How to measure the Burden Voltage of your multimeter

[hgg]

Hi,

I have the UNI-T UT61E multimeter and many people complain about its high burden voltage
of around 0.85 Volts.

When measuring current the multimeter adds a small resistance in the circuit under measurement
that 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.

[Balaur]

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 current

In your case, for the mA scale:

Burden voltage (mv) = 11.84 * Indication in mA


Cheers

[hgg]

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 terminals
of the multimeter that was reading 4.455 mA.  So If I multiply the multimeter
measured 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.

[IanB]

I have the UNI-T UT61E multimeter and many people complain about its high burden voltage
of around 0.85 Volts.

When measuring current the multimeter adds a small resistance in the circuit under measurement
that 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?

The 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 mV

Note 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.

[Balaur]

Sorry for my previous message, got things mixed up in my head. Please ignore it.

Let's start over:

If

I. Burden voltage = Voltage drop/Current

and

II. You measured an internal resistance of 11.84 ohms

then:

Burden voltage = 11.84 mV/mA

Maybe there is a 10 Ohm shunt on this scale + fuse + leads

[hgg]

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.

[kfitch42]

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.

Mr. Ohm's said so
1 Ohm = 1V/1A = 1mV/1mA

So, the units of "Burden Voltage" could be expressed as mV/mA with IS THE SAME as Ohms.

[IanB]

IanB, what are the units of measurement of Burden Voltage?
They are not mV.  They are mV/mA, correct?

As 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 burden for that measurement is 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.

[hgg]

kfitch42, I see what you are saying, so maybe the term Burden Voltage is a misleading term.
It should have been called the Burden 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 the resistance and not the voltage drop,
since the voltage drop is variable? 

So the burden resistance across 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 example
with the 1.8mV/mA of the Fluke 87V...

Can anybody with a Fluke 87V measure the resistance of the mA scale?

[Telequipment]

So many worries

[hgg]

Ok, it turns out that in my little collection of crappy multimeters    the tiny UT120C
will make the most accurate low current measurements!

Its internal resistance in the mA scale is 3.84 Omhs and the current that it measured
in the same circuit was 4.55mA.  Burden Resistance = 17.5mV/mA   3 times better than
the UT61E.  I actually measured the voltage drop of the UT120C with the UT61E and
it gave me a reading of 16.5mV/mA.    Still 9 times worse than the Fluke 87V.  Oh, well...

[IanB]

11.84 Ohms = 52.7mV/mA

You 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...

[Balaur]

So the burden resistance across 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 example
with the 1.8mV/mA of the Fluke 87V...

Sorry, 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/mA

Here 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

[hgg]

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 something
other than Voltage?

(sorry for my many questions...)

[darrylp]

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 ohms
uA = 1000 ohms.

--
  Darryl

[ivan747]

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 ohms
uA = 1000 ohms.

--
  Darryl

Don't forget to add the resistance of the test leads, fuses and PCB tracks connecting these resistors to the outer world.

[hgg]

Darryl, indeed that was what I actually measured on the mA & uA scale.

[ModemHead]

The UT61E is an odd bird when it comes to burden voltage. It The 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 ohms
2. 106mV / 10.22mA = 10.37 ohms
3. 520mV / 50.46mA = 10.31 ohms
4. 862mV / 100.32mA = 8.59 ohms
5. 1072mV / 200.79mA = 5.34 ohms

[darrylp]

The 2ohm resistor is actually two in parallel. Thus 1ohm resistance.

--
  Darryl

[PA4TIM]

From the Keithley 2000 manual:
Current  range      burdon voltage
10  mA                  < 0.15 V
100  mA                < 0.03 V
1  A                        < 0.3 V
3  A                         <1 V
SHUNT RESISTOR: 0.1? for 3A, 1A and 100mA ranges. 10? for 10mA range.

From the Agilent U1252A manual:
range              burdon voltage
500.00 ?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 125mV

Ptrema 5017: (7,5 digit meter, 0,03% reading  + % 0.002% range)
200 uA      350 mV    1000 Ohm
2 mA         350 mV      100 Ohm
20 mA       350 mA        10 Ohm
200 mA     350 mA          1 Ohm
2A               600 mA      0.1 Ohm

I 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

[hgg]

Ok, now I am really confused   

Especially after reading the following definition as well:
http://www.ni.com/white-paper/3296/en#toc6
He 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 been
subtracted from the voltage available to the circuit being tested."  He subtracts Voltage from Voltage
and 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.htm

Voltage 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'...

[ModemHead]

Ok, now I am really confused   

I 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.

[ModemHead]

The 2ohm resistor is actually two in parallel. Thus 1ohm resistance.

I 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. 

[hgg]

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 the Voltage Drop caused
by the internal resistance of the multimeter.  Volts.   Now mV/mA is another way to talk about
the internal resistance of the multimeter.   mV/mA does NOT represent 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 a
constant because the multimeter's internal resistance changes between current scales and
some 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 low
currents is not the burden voltage but its internal resistance.  That is what has to been listed
in its performance specifications.   Especially in multi-meters sold for electronics.

[PA4TIM]

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.