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.
