Frequency-to-Current Converters Information
Frequency-to-current converters are electronic circuits that produce an output current proportional or related to the frequency of an alternating current (AC) input of a specific frequency. The input signal can be of any type, as long as it is a periodic wave: for example, a sine wave, square wave, or triangular wave.
Block diagram of an F-to-C converter.
There are many different F-to-C converter designs. Integrated circuits (IC) can create converters with the specific characteristics desired when combined with external components. Some of these ICs include the pervasive 555 timer, the TC9400, and the LM331. The latter two chips can be wired as voltage-to-frequency converters or as frequency-to-voltage converters. By adding components it is possible to create a circuit that is stable, accurate, and designed for a specific application.
As an example, the following circuit is a frequency-to-voltage and/or current converter. The basic internal structure of the LM331 consists of a clock generator, a voltage to frequency modulator, a voltage reference circuit, and other devices. By adding components as is shown in the following figure, an acceptable frequency-to-voltage and frequency-to-current converter can be achieved.
In this design the LM331 is wired as a single-source frequency-to-voltage converter. The input is a periodic signal that can be a square or sinusoidal wave, and the output voltage is proportional to the input frequency. The output voltage is across the load resistance (RL); the output can also be the current through this load, making this circuit a frequency-to-current converter.
A practical F-to-C converter.
Output voltage can be found using the following equation:
The output current equation is:
Where K is a constant and its value is dependent on the values of the external components.
The following specifications are important to consider when selecting a frequency-to-current converters:
- Form factor:
- PCB. Devices are printed circuit boards (PCB) that attach to enclosures or plug directly into computer backplanes.
- Panel/chassis mounted. Devices attach to a panel or bolt onto a chassis.
- Stand alone. Devices are benchtop or floor-standing units with a full casing or cabinet, and an integral interface.
- DIN rail. Devices are mounted on a standard DIN rail. DIN is an acronym for Deutsches Institut für Normung (DIN), a German national organization for standardization.
- Rack mount. Devices are rack-mounted and fit inside enclosures such as a standard 19” telecommunications rack.
- Maximum input frequency, the highest frequency the converter will accept.
- Resolution refers to the degree of fineness of the digital word representing the analog value. A 10-bit number contains 210, or 1024, increments. A 0-10 V signal could therefore be resolved into approximately 0.01 V increments. A 12-bit representation would be in 212 (4096) increments, or divisions of 0.0024 V for the same signal. Each additional bit doubles the resolution, and one bit is required for the polarity (sign) of a number.
- Operating temperature, the full required range of ambient operating temperature.
Some applications for frequency-to-current converters are:
- Switching applications, including converters, power supplies, and motor control devices.
- Voltage conversion: 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.
- Converting power from one standard or level to another.
- Controlling AC motor speed and torque.
- General motor control, typically using a three-phase voltage inverter in which the phases are controlled using semiconductor switches and pulse width modulation devices.
- Monitoring 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. Heavy-duty frequency-to-current converters used for these applications typically have an input frequency range from 0.5 to 700 Hz and an output frequency range from 4 to 20 mA DC.
- Producing the current for standard 4-20 mA loop circuit monitors. The 4-20 mA loop is widely used in process automation applications because it is reliable, inexpensive, and resistant to electrical noise.