DC Panel Meters Display Voltage, Current and Power

Murata Power Solutions has introduced its DCM20 series of multifunction panel meters. For DC systems, these meters measure DC voltage and current, calculate power up to 96 kW, and display values either manually selected or continuously cycling. The miniature panel-mount product provides an input voltage range of 0.5 VDC to 72 VDC, with 10 mV of resolution. The meter also supports current measurement ranges from 5 A to 1,200 A when used with an external user-supplied resistive shunt. Targeted for use in 12 V, 24 V or 48 V systems, out-of-the-box accuracy of the product is +/-1 % for voltage and +/-2 % for current.
Packaged in a rugged, one-piece polycarbonate housing, with dimensions of 2.1″  x 1.43″ or 53.3 mm x 36.3 mm, the DCM20 fits in ‘0U’ and ‘1U’ racks making it well-suited for laboratory instrumentation as well as industrial and telecom equipment. Threaded mounting studs and caged terminal blocks for application wiring ensure reliable operation in harsh environments.

Applications for the product include, but are not limited to, real-time monitoring and display of DC power in telecom power distribution systems, battery management/backup systems, laboratory instrumentation and alternative energy and marine installations.

The DCM20 features a large (0.36″ /9.2 mm) bright red display easily readable at 15 feet (5 m), with green or blue displays a future option. A front-panel capacitive touch sensor is incorporated for selection of operating mode, avoiding wear-out issues possible with a membrane of other mechanical switches. Using the touch sensor control, the user may configure the unit to display voltage, current or power, or set the unit to continually cycle between the three measurements.

The unit can be self-powered from the measured voltage or powered separately from an external power supply, which can range from 9 VDC to 72 VDC. When self-powered, the input voltage range that can be measured is 9 VDC to 72 VDC and when externally powered the lowest measurable input voltage extends down to 0.5V. Current consumption of the DCM20 is generally negligible compared with the measured current being typically 6 mA at 12 V and only 2 m A at 72 V input.

A DIP switch on the DCM20 allows selection of 16 different full-scale current readings from 5 A to 1,200 A providing compatibility with a wide range of external shunt resistors, available both from Murata and other manufacturers. A fine adjustment potentiometer is also provided to calibrate the unit to compensate for shunt resistor tolerance for improved system measurement accuracy. The external shunt resistor may be placed in either the ‘high’ or the ‘low’ side of the power system, as the DCM20 has a common-mode voltage range of 72 V. A jumper is available to set where the voltage is actually measured, either remotely or at the shunt resistor. In this way, high or low side current sensing is practical and power measurement can exclude losses in wiring and the shunt resistor itself.

Murata Power Solutions | www.murata-ps.com

Real-Time Trailer Monitoring System

Dean Boman, a retired electrical engineer and spacecraft communications systems designer, noticed a problem during vacations towing the family’s RV trailer—tire blowouts.

“In every case, there were very subtle changes in the trailer handling in the minutes prior to the blowouts, but the changes were subtle enough to go unnoticed,” he says in his article appearing in January’s Circuit Cellar magazine.

So Boman, whose retirement hobbies include embedded system design, built the trailer monitoring system (TMS), which monitors the vibration of each trailer tire, displays the

Figure 1—The Trailer Monitoring System consists of the display unit and a remote data unit (RDU) mounted in the trailer. The top bar graph shows the right rear axle vibration level and the lower bar graph is for left rear axle. Numbers on the right are the axle temperatures. The vertical bar to the right of the bar graph is the driver-selected vibration audio alarm threshold. Placing the toggle switch in the other position  displays the front axle data.

Photo 1 —The Trailer Monitoring System consists of the display unit and a remote data unit (RDU) mounted in the trailer. The top bar graph shows the right rear axle vibration level and the lower bar graph is for left rear axle. Numbers on the right are the axle temperatures. The vertical bar to the right of the bar graph is the driver-selected vibration audio alarm threshold. Placing the toggle switch in the other position displays the front axle data.

information to the driver, and sounds an alarm if tire vibration or heat exceeds a certain threshold. The alarm feature gives the driver time to pull over before a dangerous or damaging blowout occurs.

Boman’s article describes the overall layout and operation of his system.

“The TMS consists of accelerometers mounted on each tire’s axles to convert the gravitational (g) level vibration into an analog voltage. Each axle also contains a temperature sensor to measure the axle temperature, which is used to detect bearing or brake problems. Our trailer uses the Dexter Torflex suspension system with four independent axles supporting four tires. Therefore, a total of four accelerometers and four temperature sensors were required.

“Each tire’s vibration and temperature data is processed by a remote data unit (RDU) that is mounted in the trailer. This unit formats the data into RS-232 packets, which it sends to the display unit, which is mounted in the tow vehicle.”

Photo 1 shows the display unit. Figure 1 is the complete system’s block diagram.

Figure 1—This block diagram shows the remote data unit accepting data from the accelerometers and temperature sensors and sending the data to the display unit, which is located in the tow vehicle for the driver display.

Figure 1—This block diagram shows the remote data unit accepting data from the accelerometers and temperature sensors and sending the data to the display unit, which is located in the tow vehicle for the driver display.

The remote data unit’s (RDU’s) hardware design includes a custom PCB with a Microchip Technology PIC18F2620 processor, a power supply, an RS-232 interface, temperature sensor interfaces, and accelerometers. Photo 2 shows the final board assembly. A 78L05 linear regulator implements the power supply, and the RS-232 interface utilizes a Maxim Integrated MAX232. The RDU’s custom software design is written in C with the Microchip MPLAB integrated development environment (IDE).

The remote data unit’s board assembly is shown.

Photo 2—The remote data unit’s board assembly is shown.

The display unit’s hardware includes a Microchip Technology PIC18F2620 processor, a power supply, a user-control interface, an LCD interface, and an RS-232 data interface (see Figure 1). Boman chose a Hantronix HDM16216H-4 16 × 2 LCD, which is inexpensive and offers a simple parallel interface. Photo 3 shows the full assembly.

The display unit’s completed assembly is shown with the enclosure opened. The board on top is the LCD’s rear view. The board on bottom is the display unit board.

Photo 3—The display unit’s completed assembly is shown with the enclosure opened. The board on top is the LCD’s rear view. The board on bottom is the display unit board.

Boman used the Microchip MPLAB IDE to write the display unit’s software in C.

“To generate the display image, the vibration data is first converted into an 11-element bar graph format and the temperature values are converted from Centigrade to Fahrenheit. Based on the toggle switch’s position, either the front or the rear axle data is written to the LCD screen,” Boman says.

“To implement the audio alarm function, the ADC is read to determine the driver-selected alarm level as provided by the potentiometer setting. If the vibration level for any of the four axles exceeds the driver-set level for more than 5 s, the audio alarm is sounded.

“The 5-s requirement prevents the alarm from sounding for bumps in the road, but enables vibration due to tread separation or tire bubbles to sound the alarm. The audio alarm is also sounded if any of the temperature reads exceed 160°F, which could indicate a possible bearing or brake failure.”

The comprehensive monitoring gives Boman peace of mind behind the wheel. “While the TMS cannot prevent tire problems, it does provide advance warning so the driver can take action to prevent serious damage or even an accident,” he says.

For more details about Boman’s project, including RDU and display unit schematics, check out the January issue.