AC/DC ammeter for load box. The circuit begins with a pair of terminals on the left. These are where the current to be measured is connected. There is a resistor connected between these two terminals that is labeled R 1. The bottom terminal also connects to circuit common. The top terminal also connects to one end of a 10 k ohm resistor. The other end of this resistor connects to the noninverting input of op amp 2, pin 5. The op amp IC is a dual amplifier L M 3 5 8. The output of op amp 2, pin 7 goes to the top AC input of a bridge rectifier, anode and cathode. The negative output of the bridge, two cathodes, goes to the negative of the microamp meter and also to one end of a 0.1 u f capacitor. The positive output of the bridge goes to the positive side of the microamp meter and also to the other end of the 0.1 u f capacitor. The meter is a 100 microamp full scale meter movement. The bridge is made up of 1N4148 diodes. The bottom AC connection of the bridge, anode and cathode, goes to one end of a resistor labeled R2. The other end of R2 connects to the top of a pot which is labeled R3. This is a trimming pot which is connected as a rheostat. The other end of the rheostat connects to circuit common. The junction of the bridge and R2 goes through a 10 k ohm resistor to the inverting input, pin 6. A 47 p f capacitor connects from pin 7, output of op amp 2, to its inverting input, pin 6. This pin, inverting input of op amp 2 goes to one end of a 0.1 u f capacitor. The other end of the capacitor goes to one end of a resistor, R4. The other end of R4 connects to common. The circuit is power by a 9 volt battery. There is a switch in series with the positive side of the battery and the negative side of a 220 u f capacitor connects to the negative side of the battery. The positive side of the capacitor connects to the end of the switch away from the battery. The positive side of the capacitor connects to pin 8 of the IC, Vcc input. The negative side of the capacitor connects to pin 4, V e e. Pin 8, the Vcc connection will be referred to as the positive rail. Pin 4, the V e e connection will be referred to as the negative rail. One end of a 220 k ohm resistor connects to the positive rail. The other end of this resistor connects to a second 220 k ohm resistor. The other end of this second resistor connects to the negative rail. The junction of the two resistors connects to pin 3, noninverting input of op amp 1. The output, pin 1, of O A 1 connects through a 1 k ohm resistor to circuit common. A 47 p f capacitor connects between output, pin 1, and inverting input, pin 2. Pin 2 also connects through a 100 k ohm resistor to common. There are two legends on the diagram. One gives the values of the resistors that did not have given values in the schematic. First is for a 1 volt drop across the shunt resistor, R1, at the full scale reading of 1 amp. The values for this condition are, R1 = 1.0 ohm, R2 = 4.7 k ohms, R3 = 10 k ohms, and R4 = 200 k ohms. Second are the values for a voltage drop of 0.1 volt across R1 at full scale. R1 = 0.1 ohm, R2 = 470 ohms, R3 = 1 k ohm, and R4 = 180 k ohms. Another legend gives the frequency limits for AC. The limit was called when the meter pointer had moved from the calibration point by an amount equal to its own width. For the resistor values giving a 1 volt drop the frequency limits were from 40 Hz to 2 k Hz. For the resistor values giving a 0.1 volt drop the frequency range is from 40 Hz to 300 Hz. The calibration point was one half scale. This completes the verbal description.
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