- #1
fourthindiana
- 162
- 25
I am a student at a trade school, majoring in HVAC. My electricity for HVAC textbook has a chapter on electric meters. However, my textbook does a poor job of explaining how a digital ammeter works.
My textbook has the following description of how digital ammeters work: "To make meters useful in line-current measuring systems, a shunt is installed within the meter. The shunt is a low resistance current path that allows a proportional amount of current to flow through the meter movement...Suppose that a 1-ampere full-scale meter is designed using a meter movement that requires .001 amperes (1 milliampere, mA) full scale deflection. The meter movement resistance is 100 ohms. According to Ohm's Law, there will be a 0.001 A X 100 ohm = 0.1 V across the meter movement at full-scale deflection. The scale of the meter would be changed to indicate 1 ampere instead of 1 mA, but it would still take only 1 mA through the meter movement to cause full-scale deflection."
I understand that most of the current goes through the shunt, and a minority of the current goes through the "meter movement". However, I don't know exactly what "meter movement" means in this context. I know that analog ammeters have a coil that is caused to move by magnetic forces when a current flows through the coil. The coil is attached to the dial of the analog ammeter in such a way that when a magnetic field moves the coil, the dial is also moved, calibrated in such a way as to indicate the current going through the analog ammeter. Perhaps a digital ammeter also has a coil that measures current by being deflected by magnetic fields from current. However, my books never mentions that a digital ammeter also has a coil that is deflected by magnetic fields from electric current.
Does a digital ammeter measure current by having a coil that is deflected by magnetic forces from currents in a way similar to how analog ammeters measure current by having a coil that is deflected by magnetic forces from currents? If not, how do digital ammeters measure current?
My textbook has the following description of how digital ammeters work: "To make meters useful in line-current measuring systems, a shunt is installed within the meter. The shunt is a low resistance current path that allows a proportional amount of current to flow through the meter movement...Suppose that a 1-ampere full-scale meter is designed using a meter movement that requires .001 amperes (1 milliampere, mA) full scale deflection. The meter movement resistance is 100 ohms. According to Ohm's Law, there will be a 0.001 A X 100 ohm = 0.1 V across the meter movement at full-scale deflection. The scale of the meter would be changed to indicate 1 ampere instead of 1 mA, but it would still take only 1 mA through the meter movement to cause full-scale deflection."
I understand that most of the current goes through the shunt, and a minority of the current goes through the "meter movement". However, I don't know exactly what "meter movement" means in this context. I know that analog ammeters have a coil that is caused to move by magnetic forces when a current flows through the coil. The coil is attached to the dial of the analog ammeter in such a way that when a magnetic field moves the coil, the dial is also moved, calibrated in such a way as to indicate the current going through the analog ammeter. Perhaps a digital ammeter also has a coil that measures current by being deflected by magnetic fields from current. However, my books never mentions that a digital ammeter also has a coil that is deflected by magnetic fields from electric current.
Does a digital ammeter measure current by having a coil that is deflected by magnetic forces from currents in a way similar to how analog ammeters measure current by having a coil that is deflected by magnetic forces from currents? If not, how do digital ammeters measure current?