Digital panel meters (DPMs) are useful for displaying electrical quantities in an easy-to-read digital format. Circuit Specialists sells a variety of these DPMs with either LED or LCD displays. Many of our customers are unsure of which display is best for them and, furthermore, are unsure how to use these units. I will provide simple instructions on how to utilize our most popular model, the PM-128E. This model is a universal type that will work in almost all applications without needing external parts.
The PM-128E and PM-128E-BACKLIT are both DPMs that use an LCD screen to display the desired electrical quantity. The advantage of the LCD-type display is the extremely low amount of power required for operation. These two models are identical in operation with the exception that the -BACKLIT version incorporates — you guessed it — a backlight, which allows the user to see the display in dark environments. This model obviously requires more power to operate than the standard version. The PM-128E series digital panel meters are completely self-contained and require no external components for operation. In addition these units can be used to measure DC current up to 200 mA without an external shunt.
To set up the PM-128E to correctly display a DC voltage, the user must determine how the DPM is to be powered. If the signal that is being measured has a common connection to the power source the unit must be supplied with a regulated voltage (5V). This would be what would be required in an automotive application, e.g., monitoring the battery voltage in a vehicle. If the signal to be measured is isolated from the power source the unit must be supplied with a 9-11V power source (e.g., a 9V battery). Next the user must choose the full-scale voltage range desired. The choices are 200 mV, 2V, 20V, 200V, or 500V. These parameters are set by simple shorting jumpers on the back of the DPM. As an example we will describe the correct jumper settings for a battery voltage monitor in a vehicle.
Because the vehicle uses a 12V battery system, we will select the 20V range and select the DC setting. This requires shorting across these two solder pads. We will power the DPM using the same battery that we are monitoring, so we will need to select the 5V common-ground power supply. To accomplish this we must short jumpers J1 and J2 and make sure that J3 and J5 are not shorted. The final jumper selection that must be made is to set the correct position of the decimal point. Since we have selected the 20V range we will want two digits to be displayed to the right of the decimal point. To accomplish this we will solder a short piece of wire between the center and rightmost hole (ON) for the P2 connections on the backside of the DPM. This will provide for a maximum display of 19.99 on the panel meter’s display. All that is left to be done is to provide the regulated 5V power to the POWER terminals and connect the signal to be measured to the IN terminals.
Thus we complete the simple steps required to set up and connect one of the many digital panel meters available from Circuit Specialists. The PM-128E unit was chosen because it is suitable for use in any application and requires no external components for operation. Some of the other models available may require the user to supply their own divider resistors for different voltage ranges.