Dimming Drivers and Ballasts

Previously, we had discussed LED drivers and fluorescent ballasts: a brief history, how they work, typical installs, and the differences between them. We spoke of the dimming capabilities of each in passing. Today, we are going to dive deeper than the Mariana’s Trench into the theory, the who, what, when, where, why, and how, and why it matters that you know. Without further ado, let’s dig into today’s topic: driver and ballast dimming.

Let’s quickly revisit some basics of both fluorescents and LEDs; we can use this as a refresher. To those who haven’t read the pilot edition of the “Drivers V Ballasts”, I would highly recommend reading that article first to get an in-depth understanding of both methods of lighting.

Fluorescent Ballasts

Fluorescent tubes have a gas, typically argon or neon, that enable them to emit a light from them when excited by a current. These tubes require incredibly high frequency to operate smoothly without flicker. Typically, electronic ballasts can produce in the area of 32kHz, whereas magnetic ballasts can only obtain around 120Hz. A T12 rapid start electronic ballast will typically have an input voltage of 120/277V AC and an output voltage of 100V> DC after the initial strike.

LED Drivers

LED Drivers convert the input 120/277V AC into either 4-10V DC or 12V DC for either constant current driver or constant voltage, respectively. Constant current drivers maintain their currents but fluctuate their voltages; Constant voltage drivers maintain their voltages but fluctuate the currents. Typically, constant voltage drivers are more widely preferred, as they can provide a larger total load sum. 

The Main Course

Now that we are caught back up and refreshed, let’s feast upon a better understanding and knowledge of how dimming controls work in both ballasts and drivers. There is no debate of which of these two have a smoother operation from full light, down to low level and off. LEDs are a newer technology with a major advantage over their fluorescent counterparts. For the most part, new construction or remodels will incorporate LED fixtures over fluorescent tubes because they are cost effective and provide savings in energy cost to the end user. Although fluorescents are cheaper up front on cost, the maintenance and energy spending is by far outweighed by its competitor, LEDs. Whereas LEDs are more expensive upfront, their energy savings and low maintenance makes for a better product; however, that’s not what we are here for. Let’s take a look at how these dimming controls function and their compatibilities with different devices.

Dimming ballasts

Compact Fluorescent (CFL) Dimming Ballast – Source: ballastshop.com

Separate dimming ballasts are required in most fluorescent luminaires for them to function properly. Most of these ballasts will be of the electronic ballast configuration compared to the magnetic ballast. Standard ballasts are not capable of dimming the fixture because they can’t produce the electrode heat needed to discharge the gases in the tubes. Besides tube lamps, there are compact fluorescent lamps, or CFLs, that come in two types: integrated and non-integrated.

Integrated bulbs

  • Have built in ballast in the base of the lamp
  • Can be used as a replacement for Edison screw base or Bayonet lamps
  • Do not provide a smooth transition from dimming levels

Non-Integrated bulbs

  • Require a separate dimming ballast, like the fluorescent tubes
  • Require at least 100 hours of full power illumination before dimming; failure to do so will result in premature failure of the lamp
  • Offer a wide range of dimming capabilities
Compact Fluorescent Lamp with integrated ballast – Source: www.hdsupplysolutions.com

Remember when dimming fluorescents, the illumination is a result of a discharge of gases which require an initial strike to light. Because of this, there will always be a “jump” when the light is turned on a low level from the off position. Some ballasts require a partnered dimming switch in place of a standard dimmable switch, while some ballasts on the market can be controlled by a low voltage switch. These models are typically compatible with occupation/vacancy sensors. 

Dimming Drivers

Dimmable LED Driver – Source: merrytek.com

Unlike the dependability of discharging gases in fluorescents, LED drivers are a solid state device, meaning they only rely on current to power the luminaires. With the use of Pulse Width Modulators, or PWM, we can control the frequency of which current is carried through the LEDs. As with it’s fluorescent counterpart, LEDs have two types: integrated and non-integrated. The integrated type lamp has a built in driver and can also replace Edison screw base bulbs. They are available in dimmable models, but not all LED bulbs are dimmable. Some dimmable LEDs also require a specific dimming control switch to be partnered with. Non-integrated LEDs have a separate driver from the luminaire, giving access to dimming control wiring. 

The most common dimmers on the market today are phase-cutting dimmers. There are two types of phase-cutting dimmers: leading edge and trailing edge.

Leading Edge Phase-Cutter

  • Cuts the current off at the zero point in the sine wave
  • Re-energizes back on later in the same mains cycle
  • The longer the “off”, the dimmer the LED appears

Trailing Edge Phase-Cutter

  • Energizes the current on at the zero point in the sine wave
  • Cuts the current off at the trail end in the same mains cycle
  • The longer the “off”, the dimmer the LED appears
Leading-edge Phase Cutting – Source: electricalcounter.co.uk
Trailing-edge Phase Cutting – Source: electricalcounter.co.uk

An important note to remember here, is this phase-cutting happens on the input or AC voltage side of the driver. DC does not operate in a sine pattern but a square pattern: starting at zero, up to one, and back to zero in a pulse; whereas a sine wave starts at zero, rises to maximum potential, back to zero, then rises to maximum potential in the opposite direction. . 


Dimming is not as easy as one may have anticipated. As we learned today, there is a plethora of complications going on in both ballasts and LED drivers. Drivers can have both AC and DC dimming applications. A PWM can vary the frequency at which the DC output is carried, and a phase-cutter is designed to ‘cut” either the leading edge or trailing edge of a sine wave.

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