Zero-Crossing detectors are commonly employed to convert sinusoidal voltage signals into square waves. In this configuration, the operational amplifier (Op-Amp) operates in an open-loop comparator mode. The power supply for this circuit can be either single or dual. Typically, this circuit is utilized to initiate a process after the zero-crossing point, such as activating a load or generating a firing pulse through a microcontroller by initiating timers, counting cycles, or monitoring the frequency of the alternating voltage signal.
When the input signal is linked to V+ and V- is grounded, as the input voltage surpasses the ground voltage (i.e., when the input is positive), the Op-Amp’s output becomes +Vsat. Conversely, the output becomes -Vsat when the input is negative.
Op-Amp as a Non-Inverting Zero-Crossing detector
In an alternate configuration, if the input signal is linked to V- with V+ grounded. When the input voltage exceeds the ground voltage (i.e., when the input is positive), the Op-Amp produces an output of -Vsat. Conversely, the output becomes +Vsat when the input is negative. This distinction is applied when the input signal is derived from a transformer’s secondary and a square wave output is needed for different applications.
Op-Amp as Inverting Zero-Crossing detector
However, it’s essential to incorporate appropriate filter circuits between the input signal and the Op-Amp circuit to eliminate harmonics.
A sinusoidal alternating current can be expressed as i = I sin ωt, where i represents the current at time t, and I is the maximum current. Similarly, for a sinusoidal alternating voltage, we can write v = V sin ωt, where v stands for the voltage at time t, and V is the maximum voltage.
We can visualize a sinusoidal signal y = Y sin ωt, with amplitude Y and angular frequency ω, as generated by a radial line of length Y rotating with a constant angular velocity ω (refer to Figure 11.2). At any given moment, the vertical projection y of the line represents the value of the sinusoidal signal.