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How to Measure Inrush AC Current

Blog Post created by bill griffith Employee on May 23, 2018

Measuring inrush current is always interesting. In some devices, inrush current can be surprisingly high (10x or higher than their steady current). Excessive inrush current can damage components and pc boards designed for lower steady-state currents. To avoid damage, many devices include a protection circuit to limit the inrush current. Typically, large inrush current lasts for a few cycles before returning to a steady-state current. Inrush current is measured as a peak current and is useful for sizing fuses or designing additional protection circuitry.

 

An inrush current of an inductive load.

Figure 1. An inrush current of an inductive load.

 

It is also important to consider what phase the AC voltage is at when it is applied to the DUT. The turn-on phase can significantly affect the inrush current. A couple of voltage waveforms with different turn-on phases are provided below.

 

Various turn-on phases of an AC voltage. Left 0 degrees, middle 30 degrees, and right 180 degrees.

Figure 2. Various turn-on phases of an AC voltage. Left 0 degrees, middle 30 degrees, and right 180 degrees.

 

When using a mechanical switch, you have no control over the turn-on phase. The device’s inrush current will be completely unpredictable since it is dependent on the phase of the AC voltage. To control the voltage turn-on phase and thus predict the inrush current, you can use Keysight’s AC6801B AC source. A peak hold measurement determines the inrush current while a peak measurement is used for steady-state current.

 

Current waveform with peak hold and steady state peak current.

Figure 3. Current waveform with peak hold and steady state peak current.

 

AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms.

Figure 4. AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms.

 

Determining the maximum inrush current

 

Most electronics contain switching power supplies as they are incredibly efficient. The test setup below tests an external switching power supply with an AC source and a 30 W load.

 

Test setup to measure the inrush current of the 12 VDC supply.

Figure 5. Test setup to measure the inrush current of the 12 VDC supply.

 

The steady state 2.26 A peak current drawn from the AC source.

Figure 6. The steady state 2.26 A peak current drawn from the AC source.

 

A series of measurements are made to determine the maximum inrush current. The first measurement uses a 0-degrees turn-on phase, and the second uses 10 degrees. Each subsequent measurement uses a 10-degree higher turn-on phase. Configuring the AC source for inrush current measurements is a two-step process.

 

1) Setting the turn-on phase from the front panel of the AC source.  

Setting the turn-on phase from the front panel of the AC source.

2) Clear the peak hold measurement from the front panel.

Clear the peak hold measurement from the front panel.

 

The 12 VDC power supply output will turn on 1.3 seconds after the AC power is applied. Only after turning on its output will it be drawing steady state current. In Figure 7, two different time scales are used to display the inrush current. The screen capture on the left shows the voltage phase and the inrush current. The screen capture on the right shows the inrush current and the 1.3 second delay before the steady-state current.

 

The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. You can see the details of the inrush current spike on the left. On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current.

Figure 7. The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. You can see the details of the inrush current spike on the left. On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current.

 

Graphing the inrush measurements versus phase for the 12 VDC power supply reveals a trend. The inrush currents are lower when the voltage is turned on with a phase of 0 and 180 degrees. This is because at a phase of zero and 180 degrees, the voltage is turning on at zero volts.

 

Inrush current for a capacitive device, 12 VDC power supply vs. phase.

Figure 8. Inrush current for a capacitive device, 12 VDC power supply vs. phase.

 

Devices with capacitive input will have low inrush currents when the voltage is turned on at zero volts. At zero volts a sinewave has its maximum rate of change, this change causes an inductive load to create their largest inrush current. An inductive load will have its maximum inrush current at zero and 180-degrees.

 

A simulated graph of inrush current vs. phase for an inductive load.

Figure 9. A simulated graph of inrush current vs. phase for an inductive load.

 

Limiting inrush current

 

Inrush current can be limited by designing a device with lower reactance. An example is lower capacitance or lower inductance. Another possibility is to turn on a small part of the device and synchronize the rest of the turn-on to the AC line, taking advantage of the phase with the lowest inrush. The 12 VDC power supply tested delayed the turn on of its output. A third possibility is adding a negative temperature coefficient (NTC) current limiting device to your design. The NTC device initially has a high impedance, which reduces the inrush current. As the NTC device warms up, its impedance is reduced. The steady-state current is not affected by the NTC. Knowing the steady-state current and maximum inrush current helps in selecting the right NTC.

 

Using an NTC to reduce the inrush current into capacitive device.

Figure 10. Using an NTC to reduce the inrush current into capacitive device.

 

Summary

 

To accurately measure the maximum inrush current, it is essential to consider the turn on phase. The design of the device will affect the phase at which the maximum inrush current occurs. Some devices will have to be designed to limit the inrush current. Adding an NTC current limiting device to your design will limit inrush current. Several measurements need to be made to select the right NTC. An AC6801B AC source can be used to characterize the inrush current of a device quickly, helping you design a device with inrush current that you and your customers can trust.

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