Often, raw power supply properties are needed: voltage can estimate the current state of charge of a battery, for example, and current indicates the power draw.
By measuring current and voltage, you can also calculate the total power. Coloumb meters do this over time and work like a “power meter”, indicating the total power drawn or produced over a given period of time, for example the yield of a solar panel.
Measuring Voltage
Both voltage and current are measured as voltages: an Ampere-Meter internally is a Volt-Meter, too. So let’s first look at how voltages can be measured.
To measure a voltage, connect an analog input pin of a microcontroller to the power source, but make sure the expected voltage is within the range that this analog pin can handle.
If you need to measure higher voltages, use a simple voltage divider consisting of two high-ohm resistors. This way, your microcontroller can measure only a given “percentage” or “fraction” of the total voltage, staying within its limits, and you can later extrapolate the real voltage in your firmware.
Voltmeters (as well as analog input pins and voltage dividers) have a high resistance. This is necessary because with a Voltmeter you are short-circuiting your circuit.
Due to the high resistance, nothing bad happens, and only a very small current flows. By using an analog input pin, you can directly measure this voltage (if it is within the acceptable voltage range), and with a voltage divider, you can measure the voltage drop at one of the two resistors.
Analog Voltmeters
Classic analog voltmeters use a coil or iron part that responds to the magnetic field produced by the electrical current. The amount of deflection of the pointer is proportional to the current flowing through the coil, which, in turn, is proportional to the voltage being measured. Since the internal resistance of the voltmeter is known, the amount of current flowing through it is directly proportional to the voltage:
- 5V: When the voltmeter is connected to 5V and has a known internal resistance of 500 Ohm, according to Ohms law the current then is U / R = I, thus 5V/500Ohm = 10mA. The current of 10mA produces a magnetic field of 4x10-8 Tesla which moves the indicator of the analog voltmeter.
- 10V: When the voltmeter is connected to 10V and continues to have a known internal resistance of 500 Ohm, according to Ohms law the current now is U / R = I, thus 10V/500Ohm = 20mA. The current of 20mA produces a magnetic field of double the strength: 8x10-8 Tesla which moves the indicator of the analog voltmeter twice as much.
Digital Voltmeters
Digital voltmeters add an ADC (Analog-Digital-Converter) to convert analog voltage to a digital signal that is then displayed. When you are using a microcontroller and one of its analog-input pins, you are essentially building a digital voltmeter.
Measuring Current
Current cannot be measured directly. Instead, there are two physical effects that can be used to measure current indirectly through a voltage, using the principles outlined above:
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Voltage Drop:
Every load causes a voltage drop. By inserting an artificial load (called Shunt resistor) with a very precise known resistance in series with the real load, the current can be measured via the voltage drop.In essence, you are building a voltage divider where one resistor is the Shunt and the other resistor is the load that you want to measure.
The Shunt resistance must be very low in order to minimize energy losses through heat.
A shunt can measure current very precisely and is immune to noise and magnetic fields in the vicinity. However, it requires the insertion of a shunt resistance into the circuit and is not isolated from the circuit that you want to measure.
For this reason, shunts are typically used with low currents and voltages.
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Magnetic Field:
Every electrical current produces a magnetic field. In AC current, the magnetic field is alternating and can be measured using a simple coil. With DC, the magnetic field is constant. It can be measured using more expensive hall effect sensors.Since magnetic fields can be measured “wirelessly” in the vicinity of currents, the measuring setup is isolated from the circuit you measure. Typically, such sensors require the wire with the current to be fed through a coil or hall sensor, and clamps simplify this even further.
This is why this approach is safer, especially with high currents and voltages. However, it is also less accurate, especially with low currents, and subject to noise produced by other magnetic fields in the vicinity.
Coloumb Meters
In some instances, you may not be interested in the momentary voltage and current. Instead, you’d like to know over some time, how much total power a power source produces (i.e. a solar panel or a battery under test), or how much energy a load consumes.
This is what coloumb meters do: they, too, measure voltage and current, but in addition they accurately calculate the total power and often can also accumulate it over time.
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(content created Mar 10, 2024 - last updated Oct 29, 2025)