The accurate and reliable operation of electrical equipment is a fundamental requirement for their application in precision engineering. Simple error formulas allow the easy establishment of functional interdependencies between outputs and input quantities. Analytical methods enable sensitivity analysis to be performed successfully and with relative ease when these dependencies are explicitly given, or when combined with experimental techniques for application in more complex scenarios. However, accurately testing inherently non-linear devices poses serious challenges, particularly when it comes to going beyond simple accuracy characterization and into active error source correction. These issues are the primary obstacles in defining a general engineering framework that uses possible modeling, calculation, and optimization approaches for irregular complex non-linear systems. The present paper presents a systematic approach to error sources in secondary power supply units, especially within functional converters with a building block made up of passive non-linear elements and supplied from DC. The method builds on constructing a causal graph of a magneto-transistor voltage divider and analyzing circuit segments in relation to their voltage dividing properties. This graphical representation comes with significant simplicity, clarity, and interpretability benefits that should enable easier recognition of primary error contributors. The presented approach is not only capable of identifying sensitivity coefficients accurately, even under certain non-linear operating conditions, but also allows for directed error compensation. These results are anticipated to provide a basis for the development of engineering methodologies in precision electrical device and system design and optimization.