How did you measure ac accurately before there were true-rms DMMs? There were three kinds of precision rms meters available to me. The available types were all analog; the electrodynamic, the iron vane, and the thermocouple. They were all fragile, requiring careful handling to achieve accurate measurements and avoid damage. They were also slow to respond and presented a significant load to the measured circuit.

How different it is today, with many digital multimeters featuring true-rms ac measurement capability. Today’s meters are rugged, respond quickly, are more sensitive, and present the results with easy to read, digital clarity. But there are some subtle points you should be aware of when making rms measurements.

Most DMMs today measure the ac-coupled true-rms signal, rejecting the dc component if any is present. A few models, such as the Fluke 189, offer the additional option of measuring ac+dc true-rms, calculating the total rms value with the dc component present. Why the difference? When should you use one versus the other?

In a properly operating power distribution system supplied through transformers, only ac should be present, so the ac-coupled measurement offered by most DMMs is appropriate. And, when a diode fails in a motor drive rectifier circuit, it’s usually more effective to detect the failure by observing a reading in the dc voltage function which rejects the ac component.

Then, there’s the case where you would like to trace an audio signal through an amplifier circuit. That signal might exist on the collector of a transistor where a dc bias current is also present. Here again, when you want to separate the ac and dc components, the ac-coupled mode of the DMM is the correct one to use.

One case for measuring ac+dc occurs in the output of unfiltered dc power supply where a significant ac ripple voltage is present on the dc signal. Since a resistive heater or incandescent lamp connected to such a supply will respond to the total energy available, an ac+dc measurement will more accurately indicate how the load might react.

What can you do if you have only the ac-coupled rms measurement capability in your DMM and you want to measure the combined ac and dc energy? Bring on the calculator.

You can first measure the ac voltage and then the dc, recording both values. Next, you square both terms and add these squared values together. Finally, you take the square root of the sum and you have the ac+dc true-rms value of the signal.

A variation of this technique may be used to investigate the ac output of an electronic motor drive by using the low pass filter function available on the Fluke 87V.

The primary signal of interest on the output of such a drive is the low-frequency signal that drives the motor. The low pass filter of the 87V isolates this signal so you can make accurate measurements of its frequency and true-rms voltage.

When you don’t use the filter, you get a much higher reading, because the measured signal now includes all the switching voltages used to create the motor drive voltage.

To determine the rms value of this difference, you can do the following:

1. Measure the total voltage and record the value.
2. Measure the voltage with the low pass filter on and record that value.
3. Now, square both terms and subtract the second squared value from the first.
4. Finally, take the square root of the result and you will have the true-rms value of all the excess energy that was not filtered, within the bandwidth of the meter’s input.

One case where rms results may be added or subtracted directly is at your household panel, where the sum of the two 120V legs is indeed 240V. Those two signals are coherent. Visit https://presidium.ph/ to get a compact true-rms meter for accurate electrical installation and troubleshooting.

Oscilloscopes track signals as they shift over time and indicate the signals on display. The amplitude of the signal is indicated on the vertical axis and time is shown on the horizontal. The device plots a graph of the instantaneous signal voltage as a function of time. In order to analyze waveform traces meticulously, here are the four characteristics of the waveform that you should look for amplitude, time, waveform shape and distortion, and waveform distances, specifically from the outside sources.

Here are the common variations to look for:

Symmetrical shape

Continuous waveforms should always be symmetrical. If there is an instance wherein you have to print traces and cut them into two pieces, both sides should be identical. A small difference can cause a problem. If the two components of the waveform are not symmetrical, there may be a problem in detecting the signal.

Rise and fall, edges

Specifically, with square waves and pulses, the rising or falling sides of a waveform can immensely impact the timing in the digital circuits. It may be essential to lessen the time per division to see the edge with greater resolution. The use of cursors and gridlines will help you measure the rise and fall times of leading and trailing edges of a waveform.

 Amplitude

Always double-check that the intensity of amplitude is within the circuit operating requirements. Test the constancy, from time to time. Meticulously track the waveform for a long period of time; also monitor changes in the amplitude. Use horizontal cursors to identify if there are any amplitude fluctuations.

 Noise or glitches

When waveforms are acquired through active devices such as transistors or switches, transients or other incongruities can have a result of timing errors, propagation delays, bad contacts, or another incident. Noise will overlay the acquired signal and it will be difficult to see the real data behind the noise. I can be generated externally from DC-DC converters, lighting systems, and high-energy electrical circuits.

Excessive ringing

Ringing can be visible most of the time in digital circuits and in radar and pulse-width-modulation applications. It occurs at the transformation from a rising or falling side to a flat dc point. Test for immoderate ringing, balance the time base to provide a clear illustration of the transition wave or pulse.

Momentary fluctuation

Momentary changes in the measured signal are usually the result of external causes such as sag in the main voltage, invigoration of a high – power device that is attached to the same electrical grid, or a loose connection. Use the ScopeMeter to watch the acquired waveform for a long period of time to depict the main cause of the problem.

Drift

Minor changes in a signal’s voltage over time can be difficult to detect. The change is slow that it is hard to distinguish. The changes and aging of temperature can impact passive electronic components which are resistors, crystal oscillators, and capacitors. The drift in a reference dc voltage supply or oscillator circuit is one of the main factors to diagnose.  Sometimes the only option is to track the calculated value (V dc, Hz, etc.) over an extended period of time.

In summary, it is necessary to practice good troubleshooting skills to save time and simplify the process of determining common waveform variation before the problem occurs. Try to discover more about troubleshooting methodology and make it a habit to always document key waveforms and measurements for future reference. Interested in getting an Oscilloscope? Get yours now at https://presidium.ph/?s=oscilloscopes