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Brymen, difference in reading between mA and uA scale
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BeBuLamar:

--- Quote from: MarioBros69 on August 13, 2023, 02:22:47 pm ---
The truth is that I don't understand this very well..., if the measurements differ, which of the two is correct, the one on the mA scale or the one on the uA scale?

How do we know then the current that circulates through the circuit when we are not measuring?

--- End quote ---

Both readings are correct. They indicate the actual current that flow thru the circuit but neither is the current flowing thru the circuit without the meter. The meter causes the current to flow slightly less than if there is no meter.
TimFox:
Note that the "burden" resistance, which is the actual resistance between the two ammeter terminals, is higher than the "shunt" resistance, where the voltage due to the applied current is measured.
(Not necessarily a large difference, note J-R's measurements above.)
In your circuit, the total resistance seen by the voltage source is the sum of your external resistor plus the burden resistance.
MarioBros69:

--- Quote from: IanB on August 13, 2023, 03:32:41 pm ---
--- Quote from: Shonky on August 13, 2023, 02:04:26 pm ---OP: just use a lower case u in place of mu for micro. Lower case y = 10^-24 not 10^-6

--- End quote ---
Or even better, use the actual µ character for µA.


--- Quote from: MarioBros69 on August 13, 2023, 02:22:47 pm ---The truth is that I don't understand this very well..., if the measurements differ, which of the two is correct, the one on the mA scale or the one on the uA scale?
--- End quote ---
Both are correct. The truth is that inserting a current meter into a circuit will affect the current. So such measurements have to be taken with care.

--- End quote ---

Shouldn't the tester compensate the measurement based on its internal resistance to show the actual current flowing through the circuit?

Something like..., if my internal resistance on the uA scale is 100 ohms I add 200 uA to give the actual current going through the circuit when it is not being measured.
wasedadoc:

--- Quote from: MarioBros69 on August 13, 2023, 05:58:10 pm ---
--- Quote from: IanB on August 13, 2023, 03:32:41 pm ---
--- Quote from: Shonky on August 13, 2023, 02:04:26 pm ---OP: just use a lower case u in place of mu for micro. Lower case y = 10^-24 not 10^-6

--- End quote ---
Or even better, use the actual µ character for µA.


--- Quote from: MarioBros69 on August 13, 2023, 02:22:47 pm ---The truth is that I don't understand this very well..., if the measurements differ, which of the two is correct, the one on the mA scale or the one on the uA scale?
--- End quote ---
Both are correct. The truth is that inserting a current meter into a circuit will affect the current. So such measurements have to be taken with care.

--- End quote ---

Shouldn't the tester compensate the measurement based on its internal resistance to show the actual current flowing through the circuit?

Something like..., if my internal resistance on the uA scale is 100 ohms I add 200 uA to give the actual current going through the circuit when it is not being measured.

--- End quote ---
It is impossible for a meter to calculate the required correction because the size of the "correction" depends on the impedance and voltage of the source of the current.  For example both a 10 Volt source with 10k resistor and a 1 Volt source with a 1k resistor would produce 1 mA current when the meter is not in circuit.  Inserting a meter with 100 Ohm shunt has a different and larger effect in the 1V and 1k case. The meter cannot know which of the two cases or any of the infinite number of other possibilities (X Volts and X Ohms) to compensate for.
MarioBros69:


--- Quote ---It is impossible for a meter to calculate the required correction because the size of the "correction" depends on the impedance and voltage of the source of the current.  For example both a 10 Volt source with 10k resistor and a 1 Volt source with a 1k resistor would produce 1 mA current when the meter is not in circuit.  Inserting a meter with 100 Ohm shunt has a different and larger effect in the 1V and 1k case. The meter cannot know which of the two cases or any of the infinite number of other possibilities (X Volts and X Ohms) to compensate for.

--- End quote ---

Thanks for the explanation, now I understand.
What is paradoxical is that it is the expensive meters that come with the uA scale and we find that the uA scale of a meter is generally less accurate than the mA scale since its internal resistance is usually higher.
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