In this paper the equivalent sources method which is intended to calculate the periodically switched electric circuits using harmonic analysis method is described. It is not possible to apply 'the harmonic analysis method to analyze such circuits, since the circuit configuration varies over time. When using the offered method, the switched source of direct voltage is replaced using the equivalent source of variable voltage, for which electric currents, voltages power and other electrical magnitudes are completely the same as in the case of the switched direct voltage source; the main difference is that after such substitution the voltage source becomes non-switched. Then the harmonic analysis method can be readily applied for calculation of the input of separate harmonics into the overall result. The essence of the method is illustrated by calculating the equivalent source for a simple electric circuit operated by first law of commutation. Ill. 6, bibl. 5 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.114.8.697