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Microamps and Millivolts Measurements on UT61E

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JenniferG:
Say I only had the UT61 -- I really wish I wouldn't of brought up the GDM-8251A now as it seems to be convoluting things.

So you wire the UT61E in series to find out the current.  You turn the dial between amps, ma, and ua.   For any mode you still have it wired the same.. in series.

What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

rsjsouza:
Attached are the measurements of the three ranges of the UT61E using a separate meter. You can tell how different their shunt resistance values are.

On the circuit, the shunt resistor will be placed between the power supply and the circuit being tested. Depending on its value, a proportional voltage will be dropped by it (Ohm's law). Now, think the voltage that is powering your circuit is slightly lower as you progress from A --> mA --> µA ranges. That slight lower voltage will cause your circuit to draw a bit less current.


                       shunt resistor
                            on DMM
    |---------------------/\/\/\/---------------------|
    |                                                 |
Vsupply                                            Circuit
    |                                             under load
    |                                                 |
    |-------------------------------------------------|

joeqsmith:
You're changing ranges, you're selecting 61E's different internal shunts.  Different shunts have a different resistance that you now have in series with your circuit.   Of course the current changes. 

I modified a UT61E to reduce the burden voltage in the uA range.  The video goes over how this circuit works if interested.

colorado.rob:

--- Quote from: JenniferG on December 07, 2022, 09:33:38 pm ---What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

--- End quote ---
Burden voltage. EEVBlog #2. https://youtu.be/u2yRR4G3yTA?t=202
Look how young he is!

bdunham7:

--- Quote from: JenniferG on December 07, 2022, 09:33:38 pm ---What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

--- End quote ---

If you include the GDM-8251A in the circuit, then you will actually see the current change on its display even though you only changed the UT61E dial and you'll realize that the current is actually changing and it isn't a reading error.  Try it!

Lets say you take your original circuit and in addition to everything else, you put a 5 ohm resistor in series.  You take a measurement, then you swap out the 5 ohm resistor for a 500 ohm.  You'd expect less current, right?  So when you turn that dial on your UT61E, that is exactly what you are doing--swapping out shunt resistors that are in series with your circuit.

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