Digital Panel Meters (DPMs) are strictly DC meters due to the digital circuitry used. Oftentimes it is desired to utilize a DPM to measure AC voltages and currents to take advantage of the improved accuracy and readability of a DPM. This application note will describe a method to accurately display AC current values on a DPM using a current transformer.
The PM128E Digital Panel Meter from Circuit Specialists is ideal for this application, as it is designed to use in a system that has the measured signal isolated from the power supply voltage. The application is for a 0-20 Amp AC meter powered by an external 9 volt battery. This application could also be powered by a “wall-wart” type of AC adapter if desired.
The PM128E DPM is a universal unit with all built in divider resistors and rectifier circuitry to allow AC voltage measurements with no external components required. This meter can be used to display AC current values with the use of a low cost current transformer and load resistor. The current transformer used is a low cost device with a 1000:1 current ratio capable of operation from 0-100 Amps.
A wire with the the AC current to be measured is simply passed through the center of the transformer with the transformer secondary winding connected across a 1.1K ¼ watt resistor so that the transformed current value will be converted to a voltage that can be read by the PM128E DPM in AC voltage mode of operation.
The settings for the PM128E meter should be J3 and J5 shorted with the P1 decimal point location shorted and the AC and 200V points also shorted. This will put the DPM in AC voltage measuring mode with a 200 V full scale reading and set for operation from an isolated 9 V DC power source. The display will read in AC Amps with 0.1 Amp resolution.
The complete implementation of this circuit is included for reference.
This application note has shown how a Digital Panel Meter may be used to measure and display AC current values using a low cost current transformer.