Why do “Reverse Phase” Dimmers work better than traditional “Forward Phase” Dimmers for LED Lighting?

Today, I thought we would talk about the difference between “traditional” Forward Phase Dimming and newer Reverse Phase Dimming technology.  Why is this important?  The answer is because of LED Lamps and Drivers (Driver is the electronics that runs the LED Lamps), which are now trying to be used and dimmed by everyone around the world.  In previous posts I have talked about the lack of resistance in LED Drivers, which can cause problems in the dimming of LED lamps.

How can a Reverse Phase Dimmer help to correct the issue of a lack of resistance in the LED lamps driver and electronics?  The simple answer is the cycle of power and how it is applied with a Reverse Phase or ELV Dimmer.  The diagram below shows both a Forward Phase (Triac based)(Diagram A-on left) and a Reverse Phase (ELV based) (Diagram B-on right) Dimmer Curve.Forward Phase and Reverse Phase Dimming ImageNow that you have seen the images of the Forward Phase and Reverse Phase Dimmer Curve.  What are we looking at in the image?  If you think left to right (just like in reading), when you look at the images.  The power is coming from the left edge and then is flowing out of the right edge.

In Image A (Forward Phase Dimming), you see that the power comes in from the left, but is suppressed or prevented from operating until it has gone through approximately 1/3 of the cycle (this is shown with the dots on the curve).  Then the power comes in quickly from the crossover line (center horizontal line) and goes to the correct level.

Whereas on Image B (Reverse Phase Dimming), the power comes in normal through approximately 2/3 of the cycle’s wave and then is cut out or suppressed for the last 1/3 of the cycle’s wave.

Both Forward Phase Dimming and Reverse Phase Dimming provide the exact same amount of energy to the light fixture, which they are powering.  The BIG difference is that with Reverse Phase Dimming, the electronics in a LED Driver and/or Fluorescent Ballast are powered FIRST and then the dimming occurs second.  Physically, we can not see the difference in the two types of Dimming, but the LED Driver and/or the Fluorescent Ballast sure notices the difference and will respond VERY differently to the two types of Dimming Styles.

Almost all dimmers made in the world prior to the year 2010 were Forward Phase (Triac Based) Dimmers.  That is why a lot of time, the LED Drivers and/or Fluorescent Ballast do not work well when connected to existing dimmers.  Since there are so many Forward Phase Dimmers around the world, the leading LED manufacturers are working with the Dimmer Manufacturers to try to make their LED Lamps/Drivers work with Forward Phase Dimmers.

However, since LED Lamps/Drivers is a growth industry and it seems like everyone is getting into the industry, there is a lot of room for error in the design of the LED lamps and/or drivers to work with Forward Phase Dimmers.  When you are choosing an LED Lamp, LED Driver, Fluorescent Lamp, and/or Fluorescent Ballast make sure that is Forward Phase Dimming Capable.  This is really important if you have existing dimmers.

One note is that the Fluorescent lamp and ballast manufacturers have already worked out a lot of these issues and the vast majority of Fluorescent lamps and ballast that say “dimming capable” are Forward Phase Dimming Capable.

If you are doing new projects and can select both the LED Lamps and Drivers for your project, then you should take some time and plan accordingly.  Yes, you can purchase Reverse Phase Dimmers from several lighting manufacturers, which will work very well with dimming capable LED Lamps and Drivers.  Again, you must make sure that the LED Lamps and/or Drivers are dimmable, even with a Reverse Phase Dimmer.  Several LED Lamps and Drivers are NOT dimmable and will not work, whether you use a Forward or Reverse Phase Dimmer.

Hopefully, this very basic explanation of Forward (Triac) and Reverse (ELV) Phase Dimming Blog will help you with your project.  Please keep in mind that Knight Sound & Lighting Inc, (www.knightsoundandlighting.com) and http://www.GoKnight.com can help you with your technical issues and provide you with dimming and control options for your facility.

Thank you for reading this blog and please feel free to ask any questions and/or make corrections to my blog post.

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