ABSTRACT

LINEAR POWER CONTROL OF AN AC LINE-POWERED LOAD

By Terry L. Fleischman

The purpose of this research was to develop a means to control the power delivered to a load in a linear manner.

Load power was determined by sampling load voltage and current. Load power was then controlled by a computer, by varying the conduction time of a triac in each alternation of the AC line, using a zero cross detector to provide synchronization of the triac trigger pulse. Load power is a function of the conduction time of the AC switching device.

Three different methods of sensing current were tried: a Hall-effect sensor, a hand-wound transformer, and a simple current-sensing resistor in series with the load. With each sensor, there were serious differences between the predictions of power delivered to the load from a computer program and results measured with a digital and with an analog meter. These discrepancies led to an investigation of how the meters measure AC current.

It was found that RMS meters do fine with sinusoidal currents, but become more inaccurate as the shape of the waveform deviates from a sinusoid. This happens as the triac fires later in the cycle, particularly at lower powers when the triac is past the peak of the half-cycle. It was found that the best way to understand this problem was in terms of how well a meter could still provide an accurate value of the RMS voltage or current over a range of crest factor values. Crest factor is the ratio of peak level to RMS level.

The final circuit chosen to monitor load voltage and load current used the AD7750 product-to-frequency converter chip. It satisfactorily handles a range of crest factor values from 1 to 10. The output from the AD7750 is a frequency representative of true load power. This was fed back to an ECG995 frequency-to-voltage converter. This voltage, now representative of load power, is fed to an analog to digital converter in a personal computer. The computer then controls the conduction time of an AC switching device (triac).

The user enters the desired load power percentage via the computer keyboard. Load power dissipation is computer controlled utilizing the associated electronic circuitry.

The final control program was set up to re-sample the full power available to recalibrate the system. This allows for rescaling that may be needed due to changes in the resistance of the load due to heating or aging.