1ohm resistance.--Darryl

author: PA4TIM
author_rank: Super Contributor
timestamp: May 14, 2013, 08:00:50 pm
content: From theKeithley 2000manual:Current range burdon voltage10 mA < 0.15 V100 mA < 0.03 V1 A < 0.3 V3 A <1 VSHUNT RESISTOR: 0.1? for 3A, 1A and 100mA ranges. 10? for 10mA range.From theAgilent U1252Amanual:range burdon voltage500.00 ?A 0.06 V (100 ?)5000.0 ?A 0.6 V (100 ?)50.000 mA 0.09 V (1 ?)440.00 mA 0.9 V (1 ?)5.0000 A 0.2 V (0.01 ?)10.000 A 0.4 V (0.01 ?)An AVO 8 testreport wrote they measured 250 mV for all ranges except 50uA where they measured 125mVPtrema 5017: (7,5 digit meter, 0,03% reading + % 0.002% range)200 uA 350 mV 1000 Ohm2 mA 350 mV 100 Ohm20 mA 350 mA 10 Ohm200 mA 350 mA 1 Ohm2A 600 mA 0.1 OhmI looked some more manuals from my meters but no mention of mV/mA. Never measured it because I almost never use the current mode.Most brands state just one Voltage that is about 1.5 upto 4 times U=ixR

author: hgg
author_rank: Frequent Contributor
timestamp: May 14, 2013, 08:18:21 pm
content: Ok, now I am really confusedEspecially after reading the following definition as well:http://www.ni.com/white-paper/3296/en#toc6He says that Burden Voltage is the Voltage Drop caused by current.The electrocuted Fluke spaceman says that "What it all boils down to is that this burden voltage has beensubtracted from the voltage available to the circuit being tested." He subtracts Voltage from Voltageand then he says that Burden Voltage is measured in mV/mA. Crazy.http://www.fluke.com/fluke/usen/community/fluke-news-plus/ArticleCategories/Electrical/BurdenVoltage.htmVoltage is measured in Volts, not in V/A which is resistance.Really confused. That's it, I am going to sleep...(If somebody knows an 'official' definition of Burden Voltage please let me know)... at least I know why is called 'burden'...

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 14, 2013, 08:54:49 pm
content: Quote from: hgg on May 14, 2013, 08:18:21 pmI hope my clumsy wording above didn't contribute to your confusion. The burden voltage arises from the resistance of your meter's current shunt, plus the fuse, the jacks, etc. It will of course change depending on how much current is flowing, according to V = I*R. For most meters, knowing what R is will allow you to predict the burden voltage in any specific situation.So when you see mV/mA called as as the "burden voltage", technically it's not a voltage, but a resistance figure that allows you to calculate the actual burden voltage for the level of current you're measuring.My point was that for the UT61E, R is not constant, and thus the burden voltage is not exactly predictable.

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 14, 2013, 08:56:03 pm
content: Quote from: darrylp on May 14, 2013, 07:52:41 pmI figured as much with the "x2" notation, but since the components are on the bottom side of the board, I was too lazy to check.

author: hgg
author_rank: Frequent Contributor
timestamp: May 15, 2013, 03:57:07 am
content: Hi,I woke up with a clear explanation.ModemHead, you are right. That is the correct explanation.Burden Voltage, is nothing more than Voltage and its the same as theVoltage Dropcausedby the internal resistance of the multimeter. Volts. Now mV/mA is another way to talk aboutthe internal resistance of the multimeter. mV/mA doesNOTrepresent burden voltage,but is a measure of its cause.Burden Voltage is not a constant while mV/mA is a constant. (more or less) Well, not exactly aconstant because the multimeter's internal resistance changes between current scales andsome times between voltage levels as in the UT61E. (not sure about the last one though).The bottom line is that the thing that characterises a multimeter's accuracy in measuring lowcurrents is not the burden voltage but its internal resistance. That is what has to been listedin its performance specifications. Especially in multi-meters sold for electronics.

author: PA4TIM
author_rank: Super Contributor
timestamp: May 15, 2013, 06:27:39 am
content: That is wat I was telling you. It is a voltage, they do not give V/A in the specs of well known brands. They list burdon voltage and also internal resistance separate (and tempco of that internal restistance and stability over time fir that and a lot more. ) That is why I showed you the specs from those three brand.

author: hgg
author_rank: Frequent Contributor
timestamp: May 15, 2013, 06:32:12 am
content: Yes you were right PA4TIM,The fluke man confused me. Its clear now.Thank you!

author: Wytnucls
author_rank: Super Contributor
timestamp: September 11, 2013, 01:28:29 pm
content: The burden voltage varies linearly with the current being measured. The only constant is the shunt resistance for a particular range (Ohm's law). So it should be given as a ratio of voltage versus current, ie: mV/mA. A burden voltage figure by itself doesn't mean anything, unless it is referenced (by convention) to the full current range under consideration.The burden voltage of the Fluke 87V, for instance, on the milliamp range, is 1.8 mV/mA.As an example, if your multimeter mA range 5ohm shunt is placed in series to measure the current flowing through a 25ohm circuit powered with a 3V battery, you will measure a current of 100mA instead of the actual 120mA, if the meter was not part of the circuit.

author: onlooker
author_rank: Frequent Contributor
timestamp: September 12, 2013, 01:22:13 am
content: The burden voltage is not really a voltage. It is always a voltage rate against some current value. The current value may be as sample as 1mA, or something like full scale current of the range in reference.In the formal case, it is the same as the shunt resistance of the range (in general).

author: Wytnucls
author_rank: Super Contributor
timestamp: September 12, 2013, 04:11:29 am
content: Well, the effect is referred to as burden voltage, because it robs your circuit of part of the supplied voltage. The lowering of circuit voltage could have consequences for the circuit, especially if the normal operating voltage is only a few volts.

author: Unknown
author_rank: Unknown
timestamp: September 12, 2013, 04:40:07 am
content: But thinking of it as a series resistance is also useful, for example when evaluating the effect on transient response. Good power supplies often specify an output impedance in the order of tens milliOhms up to audio frequencies. Putting 1 ohm in series increases the output impedance by two orders of magnitude. Upping the voltage can compensate for the voltage drop, but not for the poor transient response.