Electronic Design

Measure True RMS Lamp Voltage

When driving incandescent lamps from a display driver such as the Allegro Microsystems UCN5824A, the true rms voltage applied to the lamp filaments may require measurement. That can be a problem, though, because the multiplexed lamps receive unipolar pulses of voltage that are switched during the multiplexing operation per each driver's Output Enable pin. Some so-called true-rms DVMs measure rms values through a dc block. Therefore, obtaining meaningful results for these lamps can be a difficult chore.

However, by setting the display for all eights and obtaining the lamp drive waveform (see the figure), any average-responding dc voltmeter can be used to measure the true rms voltage. The measurement involves some calculations using the following relationship:

Power to resistance "R" during "ton" = EPKPK2/R. Average power over full cycle time "t" = EPK2/R × ton/t.

Thus, by definition, power is Erms2/R. Therefore, Erms2 = EPK2 × ton/t and Erms = EPK (t on/t)1/2. Average dc value = EAV = EPK × ton /t.

Therefore, Erms = EAV × t/ ton × (ton /t)1/2 = EAV (t/ ton) 1/2, and Erms = EAV ÷ (duty cycle)1/2.

The true rms voltage thus equals the average dc voltage divided by the square root of the duty cycle.

Accuracy will depend on how closely the lamp waveform comes to being truly square. Ultimately, though, it won't be far off and virtually any dc voltmeter can be used for the measurement.

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