Since the shunt resistance is in parallel with the meter movement, the voltage drop across the shunt and movement must be the same. For each required value of full scale meter current, we can determine the value of shunt resistance. The current range of the dc ammeter may be further extended by a number of shunts, selected by a range switch. Such a meter is called a multirange ammeter, shown in Fig. The circuit has four shunts R 1 ,R 2 ,R 3 and R 4, which can be placed in parallel with the movement to give four different current ranges.
Switch S is a multiposition switch, having low contact resistance and high current carrying capacity, since its contacts are in series with low resistance shunts. Make before break type switch is used for range changing.
This switch protects the meter movement from being damaged without a shunt during range changing. If we use an ordinary switch for range changing, the meter does not have any shunt in parallel while the range is being changed, and hence full current passes through the meter movement, damaging the movement. Hence a make before break type switch is used. As can be seen, there is no possible way that the circuit can be left without a shunt resistance, saving you millions of pounds on replacement PMMC meters.
However, nothing in life is free : as a result of these hijinxs , the overall meter resistance is increased. To calculate the resistance valves , the best method is to start with the most sensitive range and work backwards to the least sensitive. That's the same as last time! You're correct, but remember: Everything you change changes something else. In this case, since the resistance values have changed, the I is also changed, thus giving a different result.
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