Electronic vs. Magnetic Dimming

A few weeks ago, we talked a little bit about Dimming and what can make your dimmers hum or flicker.  One of the underlying themes to that post was that dimming really isn’t that simple. 

Traditional dimming (Sine Wave Dimming) is a very simple concept.  In order to control the light output from a lamp, vary the voltage.  Ohm’s Law made the solution even simpler.  With Voltage being the multiple of Ampacity and Resistance, simply vary the resistance at the light to control its output.  Increase the resistance, decrease the output.  And, the converse is true as well, decrease the resistance, increase the output.  When looking at an electrical sine wave, you are either increasing or decreasing the amplitude of the wave to affect the output. 

Enter Solid State Dimming.  Using a different approach, solid state dimmers cut off a portion of each cycle of the alternating current.  The thermal inertia of the filament averages out the brightness at a lower lighting level, with no perceptible flicker.  This leads us to our main point, the two main types of solid state dimmers, Electronic and Magnetic. 

With Low Voltage lighting systems, the lamps are driven by a magnetic transformer or by an electronic transformer.  120V power is provided into the lamp, and the voltage is stepped down to drive the lamp.  With two different potential types of transformers in the lamp, it is very important to confirm the transformer type prior to selecting a dimmer, because of the different characteristics of the different dimming types. 

So let’s talk Magnetic first.  These transformers will step the 120V down to 12VAC or 24VAC, and magnetic transformers use an inductive core (steel wound with copper). 

Electrical transformers are similar, in that they will step the 120V down to 12VAC or 24VAC.  However, the electrical transformers are comprised of electrical circuitry that is capacitive by nature. 

Alright, so we have two different types of dimmers that step the voltage down to the same low voltages, but do it with different cores.  Because they have different cores, they dim the lights through a different process. 

If we go back to some of what was discussed in the previous dimming discussion, you’ll recall that electricity travels in a wave form.  When lights are at full output, the sine wave is continuously providing power to the light bulb.  When a dimmer is introduced to the circuit, the dimmer “chops” up the sine wave.  The dimmer will hold the electricity output to the circuit at 0 for a portion of the wave.  The longer the dimmer holds the wave to 0, the lower the amount of energy transferred to the circuit, the lower the light output of the lamp.  See the image below to demonstrate how the sine wave is affected by a dimmer. 

  So back to Magnetic and Electronic dimming, how do they dim the circuit differently?  Magnetic dimmers use forward phase dimming, and electronic dimmers use reverse phase dimming.  What is the difference between the two?  Forward phase dimming cuts off the front side of the wave, while reverse phase cuts off the back end of the wave. 

Forward Phase Dimming

Reverse Phase Dimming

So where does this leave us, and more particularly, where does it leave the specifying engineer or lighting designer?  We are left with two different ways to dim load types, and we are stuck with another form of coordination.  A reverse phase dimmer will not dim a forward phase lamp, and vice versa.  When dimming loads, coordinate dimmer type with lighting type to ensure a functioning lighting control system.


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