Backyard Meteorological Instrumentation

Power Monitor


This module monitors the incoming power to the equipment shelter by measuring the supply voltage and the supply current. This allows the system to monitor is power consumption to determine if experimental setups are drawing too much power.

Voltage Divider

The supply voltage is approximately +24V, but the Analogue to Digital Converter (ADC)  that will measure the supply voltage can only accept an input voltage of +3V. Assuming a safety margin of 6V over the nominal +24V, the measured voltage must be reduced by at least a factor of 10. This is easily done using a resistive divider. Ideally, the load on the supply is as low as possible so that power isn't wasted, but the input impedance limits the size of the divider elements to maintain accuracy.

In theory, this is a simple problem. For Vout to equal 0.1Vin, R1 just needs to be 9 times larger than R2, and for R1 + R2 to be a really large value to keep the current low. However, the ADC being used to measure Vout has a finite input impedance which limits the size that R1 can be to be 9 times the combined resistance of the R2 in parallel with the ADC. It is also complicated by the fact that preferred value resistors may not provide the exact resistance value that is needed.

In reality, the use of preferred values and the tolerances of the component values that the actual output will only be an approximation. This scaling error can be adjusted by adjusting the software to account for the actual output voltage division. The input impedance varies with the selected range, so a nominal input impedance of 3MΩ will be used in the calculations. Since the input impedance is 3MΩ, the value of R2 should be at least 10 times smaller so that the input impedance has a minimal effect of the combined value (Rx). Assuming that the total resistance of the R1+Rx should be at least 1MΩ, Rx should be 0.1MΩ and R1 0.9MΩ. However, 1MΩ is the closest preferred value to 0.9MΩ, so Rx should be 0.11111MΩ. Working backward from Rx and ADC input impedance, R2 should be 115.383kΩ. This can either be done by 100kΩ and 15kΩ resistors in series, or it could be done by a 200kΩ potentiometer connected as a variable resistor. The 200kΩ variable resistor was selected as it is available, and was adjusted so that the ADC input is scaled correctly. The scaling of this reading to the supply voltage is carried out in the software.

Current Sensor

The supply current to the enclosure is measured using a Hall-effect current sensor ACS-712. The rated capacity of the +24V supply feeding the enclosure is a maximum of 5A. A 20A version of this sensor is available and provides a level of overhead if the supply ever provides more than its rated 5A. This sensor uses a +5V supply and the output range is 0.5V (-20A input) to +4.5V (+20A input). The output voltage in the range 0A to +20V is +2.5V to +3.0V. Scale and offset adjustment is carried out in the software.

The ADC can only tolerate a maximum input of  +3.6V (VDD + 0.3V) and this corresponds to just over +10A of supply current. As a 100% current overload is not likely given that the +24V supply is regulated and protected, but to account for this unlikely possibility, the sensor is connector so that an increase in current will result a reduction of output voltage towards 0V output, rather than +5V output. it has been decided that a voltage divider on the output of the current sensor is not required.

Physical Construction & Circuit Diagram

The power monitor is located in a small plastic box that sits on the floor inside the enclosure. The +24V supply connects to terminals on one side of the monitor. Internally the +ve rail passes though the current sensor before exiting via a terminal on the other side of the box. The -ve rail passes though the box from one side to the other. The voltage divider is hard wired between the input +ve and -ve terminals.

On one of the perpendicular faces, a 4-wire cable brings in the power to the current sensor and takes out the supply voltage and current samples.

I/O cabling

The power monitor has a set of power input terminals to accept the incoming power cable; and a corresponding set of output terminals where the power is connected to the DIN rail input terminals. It also has a flying cable from the monitor box to the DIN rails terminals that brings power to the monitor and accepts the current and voltage sensor readings from the power monitor box.

Licenced under Creative Commons Attribution Share Alike 4.0 International or better by Mark Little (2022 - 2023)