# Why add extra resistance to a shunt?

simulate this circuit – Schematic created using CircuitLab

I'm using a Fluke Y5020 air cooled shunt (it's 10 mΩ, 20 A max) and putting 3 A at 60 Hz into it.

• Aside from Ohm's law, is there any particular reason I would need to add resistance in series (I had a 50 Ω max rheostat in series) through the current input into the Fluke Y5020? (I couldn't get this particular shunt to work otherwise like a normal shunt, just hook up and go, sorry no pic.)

• I'm using a 34401A to read the mV. (I realize this is only 10 MΩ which is not enough for this shunt, that's why my mV readings initially looked bad.)

Some circuit analysis with diagram would be great.

The manual says Accurate current measurements require the use of a highimpedance (>10 MΩ) digital voltmeter to measure the shunt voltage appearing at the OUTPUT TERMINALS."

• At 10 mΩ you're going to get 100 mV/A from the shunt. What do you mean by, "I realize this is only 10 MΩ which is not enough for this shunt"? Jan 1, 2023 at 15:22
• Putting a 10 MΩ voltmeter in parallel to a 0.01 Ω shunt will reduce the resistance by 0.000,000,000,010 Ω. Does that matter to you? Jan 2, 2023 at 12:19
• @Doug 10 MΩ in parallel with 10 mΩ = 9,999999990 mΩ, which is 10 pΩ less. Jan 2, 2023 at 14:22
• @Doug - Hi, Your question (which I think is something like - why did you apparently need to add a 50 Ω rheostat in series with the shunt for it to "work") seems to be mixed with your concerns about the voltmeter impedance requirements (where you seem to be saying that a 10 MΩ voltmeter isn't suitable). If your main question is why you apparently needed a series resistance, please edit your question & add more details of what happened without that series resistance. Can you reproduce the "not working" situation again please, then supply photos? (I know you said you don't have any now.) TY Jan 2, 2023 at 14:32
• Additionally to what Sam mentioned, an expression like "my mV readings initially looked bad" leaves us in the dark about what actually is happening. Try to describe the setup in more precise terms, what you expected and what you got as measurements. Jan 2, 2023 at 14:38

Is there any particular reason I would need to add resistance in series

Yes. That "added resistance" is the load whose current you wish to measure. Without that load, a) the shunt acts as a short circuit across the voltage source, with dire consequences; and b) what's the point of the measurement if there's no load, which is what you wanted to measure in the first place?

I'm using a 34401A [voltmeter] to read the mV

That voltmeter has an input resistance of 10 MΩ. That's nine orders of magnitude more than the shunt resistance. That means that the meter will have no discernible effect on the reading. You're fine.

• Thank You, that's why I was baffled by the 34401A.... Jan 2, 2023 at 20:17

From here is a photo of the same part number Fluke shunt:

As you can see, it's about 0.01Ω, so the 10MΩ input impedance of the 34401A will not load the shunt significantly, even to the 6 digit resolution of the DMM.

The shunt appears to be isolated from ground (though they recommend no more than 20V RMS between shunt and earth ground, probably for legal liability/safety reasons).

Here is the manual and you can see that it's a rather simple circuit:

So there's no reason this would not work as an ordinary shunt, since that's all it is. It's up to you to limit the current through the shunt to an appropriate level and that would typically involve both the source voltage and the load resistance. And typically the load resistance would be >> 0.01Ω so as to minimize the effect of the shunt on the overall circuit.

simulate this circuit – Schematic created using CircuitLab

Here the voltage source is 12VAC RMS so the voltage across the shunt resistor is about 2mV RMS since the total load resistance is 6.01Ω.