The datasheets tell us what the manufacturer wants us to know about their parts. Venable's RLC Measurement Set will tell you the rest of the story, revealing important component characteristics that play a significant role in ciruit performance.
In a single sweep, one can see the performance of capacitors, inductors and resistors over frequency. ESR, ESL and self resonant frequency are revealed in graphical and numeric detail.
Measure the DC resistance, open circuit inductance, leakage inductance, self-resonant frequency and winding capacitance of any transformer.
Sometimes you have to have an optocoupler in you feedback loop. Measure the current transfer ratio (CTR), including AC effects and bandwidth.
Though much attention has been focused recently on modeling of power supply circuits, parasitic elements are a major drawback, and have greatly diminished the success and effectiveness of modeling. Values of parasitic elements are not easily determined, but can be measured and accounted for.
