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Electronic Theatre Controls Inc

Efficiency, Noise, and AIC Considerations for ETC 800 Microsecond SCR Dimmers vs. IGBT Dimmers

When comparing 800 microsecond SCR dimmers to alternative-technology IGBT or MOSFET dimmers, the subject of efficiency often comes up.  In order to perform a real-world analysis of SCR dimmer efficiency, a number of factors must be considered.

SCR Dimmer efficiency is superior with lower loads

Unlike IGBT dimmers, which have a relatively constant efficiency, SCR dimmers exhibit different efficiencies depending on their connected load. This is due to the non-linear I2R losses of the dimmer's choke. This "variable efficiency" becomes even more important in a typical dimmer-per-circuit performance lighting system, where connected loads rarely, if ever, approach the nameplate rating of the dimmer. Most often, a 2.4 kW dimmer will have somewhere between 575W and 1725W connected to it, because 575W is the rating of the HPL lamp most often used in a performance space.    

ETC publishes efficiency numbers for full load.  In more detail, the efficiency of an ETC D20DHR 2.4kW 800 microsecond dimmer versus an IGBT dimmer at various loads looks like this:

ETC D20DHR Dimmer     IPS Capio IGBT Dimmer  
Load  Loss Efficiency Loss Efficiency
2400W 100W  95.8% 72W 97%
1725W 55.8W 96.8% 52W 97%
1150W 28W 97.6% 35W 97%
575W 9.4W 98.4% 17W 97%

Since the efficiency of an IGBT dimmer is constant, no matter what the connected load, the SCR dimmer becomes more efficient than the IGBT dimmer at loads below approximately 50%. Given the preferred operation mode of most theatres and concert halls (lower loads on each dimmer, more control of individual fixtures), it is likely that SCR dimmers will provide a more efficient system.

Total system efficiency increases for SCR Dimmers at lower loads

For purposes of comparison on a large system, the two technologies would look like this:

Assume 416 dimmers of 2.4kW capacity.

Total Dimmer loss      Heat Load (BTU)  
Load per Dimmer SCR IGBT SCR IGBT
2400W 41.9kW 30kW 142,968 102,372
1725W 23kW 21.5kW 78,479 73,361
1150W 11.5kW 14.4kW 39,240 49,135
575W 3.8kW 7.2kW 12,966 24,567

Thus, it can be seen that when SCR dimmers are operated at loads less than about 50%, which is the norm in performance lighting systems, the heat load of the SCR dimmer is less than an IGBT design, and the overall system efficiency is higher using SCR dimmers.

IGBT Dimmers are least efficient at settings below 100%.

Efficiency specs for ETC SCR dimmers have always been published for worst-case full load, 100% control setting. Many IGBT manufacturers take advantage of this convention and also report their efficiency at 100% control setting.  However, IGBT switching losses are much greater at settings below 100%, and thus dimmer efficiency is substantially lower at these reduced settings. When comparing IGBT to SCR dimmers, it's important to determine whether the published efficiency specs apply to all control settings from 0 to 100%.

High risetime SCR technology is consistently quiet, without any noise performance compromises for  temperature rise. 

ETC D20DHR dimmers provide constant risetime (and therefore quiet lamp filaments) no matter what the ambient temperature or total dimmer rack load. These dimmers are rated up to a dimmer room ambient temperature of 40o C.

On the other hand, IPS Capio dimmers decrease their rise time from 800 to 200 microseconds as the rack temperature rises, thus increasing filament noise. The Capio dimmers have a lower 35 o C ambient temperature requirement to begin with. As a Capio rack is more heavily loaded, dimmers go into reduced risetime mode to compensate for higher temperatures.

Are there any other benefits to SCR dimmers with chokes?

  • Increased AIC rating. ETC Sensor dimmer modules have a native short-circuit rating of 100,000 AIC, due to the current limiting combination of choke and circuit breaker. The Sensor dimmer rack also has a native short circuit rating of 100,000 AIC, without the use of current limiting fuses. Many IGBT designs without chokes have only a 10,000 AIC short circuit rating.
  • Lower parts count and increased MTBF. The robust design of Sensor SCR dimmers produces an extremely low Mean Time Between Failure rating.  Compared to the relatively high parts count and high complexity of IGBT designs, SCR dimmers have a much better track record of reliable operation.
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