Hi,
I am reading through an older electronics textbook I found in the library, and I am at the section regarding shunts and multiple-range ammeters. I think I understand the gist of what's going on here, except one aspect. The book says a multiple-range ammeter must use a shorting-type switch so that the meter movement is shunted even while switching ranges. The book further states that "with a nonshorting switch, the meter movement would carry all the current for the instant it takes to change ranges", which could damage the meter movement.

The book gives an example of a 100 micro-ampere meter movement with a 1000-Ohm resistance. If a nonshorting switch is currently selecting the 10 milli-ampere range, and then is switched to the 1 milli-ampere range, a nonshorting switch would momentarily cause the 100 micro-ampere meter movement to receive all the current, but wouldn't the 1000-Ohm resistance be high enough to limit any damage?

I have tried doing the math to determine something about the relationship among the 10 milli-ampere shunt, the 1 milli-ampere shunt, and the 100 micro-ampere meter movement, but I can't seem to make any sense of it.

To sum it up, I am wondering why a shorting-type switch must be used with a multiple-range ammeter, and why a nonshorting-type switch cannot be used.

Thanks!
- Jason

 

In your situation, assume that one end of each of the shunts is tied to the negative side of the meter AND the load side of the current with the switch wiper common to the positive side of the meter AND the source of the current If the wiper of the switch is not in contact with any of the shunts, all current will flow through the meter. As for the 1000 ohms limiting the current, the premise of a shunt type current meter is that most of the current will flow through a very low ohm shunt. Some shunts are measured in thousandths (1/10000s) of ohms. If you don't have the shunt in the circuit, it is possible for the voltage across the meter to go into extremely high levels before the meter movement actually blows open.

On edit: Are you sure the movement RESISTANCE is 1000 ohms and it is not that the meter is 1000 ohm/volt??