Measure True RMS Lamp Voltage

Oct. 14, 1993
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

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.

Sponsored Recommendations

Highly Integrated 20A Digital Power Module for High Current Applications

March 20, 2024
Renesas latest power module delivers the highest efficiency (up to 94% peak) and fast time-to-market solution in an extremely small footprint. The RRM12120 is ideal for space...

Empowering Innovation: Your Power Partner for Tomorrow's Challenges

March 20, 2024
Discover how innovation, quality, and reliability are embedded into every aspect of Renesas' power products.

Article: Meeting the challenges of power conversion in e-bikes

March 18, 2024
Managing electrical noise in a compact and lightweight vehicle is a perpetual obstacle

Power modules provide high-efficiency conversion between 400V and 800V systems for electric vehicles

March 18, 2024
Porsche, Hyundai and GMC all are converting 400 – 800V today in very different ways. Learn more about how power modules stack up to these discrete designs.

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!