The potential of the exit point j of an arm (figure 1), can take the values:

 

By rapidly commuting from one value to the other, the potential can take, as AVERAGE, any value between and .

If T is the commutation period, we can adjust the average value of the potential by imposing the relative length during which .

The relative length during which has the value results by comparing

- a reference voltage with the period T,

with

- a command voltage , which is an image of the desired average value

With a triangular reference voltage, we have (figure 2):

By slowly varying regarding the modulation period T, we obtain a wave form that follows on average .