A moving iron instrumente gives full scale defecon 200 V. It has a coil of 20,000 turns and a resistance of 2000 ohms. If the instruments used as an ameter to give full scale deflection at 10A ,calculate the number of turns required
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Answer:
You are describing an ohm meter. Here is an example of how to do it. Just replace the values with your values and do the math. I suggest you use a 1.5 volt battery. You should Google this stuff for yourself.
To determine the proper value for R, we calculate the total circuit resistance needed to limit current to 1 mA (full-scale deflection on the movement) with 9 volts of potential from the battery, then subtract the movement’s internal resistance from that figure:
Now that the right value for R has been calculated, we’re still left with a problem of meter range. On the left side of the scale we have “infinity” and on the right side we have zero. Besides being “backwards” from the scales of voltmeters and ammeters, this scale is strange because it goes from nothing to everything, rather than from nothing to a finite value (such as 10 volts, 1 amp, etc.). One might pause to wonder, “what does middle-of-scale represent? What figure lies exactly between zero and infinity?” Infinity is more than just a very big amount: it is an incalculable quantity, larger than any definite number ever could be. If half-scale indication on any other type of meter represents 1/2 of the full-scale range value, then what is half of infinity on an ohmmeter scale?
The answer to this paradox is a nonlinear scale. Simply put, the scale of an ohmmeter does not smoothly progress from zero to infinity as the needle sweeps from right to left. Rather, the scale starts out “expanded” at the right-hand side, with the successive resistance values growing closer and closer to each other toward the left side of the scale:
Infinity cannot be approached in a linear (even) fashion, because the scale would never get there! With a nonlinear scale, the amount of resistance spanned for any given distance on the scale increases as the scale progresses toward infinity, making infinity an attainable goal.
We still have a question of range for our ohmmeter, though. What value of resistance between the test leads will cause exactly 1/2 scale deflection of the needle? If we know that the movement has a full-scale rating of 1 mA, then 0.5 mA (500 µA) must be the value needed for half-scale deflection. Following our design with the 9 volt battery as a source we get:
With an internal movement resistance of 500 Ω and a series range resistor of 8.5 kΩ, this leaves 9 kΩ for an external (lead-to-lead) test resistance at 1/2 scale. In other words, the test resistance giving 1/2 scale deflection in an ohmmeter is equal in value to the (internal) series total resistance of the meter circuit.