Digital Ammeter Top Cover

        The shunt is constructed by adding additional resistors in series for succeeding ranges. The smallest resistor connects to one of the inputs. The switch connects the other input to the taps in this ladder. The amplifier connects to the end of the ladder.

The purpose of this arrangement is to obtain a Wheatstone bridge with a single switch. This requires that:

• Two independent connections to the bottom resistor.
• The amplifier input resistance to be very high compared to the ladder.

The protection diodes are actually doubled by using a bridge rectifier with the two AC shorted together and the DC connectors likewise.

The shunt resistors are not exact; the micro-controller compensates for their error.

The purpose of the position switch is to minimize port usage and cabling. It is trivial software to get the position of the switch from the voltage of the tap selected, provided that the resistors in the ladder are matched. These resistors are in SIP packages whose manufacturing processes inherently does this.