Frequency-to-Current Converters

Frequency-to-current converters accept a frequency (an AC signal) and produce an output current whose value is a function of the input frequency. Frequency-to-current converters were originally made of electromechanical components such as motors and generators, but are now composed mostly of solid-state components.

Frequency-to-current converters consist of a rectifier or diode that converts incoming alternating current (AC) to direct current (DC). The direct current is then inverted by using devices such as thyristors, which are semiconductor switches that function similarly to a transistor, or an insulated gate bipolar transistor (IGBT). These transistors are frequently used for switching applications, including converters, power supplies, and motor control devices. Frequency to current converters may also convert voltage. To convert the voltage of an incoming signal, the frequency-to-current converter includes a transformer to provide the galvanic isolation necessary between the input signal and the output signal.

Frequency to current converters are often used to convert power from one standard or level to another. Frequency to current converters can also be used to control the speed and torque of AC current-driven motors. A frequency to current converter used to control a motor typically uses a three-phase voltage inverter where the phases are controlled using semiconductor switches and pulse width modulation devices. A frequency current converter may also be used to monitor the speed of heavy equipment such as turbines or flow meters, or machinery used in hazardous environments, such as the underground pulley systems and conveyors used in transportation and mining. These heavy-duty frequency-to-current converters typically have an input frequency range from 0.5 to 700 Hz and an output frequency range from 4 to 20 mA DC. A 4-20 mA loop circuit monitors the analog signal and transmits the information back to the converter’s controller. The 4-20 mA loop is widely used in process automation applications because it is reliable, inexpensive, and resistant to electrical noise.