multirange ammeter

[cinderella1]

helloooo everyone, can kindly help me a bit in to answer this question. I just need a brief explanation. How to know the limiting error of each measurement range on the ammeter eg: 0-10mA, if all resistors i use in my design has 10% limiting error?? btw,it has FSD of 0.1mA and internal coil resistance of 500ohm.. thank you

 

[duffy]

I'm sure they want you to assume the 500Ω coil resistance is precise, so you have a 100% precise resistor in parallel with a 90% precise resistor, right?

Since the meter only goes to .1 ma, and you want measure to 10 ma, most of the current will be going through that imprecise shunt resistor, see? That proportion between the 100% accurate meter resistance and the 90% accurate shunt resistance is telling you your answer.

 

[bountyhunter]

helloooo everyone, can kindly help me a bit in to answer this question. I just need a brief explanation. How to know the limiting error of each measurement range on the ammeter eg: 0-10mA, if all resistors i use in my design has 10% limiting error?? btw,it has FSD of 0.1mA and internal coil resistance of 500ohm.. thank you

You have to do a "worst case" calculation. To do that, shift resistor tolerances both ways like go +10% on half and -10% on the other half. This kind of calculation is essential in knowing if a design is robust.

 

[cinderella1]

ya..i already calculate my shunt resistors.... for 0-10mA, i have resistor shunt of 5.05ohms. So how should i calculate for the limiting error for my measurement range if my resistors used came with 10%limiting error. can give me example?

 

[colin55]

When I turn a movement into an ammeter, I place the movement in series with an ammeter of known accuracy and a power resistor. I then place a small value resistance across the movement and adjust the power resistor to create a flow of say 2 amps as shown on the main ammeter.
I then gradually increase the value of the shunt resistance (across the movement) until both meters read the same value.

 

[cinderella1]

ya i know that...then??

 

[duffy]

Look at it this way - if ALL the current was going through the shunt resistor, your measurement accuracy would be limited to +/- 10% of the actual value. If ALL the current was going through the known meter resistance, the accuracy would be +/- 0%. If half the current was going one way, and half the other, the accuracy would be +/- 5%. Get it?

 

[colin55]

99% goes through the shunt

 

[cinderella1]

this is the current range