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Dimmer heat load

I am curious at to the source of heat from the Dimmer Paks.  Typical efficiency is rated at about 96.9%, and a 20 amp dimmer pack is reported to generate about 77 watts - and a dual-channel dimmer pack would generate about 154 watts.  [This means 40 amps creates about 160 watts heat - or 4 watts of heat per amp of light controlled.]

Many dimmer packs use CRYDOM SCRs - and the Crydom Tech sheet on a typical SCR has the following information:  

     " Due to the forward voltage drop of the output SCRs, solid state relays generate an internal power loss. The amount of power generated is a function of the load current. The manufacturer provides power loss curves, as shown in Fig 1. At normal load currents the power loss can be estimated at 1 Watt for every 1 Arms of load current. "

So a Crydom SCR yields about 1 watt per AMP - but the dimmer pack is generating almost 4 watts per Amp - almost 160 watts for a 40 amp dimmer module (two 20 amp circuits.)

QUESTION - what is generating the other 3 watts per AMP of dimmer circuit??  [Now - maybe the Crydom is per SCR - and a single dimmer channel has 2 SCRs - and thus we are looking at a 2 to 4 watt disparity vs. a 1 to 4 watt disparity.]

Any thoughts??

  • A dimmer is not just the SCRs!

    The chokes also dissipate heat due to resistive and iron losses - good chokes are necessary for lamp life and to meet electrical standards, and also reduce the noise certain types of lamp make (esp. parcans).

    It also sounds like the comment you've quoted is for an SCR that is already fully conducting (turned-on).
    There are additional losses involved in the turning on of an SCR, and keeping it turned on when used to dim small loads.
    The SCR junction temperature also has an effect, you'll probably be able to find those derating tables in the datasheet you've been looking at.

    Finally, the efficiency we state is the absolute 'worst case' (max. load, hottest etc), and is therefore notably higher than the quoted estimate.
    Most installations actually acheive much higher efficiencies.

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