Digital Panel Meters (DPMs) are voltage measuring devices and are unlike analog meters which are current measuring devices. When current measurement is required from a digital meter, the current must be converted to a voltage to display a measurement.
So, here is how to accurately display DC current values on a DPM:
There are several methods that can be used to convert current to voltage such as Hall effect devices and shunt resistors. Since shunt resistors are the easiest to use and provide the greatest amount of accuracy, this technique will be examined. The shunt resistor is placed in series with the applied current which causes a voltage drop to occur across the shunt. To minimize the voltage drop in the circuit, the smallest resistance value possible should be chosen. This value depends on the maximum current value that will be encountered. For relatively small current values (below 1 Amp) a 0.1 ohm shunt resistor should perform adequately. This value will minimize any loading on the circuit but will still produce a reasonable reading on the DPM. If higher current levels will be encountered, a 0.01 ohm or lower value should be used.
The CX102A 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 0-1 Amp DC meter powered by an external 9 volt battery. This application could also be powered by a “wall-wart” type of AC adapter if desired.
Like all Digital Panel Meters, the full scale range of these DPMs are 200 mv full-scale. To use the DPM to measure current, we will choose our shunt resistor to assure that no more than 200 mV is developed across it. We will also set the Decimal point jumpers accordingly to indicate the correct Amp reading. For instance, if 1 Amp is the full scale reading desired, we will use the 0.1 ohm resistor and set the decimal point jumper to show three digits to the right of the decimal point. We must also determine the correct power rating of the shunt resistor by using the ohms law power formula P (Power)=E (Voltage) X I (Current).
P = V max X I max = (0.200) X (1.0) = 0.1 Watt
So we should use a 1/2 watt 1 % resistor to be safe.
We will also need to connect pin 3 to pin 6 (for proper decimal point display).
Note that the current value displayed on the meter can be fine-tuned by adjusting the trimmer potentiometer on the back of the DPM.
This application note has shown how a Digital Panel Meter may be used to measure and display DC current values. The connection diagram is shown below.
Note that pins 8 & 10 are shorted together and connected to the Negative end of the shunt resistor. Rshunt will be connected across pins 7 & 8 and will be connected in series with the load.