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Measuring the Current Draw of a Device with Multiple Power Buses

2018-09-28  |  4 min read 

When designing a device, knowing the current consumed is essential for protecting the device. As the design progresses knowing the energy consumed becomes more critical for sizing batteries and power adapters. Measuring current on devices with multiple power buses can be particularly challenging. Additionally, lower power devices use very low currents which can be difficult to measure. This article provides several insights into making better current measurements.

Measurement readback of a power supply

The simplest way to measure the current draw of a device is to use the built-in current measurement capability of a power supply. Power supplies are available with various degrees of precision. Power supplies use an internal shunt resistor to measure current.

Figure 1. Keysight E36312A measuring the voltage and current (top two lines) at the binding post.

As current flows through the shunt resistor, an internal analog to digital converter (ADC) measures the voltage drop across the resistor. Ohm’s law is used to convert the measured voltage and known resistance to current (I = V/R). Shunt resistor selection requires great care as it needs to have a near constant value. A common problem is self-heating. A rise in temperature will, in turn, cause a change in the resistance. Current running through a resistor will cause the resistor to self-heat. A power supply's current readback specification includes this error and others that will occur.

Measuring the power usage of a device

Along with current, we need to know the voltage at the device to calculate the energy. Again, the power supply can be used to measure voltage at the device. To make quality measurements, the leads should be kept short and twisted. Along with appropriate size wire 16 to 18, AWG is good for 10 A or less and only adds 13 to 21 mΩ/m. Using remote sense allows the power supply to adjust its output voltage to ensure a constant voltage at the DUT.

Figure 2. Remote sense increases the voltage output to compensate for the loss in the leads.

Measuring large swings in current

Correctly measuring a large swing in current, such as an inrush current, is difficult to make. When a load draws a large amount of current the power supply’s voltage will drop momentarily. The momentary voltage drop is known as droop. Power supplies often spec the recovery time but never the amplitude of the droop which is dependent on the cabling and load. Twisting the wiring reduces the magnitude of the droop by lowering the inductance. Any extra resistance in the cabling will increase the recovery time. Lastly, the architecture of a typical bench power supply, Figure 3., is designed to measure the average current while maintaining a stable voltage. Accurately measuring dynamic current requires a specialized power supply.

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