Electronic Design

# Circuit Combines DC High-Voltage Drop Detector, Undervoltage Alarm

Environmental systems that precipitate dust and carbon particles frequently use high-voltage dc power supplies. However, failure of high-voltage components caused by a change in their properties over time can create a large drop in voltage that prevents the system from working properly, causing severe air pollution.

The circuit in the figure lets system operators know in advance whether the high-voltage supply is functioning well enough to keep the precipitator mechanism working. It draws very little current from the high-voltage dc supply through R1, a high-voltage device with a value of 100 MΩ or more. R1 and R2 are chosen to provide a safe operating voltage of 12 V across R2:

HV · R2/(R1 + R2) = 12 V

where HV is the dc high-voltage potential, R1 is the current-limiting resistor, and R2 is the potential-forming resistor.

If the supply’s voltage suffers a voltage drop, the voltage across R2 will drop also, reducing the voltage to the base of transistor Q1. That turns on Q1, which also starts IC1, an LMC555/TLC555 oscillator. Typically, the designer can set the maximum frequency using R3, R4, and C1 to about 10 kHz. IC1’s frequency will vary from 0 Hz for a safe operating high voltage to the 10 kHz maximum for a failed high-voltage supply, with lesser frequencies for lower voltage drops.

The 16-V zener diode protects Q1’s base from excessive voltage. Designers can use a 12-V zener to match the operating voltages exactly, but the 16-V diode adds a 4-V margin of flexibility in the operating voltage. It is easy to optically isolate the frequency output.

The circuit can run off a standard 12-V battery, but the IC’s operating voltage should not exceed 16 V or the oscillator will give a false alarm for a high-voltage failure. The circuit’s minimum operating voltage is 3 V dc. The current-control resistor for the alarm LED depends on the circuit’s operating voltage and the LED’s maximum current. Normally, 470 Ω is used.

The low-cost, industrial-standard LMC555 IC was an excellent choice for this circuit. It’s easy to transmit a pulsed signal on a phone line. The device’s ac output also permits isolation with an audio transformer. Additionally, the designer can connect the LMC555 to a speaker for an audible alarm.