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Electronic Theatre Controls Inc

Dimmer Curves on an LMI SD Pack

There are 2 trim pots on the card. On the front of the pack there is a "Tune" Knob which controls the high end of the curve and inside the unit on the dimmer card is another trim pot that controls the middle trim. This was done with 2 post for a number of reasons one of which is these packs are designed to accept either 5vdc-40vdc analog or pulse width modulated wave form as a control input.

This process can take a bit of tweaking and requires both a meter and a visual of the dimmer. The pots are single turn so they are a bit sensitive. Smaller movements are better than larger movements.

Note-Icon.png Note: These 2 pots work with each other to even out your dimmer curve.


  • A RMS voltage Meter
  • A Small Screwdriver
  • A Light to attach to the pack (Might we suggest a Source Four?)
  • Some time
  1. Meter incoming voltage as this will be your max output
  2. Turn dimmer to full and using the Tune pot turn the pot until the light does not get any brighter and then turn back a small distance.
  3. Meter voltage at this point as this is your full output voltage
  4. Turn light to 50% and meter voltage. Adjust trim pot inside so displayed voltage is approx 50% of full level.
  5. Return to step 2 and repeat until you are happy with your dimmer curve.

Once you do a few of them you will get the rhythm for the adjustment.  The high end trim is really more of a visual and the middle is more of a metered adjustment. In a perfect world high would be 120vac and middle would be 77vac as metered with an RMS meter, but trying to get both the voltages perfect on the meter and the curve looking perfect to the eye can be nearly impossible. Let the visual be your final answer.

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