Measuring Range Extension "homework"

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Discussion Overview

The discussion revolves around a homework problem related to measuring the range extension of a moving-coil movement in an electrical circuit. Participants explore the effects of internal resistance changes on the voltage indication of the moving-coil meter when subjected to a specified voltage source.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant calculates the series resistance required for full deflection of the moving-coil movement at 25 V, arriving at a value of 247.3 kΩ.
  • Another participant seeks clarification on the representation of certain values and the method to determine the meter's indication at 25 V.
  • A participant explains their approach to calculating the internal resistance and its increase due to production tolerances, leading to a new resistance value of 3240 Ω.
  • There is a discussion on how the increase in internal resistance affects the current or voltage drop across the moving-coil movement.
  • One participant attempts to derive the adjusted maximum voltage indication based on the new resistance values but questions the validity of their result.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the calculations and methods involved. There is no consensus on the final voltage indication or the correctness of the derived values, indicating ongoing uncertainty and exploration of the problem.

Contextual Notes

Participants have not fully resolved the implications of the increased internal resistance on the voltage indication, and there are missing assumptions regarding the behavior of the circuit components under the specified conditions.

Apo_GER
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Moved from a technical forum, so homework template missing
a)
dimension the moving-coil movement so that it indicates full deflection at 25 V
given Values:

Voltage Source: U2Max = 25V;
moving-coil movement: IM = 100µA; UM = 270mV

Rv = ( U2Max - UM ) / IM = (25V - 270mV) / 100 µA = 247,3kΩ

b) the internal resistance is increased by 20 % due to tolerances in production. What does the moving coil movement indicate at an output voltage of 25V

My approach is the following...

RM = UM / IM = 270mV / 100µA = 2700Ω

now: it is 20% higher --> RM.Tol = RM *1,2 = 3240Ω

...
...

The Answer is : The moving coil movement indicates 24.945 V
 
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Hallo Apo, :welcome:

Nor clear what your dots are representing...
You found a new value for RM and the question is: how far does the meter that you built with this coil and the 247.3 k##\Omega## series resistance go on the scale, when the actually applied voltage is 25 V ?

(and you already know that 100 ##\mu##A gives full scale)
 
Hi BvU
The dots are representing the missing "method" to get to the answer.

In task-part a) i came to the result, that i have to add an other resistance Rv with 247,3kOhm in series so that the moving coil movement shows full deflection.

In b) the internal resistance increases by 20% and voltage source supplies 25 V.
The question is what does the moving coil movement shows?
The answer has to be 24.945 V.

My Idea:

The inner resistance increase by 20%...
--> Rm = Um/Im --> 270mV * 100µA = 2700Ohm
--> RM.Tol = Rm * 1,2 = 3240OhmNow the current strength changes? or the voltage drop? "Over moving coil movement"

--> UM.Tol = RM.Tol * IM = 3240Ohm * 100µA = 324mV

--> Rv = ( U2Max.Tol - UM.Tol ) / IM = > 247,3kOhm = ( U2Max.Tol - 324mV ) / 100µA => (247,3kOhm * 100µA ) + 324mV = U2Max.Tol
=> U2Max.Tol = 25,054 (not true)
 
Hehe, with the answer given you should be able to draw your own conclusion ...:rolleyes:

The intention of he exercise is
for an applied voltge of 25 V the current is now a bit lower. The ideal coil showed full scale at 100 ##\mu##A, but the coil with 20% more resistance will cause a smaller current, so it does not go all the way to full scale. With full scale (= 100 ##\mu##A ) marked as 25V for the ideal coil, the non-ideal coil shows ...

extra exercise: how big is now the voltage drop over the coil ?