Frequency indicator for generating equipment
This circuit was developed to adjust the operating speed (RPM)
of portable AC generators. These usually have a speed-control knob but no means
of determining the correct (50Hz) frequency. Since this sort of job is done
outdoors, a digital frequency counter is not really justified and can be
difficult to read, because of the typically long gating time at low
Two NAND gates, IC1c & IC1b, are wired as a 500Hz
oscillator and this clocks a 4017 decade counter, IC2. This drives 10 LEDs but
only one will be on at any particular time, as the counter cycles through from
1-10. However, all the LEDs are connected to 0V via a common 1kW resistor and transistor
Q1 which is turned on and off by a 50Hz signal derived from the generator.
If the 50Hz and 500Hz frequencies are precisely locked, the
display will show a stationary LED. However, if the 50Hz signal is slightly high
in frequency, the LED will appear to move in one direction and if the signal is
slightly low, the LED will appear to move in the opposite direction.
The 50Hz 240VAC input from the generator is fed via transformer
T1 and it powers the circuit via rectifier diode D1, a 100mF filter capacitor and a
7809 9V 3-terminal regulator (REG1). The 50Hz signal from the transformer is
also fed via a 10kW resistor to a squaring circuit comprising gates IC1d & IC1c. This
drives transistor Q1.
In use, the unit is plugged into the mains outlet of the
portable generator and the governor speed of the engine is adjusted to give a
stationary LED display. For 60Hz equipment, the oscillator would be set to
Note that the oscillator frequency should be adjusted precisely
with the aid of an accurate frequency meter.
Taylorville, Westland, NZ. ($60)