Building a Power Supply – Sorting Out Voltage Readings

<post photo of setup>

My previous posting/project, PIC Programmer & Experiment Board, created the opportunity to build my own power supply. A Google search will turn up many design variations, but the most popular is to use an AC transformer, 4 diodes or a full rectifier, capacitors, resister, fuse, and a switch.

As I was testing the transformer and full wave rectifier I noticed a significant difference between the AC voltage output from the transformer and the DC voltage output from the rectifier. It was large enough that it made me want to investigate.



Radio Shack – 25.2V Center Tap 2.0A Heavy-Duty Chassis-Mount Transformer with Lead
Model: 273-1512| Catalog #: 273-1512

You will often see transformers with 2 input lines and 3 output lines like this one. The voltage between both yellow lines for this model will peak at approx. 25.2V up to a certain amperage. The voltage between the center tap (CT), which is the black line, and one of the yellow lines will be at approx half (12.6V) of the full capacity (25.2V). This is because the CT is a line from the middle windings of the transformer.

Transformer Voltage Readings – From Center Tap

<picture of voltage>

Notice the multimeter voltage reading of 13.87 AC and the oscilloscope’s peak voltage reading of 18.7 AC (peak to peak = 37.4). How can this be?

Turns out most multimeters report the root mean square (RMS) of AC voltage. The accuracy of the reading is highly dependent on the circuitry of the multimeter and the wave being measured. The higher-end models will have the ability to sample more frequently thus provide more data to calculate RMS.   RMS also represents the DC voltage equivalent of the AC voltage that delivers the same energy.










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