# Circuit to Measure Power Supply Voltages

Posted on Feb 7, 2014

To obtain the power supply graphs on the previous page, I didn`t stand outside for two days straight measuring the power sources with a voltmeter. Instead, the circuit itself is able to independently monitor the power sources with the addition of a few resistors. Although diode D3 allows the solar panel voltage to charge the batteries, measuring t

he voltage behind D2 does indeed reflect the battery voltage, not the solar panel voltage. (Well, assuming there is a battery pack connected. ) If you pull the batteries out of the circuit and measure them with a multimeter, the voltage would be nearly the same as the in-circuit measurement. The device performing the measurement needs to have a fixed voltage (either regulated power or a voltage reference). In this example of the weather station device, it has a linear regulator that supplies 3. 3 V to the microcontroller. As such, we need the measurable voltages of the power sources to be less than 3. 3 V in order for the microcontroller analog-to-digital converter (ADC) to be able to safely and accurately measure the power supplies. The power source voltage is easily divided in half using a voltage divider consisting of two equal value resistors in series. In this case, R1 and R11 are 10 kilohms each, so the voltage in the middle of them will be half the voltage that is being fed into the top. For example, when the solar panel is at 5. 65 V (the maximum peak I measured in full sun with no load attached to the panel), TP1 will measure 2. 825 V. That`s less than 3. 3 V, so the microcontroller can measure it accurately. After measuring, the microcontroller simply multiplies the value by 2 to determine the true solar panel voltage. If the device is disconnected at TP3 (for example, by an on/off switch at that point), one might...

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