121GW: Shunt Resistors used in Current Measurements

[JDW]

What are the SHUNT RESISTORS used in the 121GW for each Current Measurement Range?

For example, when using my µCurrent Gold, I know the following to be true...

• 0-1250 nA Range (10kΩ 0.1%): 10μV/nA

• 0-1250 µA Range (10Ω 0.1%): 10μV/µA

• 0-1250 mA Range (10mΩ 0.5%): nominal 70μV/mA

And when using my Fluke 8845A benchtop meter, the following is true (click attachment)...

[J-R]

Have a look at page 18 of the user manual: https://www.eevblog.com/wp-content/uploads/2017/11/121GW-Manual-5.pdf

[JDW]

Have a look at page 18 of the user manual: https://www.eevblog.com/wp-content/uploads/2017/11/121GW-Manual-5.pdf

I just did.  Nowhere on page 18 is there any specific mention of the Shunt Resistance Values.

If you know the exact resistance values in Ohms, please post that in your reply.

Thank you.

[Gyro]

You can very easily derive it from the burden voltage (voltage/current) and ohms law.

[JDW]

You can very easily derive it from the burden voltage (voltage/current) and ohms law.

Nobody likes to calculate when a nice graphic plainly and quickly gives all the answers, so I have done that work for others who read through this thread in the future, as per the attachment I created below.

However, the 121GW Schematic shows the actual shunts to be:

R33: 100Ω
R43: 1Ω

And it appears that Fusing also affects the actual shunt resistance too, which in turn impacts the Burden Voltage.

[J-R]

Zero calculations are necessary, since the unit factors are the same.  Just read the numbers right off the page!  Also, there are other things at play besides the fuses, such as contact resistance, so it's always good to just measure it if you need to know for sure.
uA: 100 Ohms (measured @ 103 Ohms)
mA: 2 Ohms (measured @ 3 Ohms)
A: 0.03 Ohms (measured @ 0.04 Ohms)

[JDW]

I have a new and related question.  Please see the attachment below which I created from copied selections of the 121GW Manual and the Shunt Resistances discussed a few days ago.  It considers DC only, not AC.

In the attachment, my additions are colored in Red. 

QUESTION: In what "Dial Setting" are the 5 mA & 50 mA Ranges found?

I can only "assume" it is the "A, mA" Dial Setting because that is the only Dial Setting where "mA" can be displayed on the LCD using 1 press of the RANGE button.  However, the 5mA & 50mA Ranges shown in the attachment (as per the Manual), have a 2Ω Shunt Resistance (as per our previous discussion), while the 3 Ranges in the "A, mA" Dial Setting have a 30mΩ Shunt Resistance, which is adding to my confusion.

Another reason I am confused is because if you read the descriptions of the "A 500mA" & "mA µA" jacks at the bottom of the attachment, it lists only 6 current ranges while the chart shows 7!  Only the Chart shows the 5mA Range!  Furthermore, when using the "µA" Dial Setting, the LCD displays only the µA icon, not mA.

Any clarity you can provide on this topic would be greatly appreciated.

[ Specified attachment is not available ]

[J-R]

Yes, the 5mA and 50mA ranges are via the A/mA selector position and mA/uA jack.  The 5mA range is just not listed in that other section of the manual, which is not ideal.  But that section is more of a general operational section so not all specifications are listed out.

The mA/uA jack is 2 Ohms when the selector switch is in the A/mA position, but 100 Ohms when the selector is in the uA/uVA position.  The A/500mA jack is always 0.03 Ohms no matter the selector position.

If you connect another DMM in resistance mode to the mA/uA jack and change the selector from uA to mA, you will see the resistance change.  For fun, check out all the other selector positions...

[JDW]

Yes, the 5mA and 50mA ranges are via the A/mA selector position and mA/uA jack.  The 5mA range is just not listed in that other section of the manual, which is not ideal.

Thank you very much, but I am still curious...

When I switch the Dial to "mA/µA" I can only press the RANGE button 1 or 2 times.  Pressing a 3rd time takes me back to what I see after pressing RANGE only 1 time.  More specifically, there is the AUTO setting by default when I first switch the dial to the"mA/µA" range, then I press the RANGE button 1 time to show 00.000µA on the LCD, and when I press RANGE a 2nd time, it shows 000.00µA on the LCD.  So even if I press RANGE twice, the 121GW would measure up to 999.99µA based on what I am seeing on the LCD, not something in the mA range.  That again is the source of my confusion here.

[J-R]

I'm not sure what you are saying, there is no mA/uA dial position.  There is a uA/uVA and A/mA dial position and then the A/500mA and mA/uA jack.

When you set the selector dial to A/mA, it will automatically change the display based on the split-jack position.  So if your test leads are plugged into the A/500mA jack, it will autorange down to the 500mA range.  When you move the test lead to the mA/uA jack WITHOUT changing the selector dial, it will automatically switch to the 5mA/50mA mode, and autorange will probably pick 5mA if no current is flowing.

For all functions, if the range is set to auto, pressing the range button ONCE will simply lock it in the range is it currently in.  Basically disabling autorange.  You then have to press it again to change it, and it always goes in a certain direction.

[JDW]

Sorry, I couldn't correct my error before your reply.  In my previous post, I mean to say: When I switch the Dial to "µA/µVA"...

When I followed your instructions, I am now able to see "x0.000 mA" or "0.0000mA" on the LCD. (x = blank space instead of digit on LCD.)  Thank you!

More specifically, I did this:

1. Put Red probe into "A/500mA" (Black probe is in COM) (neither Probe connected to anything)
2. Set Dial to "A/mA" — LCD shows: AUTO DC xx0.00 mA (500mA range)
3. Pressed RANGE button 1 time — LCD shows: DC xx0.00 mA (500mA range)
4. Disconnected Red probe — LCD shows: DC xx0.00 mA (no change)
5. Inserted Red probe into mA/µA — LCD shows: AUTO DC 0.0000 mA (5mA range)
6. Pressing RANGE 1 time — LCD shows: DC 0.0000 mA (5mA range)
7. Pressing RANGE a 2nd time — LCD shows: DC x0.000 mA (50mA range)
8. Pressing RANGE a 3rd time —  LCD shows: AUTO DC 0.0000 mA (5mA range again)
9. Switching Dial to µA/µVA — LCD shows: AUTO DC x0.000 µA (50µA range)
10. Pressing RANGE 1 time — LCD shows: DC x0.000 µA (50µA range)
11. Pressing RANGE 2nd time — LCD shows: DC xx0.00 µA (500µA range)
12. Pressing RANGE 3rd time — LCD shows: DC x0.000 µA (50µA range again)

In other words, when you have the 2 probes disconnected (not actively measuring current), the only way to get "x0.000 mA" (50mA range) or "0.0000mA" (5mA range) to display on the LCD is to start off with the RED probe attached to "A/500mA" and switch the Dial to "A/mA" and then pull out the Red probe and attach it to "mA/µA".

[J-R]

No, you don't need to start with the test lead in the A/500mA jack.  Just simply connect it to the mA/uA jack and set the selector to A/mA.  And which you do first doesn't matter.

[JDW]

No, you don't need to start with the test lead in the A/500mA jack.  Just simply connect it to the mA/uA jack and set the selector to A/mA.  And which you do first doesn't matter.

Thank you very much!  I just confirmed that on my meter and updated the chart in the attachment below.

[J-R]

To clarify, the column for "Presses of "RANGE" btn" in your chart is not going to be accurate because the number will depend on the range that the DMM has auto-ranged to at the instant you press the Range button.  As I mentioned previously, the first press of the Range button simply takes the DMM out of auto-range, but it stays in the same range that auto-range had selected.

[JDW]

To clarify, the column for "Presses of "RANGE" btn" in your chart is not going to be accurate because the number will depend on the range that the DMM has auto-ranged to at the instant you press the Range button.  As I mentioned previously, the first press of the Range button simply takes the DMM out of auto-range, but it stays in the same range that auto-range had selected.

I apologize for not having clarified that my chart is going on the assumption that the two probes are connected but attached to nothing. If both probes are disconnected, then what is written in the attachment I created is absolutely correct because it will not have auto-ranged to anything other than the default range.