How to Measure Ripple Voltage Using a Multimeter

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One of the most typical specifications when assessing a power supply is ripple. While it might look like a simple measurement, there are important factors you need to remember to get the proper data. 

The performance of a switching converter can substantially lower if the output signal is influenced by the ripple. Its overall presence is visible in a spectrum meter. The reduction of output ripple and switching transients is essential for every application. 

What is a Ripple Voltage?

A ripple voltage also referred to as ripple, refers to the fluctuation in voltage output. The measurement of the output ripple is switching regulations is a delicate and essential operation for different reasons. 

Measuring that unnecessary signal needs utmost care as it’s simple to get flawed readings. Ripple is an unwelcome signal, which exists at the output of the switching regulator. It varies on the quality of the regulator itself and the qualities of the external components utilized. 

Keep in mind that ripple is the excess of the alternating voltage at the AC output. Ripple’s frequency significantly depends on the circuit switching. Further, its fundamental frequency is similar to the switching frequency.

A crucial parameter, which determines it is the ripple factor—the equivalent to the ratio between the average worth of the rectified output and the RMS value of the AC component in the output. Normally, it’s less than unity. 

It’s somewhat complicated to measure precisely. Engineers are experiencing some problems with making exact measurements. The total of the error sources should be only a few mV. However, the limits are much narrower for other more critical applications like medical ones. 

What Causes Ripple Voltage?

Are you checking the stability of the power supply voltage? Then it’s possible that you will find noise in the frequency and time domains. Your goal is to determine noise sources and take the necessary steps to minimize them.

Basic steps such as decoupling and filtration are suitable for specific noise sources. Unfortunately, they won’t give enough decoupling, especially if you are working with fast digital signals. 

Keep in mind that intrinsic noises from the power rails or on the power supply come in different forms. These include the following:

  1. Transient ringing

This oscillation looks the same as a ripple from power conversion. Nonetheless, it’s essentially different. It increases from the inductance in the PDN and parasitic capacitance. It attracts a burst of current, producing a transient oscillation as an IC switches states or output levels. That’s often referred to as ripple voltage. That transient response could show on the power rails even when the supply voltage is controlled. 

  1. Ripple from AC to DC conversion

This is normally observed from unregulated, unfiltered power supplies converting an AC output to a DC output. It’s leftover from the full-wave rectifier utilized for power conversion. 

  1. Changing noises

It shows at particular frequencies on the output from a switching regulator. That noise comes from the PWM signal utilized to switch one or more FET drivers within the converter. 

  1. Random sound within the frequency domain 

That noise shows as thermal noise through the frequency domain. That white noise is Gaussian distributed. 

Bear in mind that there’s a major difference between ripple from a transient ringing and a smoothing capacitor. That difference lies in the frequency where the oscillation is supervised. A rectifier bridge utilized for AC to DC conversion takes at least 60 Hertz or 50 Hertz sinusoidal power source and then turns it into its complete value.

That is then transformed to a DC signal along with a smoothing capacitor. Moreover, the smoothing capacitor can lower the effect of that specific frequency along with a bandstop filter. Nonetheless, that leaves the output loose. It means the voltage observed by the load will differ if the load impedance varies.  

Typically, the output from an AC to DC converts inputs into a regulator. You see, a regulator is created to hold the output voltage constant over changes in the load on the output. That’s somewhat typical when handling nonlinear circuit elements. 

The device outputs a few currents up to a maximum value while holding the voltage constant. The regulation is presented by a diode and switching a FET driver—that is driven with a PMV single. Thus, that overpowers ripple from a rectifier bridge. Still, it generates the switching sound mentioned earlier. 

Odd noise from a power regulator doesn’t show as a flawless low-frequency oscillation. As an alternative, it shows as a high Q-peak within the frequency domain. 

Contrast that with a transit ripple voltage. That shows at greater frequencies conforming to the damped transient oscillation frequency within the PDN. That normally shows as an underdamped oscillation throughout a single switching occasion. That’s especially true even if it could show as a damped oscillation. 

What is the Right Way to Measure Ripple Voltage?

At this point, you are fully aware that a ripple voltage is a tiny AC voltage put on top of a DC offset. You can measure a measured voltage by using a digital multimeter. Just follow the steps below to do it successfully.

  1. Insert the probe into your multimeter. Typically, you will notice that two probes are supplied.
  1. Attach the multimeter’s black probe in the negative terminal and the red probe into the positive terminal.
  1. Turn the dial of your multimeter’s front panel clockwise to switch the device on.
  1. Pick the AC voltage by twisting the dial on the front to the symbol of an oscillatory wave. Make sure you connect the probes on the circuit with the ripple voltage. Your meter will measure the signal’s AC component. 
  1. The display must change to present the measured value of the ripple voltage amplitude. Ensure you also measure the frequency.
  1. Turn the front dial to the frequency setting. Connect the probes into the ripple voltage circuit. The frequency will be highlighted on your meter. After doing that step, you can characterize the ripple voltage. 

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