CMOS timer
Measuring extremely high dc voltages of 10 kV or more usually requires a suitably equipped high-voltage ohmmeter or a digital multimeter and high-voltage divider, along with the proper safety precautions. As an alternative, a simple circuit based on a low-cost CMOS timer IC can also provide accurate high-voltage dc readings to 15 kV by means of direct measurements.
The timer, the National Semiconductor LMC555, is a CMOS version of the company’s popular 555 series general-purpose timers. In this high-voltage measurement application, the timer is used as a free-running oscillator circuit with a dependency on the high-voltage current from the source to be tested
The CMOS timer features frequency capability to 3 MHz with good temperature stability and can operate on supplies as low as 1.5 V. It delivers outputs that are fully compatible with transistor-transistor logic (TTL) and CMOS logic at 5-V levels. Designed for low reset, trigger, and threshold currents, the device is available in an eight-pin mini small-outine package (MSOP) as well as an eight-bump micro-SMD (surface-mount device) housing. For the high-voltage measuring circuit, an eight-pin MSOP device was used for simplicity of connections to other devices.
The high-voltage measurement circuit (see the figure) uses the LMC555 timer as a free-running oscillator circuit, with voltage/current from the source fed through a large 100-MΩ, 15-kV resistor (R1), a precision 1-kΩ (1% tolerance) metal-film resistor (R2), and a 1-nF charging capacitor (C1). Capacitor C1 is charged by energy from the voltage source to be tested and discharged by means of resistor R2, while also serving to control the frequency/timing of the LMC555 circuit.
The voltage (V) to be measured is determined by means of \\[V – VC(t)\\]/R1, where VC is the voltage of the charging capacitor, C1. For simplicity, any changes on capacitor voltage VC are ignored in making the high-voltage measurements. The free-running oscillation frequency of the LMC555 is limited to about 30 kHz, allowing the use of an additional, simple microprocessor-controlled circuit to count the output frequency of the LMC555 during high-voltage measurements.
Since the LMC555 timer achieves less than 1-mW typical power consumption (less than 5 mA current), it can be powered by a 3- to 12-V battery to ensure high isolation between the timer and the high-voltage source. Additional isolation for the LMC555’s timing circuitry is provided by selecting a component with 10-nF and 30-kV ratings for timing capacitor C1.
Perhaps the most critical companion component to the LMC555 in this circuit is the high-voltage resistor, R1. It should be designed for high-voltage use and from a reputable supplier known for reliability. As an example, model ROX200 is a metal-oxide resistor from Vishay with a voltage rating of 15 kV. It is available in a wide range of resistance values, from 1 kΩ to 1 GΩ, with tolerances of 1%, 2%, 5%, and 10% and a low temperature coefficient of resistance (TCR) (±200 ppm/°C standard). The high-voltage resistor can effectively dissipate heat in either open-air or oil-bath operation.
Almost as critical as R1 to the measurement circuit are resistor R2 and capacitor C1. R2 is a 1-kΩ metal-film resistor with ±1% precision. As an example, model CCF60 from Vishay is rated for a maximum rated voltage of 500 V and available with power ratings of 0.50, 0.75, and 1.00 W. The precision resistor has a low TCR of ±100 ppm/°C and includes a flame-retardant epoxy conformal coating for added protection at high voltage levels.
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At 1 nF, timing capacitor C1 must be stable with temperature, with NP0/C0G type dielectric material. C0G (NP0) capacitors are multilayer ceramic components designed to be stable over temperature, time, frequency, and voltage and suitable for use with low-loss tuning and timing circuits.
The Electronic Industries Association (EIA) established the C0G designation. The NP0 designation is an industry specification. Both designations refer to the slope of the device’s temperature-capacitance relationship. Capacitors made with C0G or NP0 low-loss dielectric materials can be used through gigahertz frequencies.
Additional components in the high-voltage measurement circuit include diode D1, Zener diode D2, and capacitor C2. The two diodes are provided for isolation and protection, and they should be rated for very low leakage characteristics to ensure high measurement accuracy.
Leakage through diode D1 is particularly critical to measurement accuracy, since leakage current of less than 3 pA can be a source of measurement error at high-voltage levels. Leakage current from Zener diode D2 is less critical to the measurement accuracy and can be as high as 100 pA with negligible effects on high-voltage measurement results.
Zener diode D2 is included as a safety feature for the timer IC, offering a switched path to ground for protection. It can be considered optional to the circuit, used only in situations that must adhere to the highest safety requirements.
Capacitor C2 provides additional isolation between the frequency counter circuitry and the LMX555 timer. This is not a critical component and can be any nonpolar-type ceramic capacitor with a suitable high-voltage rating (depending on the voltage to be measured). In fact, the high-voltage measurement circuit will work without this capacitor as well as without the isolation 3- to 12-V battery. In some cases, it may be possible to substitute an optical isolator for C2 to provide adequate protection and isolation for the connected circuitry.
The high-voltage measurement circuit contains many of the elements of a digital multimeter (DMM), which consist of a power supply, a voltage reference (usually a Zener diode), a digitizing circuit based on an analog-to-digital converter (ADC), and a display. Of course, without the use of an additional voltage divider, a DMM is limited to measurements of voltages of typically about 1 kV.
This simple measurement solution relies on the quality of resistor R1 and the precision of timer LMC555 to provide accurate measurements with a circuit that protects the timer by means of various isolation components. At high voltages, this added isolation also prevents corona leakage from degrading the accuracy of the measurements or endangering the safety of the test circuit or its operator.