75VAC value is different under RMS TRMS averaging peak-to-peak

QUESTION:
A 110VAC transformer delivered 32VDC with an AC ripple of 75VAC according to an obsolete analog multimeter. Is this 75VAC value different under an RMS, TRMS, averaging, peak-to-peak, or other types of meters? An automotive technician said this value could be as high as 400+ if tested with a 900Mhz oscilloscope e.g. PicoTest. Please explain with full details.

QWEN 2.5MAX AI ANSWERED:

To understand the discrepancy between the 75VAC value measured by the obsolete analog multimeter and the potential 400+ VAC reading on a modern oscilloscope we need to delve into how different types of measurement devices interpret AC signals particularly in the presence of ripple voltage.

Key Concepts:

1. Transformer Output: The transformer delivers 32VDC with an AC ripple component.

2. AC Ripple Voltage: This is the fluctuating AC component superimposed on the DC output. It's typically caused by incomplete filtering of the rectified AC signal.

3. Measurement Methods:
- Analog Multimeters: Usually measure the average value of an AC signal and then scale it to represent the RMS value assuming a pure sine wave. This method can be inaccurate for non-sinusoidal waveforms like ripple voltages.
- True RMS meters: Measure the true effective value of any waveform including distorted or non-sinusoidal waveforms making them more accurate for complex signals.
- Oscilloscopes: Can display the actual waveform allowing measurements of peak-to-peak RMS and other parameters directly from the waveform.

Why the Discrepancy?

Analog Multimeter Reading:
The obsolete analog multimeter likely uses an averaging method to measure the AC component. If the ripple voltage is not a pure sine wave which it rarely is in rectified DC circuits the meter will misrepresent the true RMS value. Most analog meters assume a sinusoidal waveform and apply a correction factor to convert the average value to an equivalent RMS value for a sine wave. If the waveform is different e.g. a sawtooth or pulsating DC this conversion will be incorrect.

For example if the ripple waveform has sharp peaks as is common in unfiltered rectified AC the average value could be much lower than the RMS value. The meter might read 75VAC but this would not reflect the true energy content of the ripple.

Oscilloscope Reading:
An oscilloscope provides a direct visualization of the waveform. When measuring ripple voltage the scope can show the peak-to-peak voltage which includes both the positive and negative excursions of the ripple.

If the ripple waveform has significant spikes or high-frequency components common in switching power supplies or poorly filtered rectifiers these peaks can be much higher than the average or RMS values. For instance a ripple with a small RMS value might have very sharp high-amplitude peaks that contribute to a much higher peak-to-peak voltage.

In your case the technician mentioned a value exceeding 400V when using a PicoTest oscilloscope. This suggests that the ripple voltage contains high-frequency components or transient spikes that are not captured accurately by the analog multimeter. The oscilloscope measures the true peak-to-peak voltage which can indeed be much higher than the RMS or average values.

True RMS vs Average Responding Meters:
A True RMS meter would provide a more accurate RMS value of the ripple especially if the waveform deviates significantly from a pure sine wave. However even a TRMS meter might not capture extremely fast transients unless it has a sufficiently high bandwidth.

Summary of Measurement Types:

- Analog Multimeter Average Responding: Likely underestimates the true RMS value of non-sinusoidal waveforms. Reads 75VAC in this case.

- True RMS Meter: Would give a more accurate RMS value potentially higher than 75VAC if the waveform is distorted.

- Oscilloscope Peak-to-Peak Measurement: Can reveal the full extent of voltage spikes possibly showing values exceeding 400V if high-frequency components or transient spikes are present.

Conclusion:
The 75VAC value obtained with the analog multimeter is likely an underestimate of the true RMS value due to the assumptions made about waveform shape. The 400+ VAC value observed on the oscilloscope reflects the peak-to-peak voltage capturing the full amplitude of the ripple including any high-frequency spikes or transients. Therefore the two readings are not directly comparable because they represent different aspects of the waveform.

Final Answer:
Yes the 75VAC value is different under RMS TRMS averaging peak-to-peak or other types of meters.