Have you ever wondered why fluorescents use AC voltage and LEDs use DC voltage? Did you know that was a thing? Have you ever wondered why old fluorescent tubes would flutter and more modern lights of the same build don’t? Today, we will be exploring the differences between drivers and ballasts and their inner workings. Each has unique characteristics but fundamentally operates in a similar manner. Sink deeper into your chair, find a more secluded corner on the job site, or lock the bathroom door, because we are diving head first into the world of lighting!
Let’s take an in-depth look at how a driver works and its basic functions. There are two types of drivers: a constant voltage driver and a constant current driver. These are exactly like what they sound like they are; a constant voltage driver maintains its voltage but fluctuates its amperage, and a constant current driver maintains its amperage but fluctuates its voltage. There are pros and cons to both styles, but the general consensus is that constant voltage drivers can provide a larger total load of the two, but the fluctuation of current results in greater heat that will cause the LEDs to burn out faster. On the flip side, constant current drivers have a specific rating they are manufactured at, making them last much longer and be more efficient than their counterpart. Although outputs vary from each other, the functionality of both of these types of drivers is the same. On the input side, they will take 120-277V AC, and the output will take either 4-10V DC or 12V DC for either constant current or constant voltage, respectively.
Now LED lights use DC, so if we have an input AC voltage, how do we end up with a DC voltage on the output? Glad you asked! We know that current has a pulse in the form of a sine wave and one full cycle starts at zero, rises to a peak in one direction, comes back to zero, then rises to another peak in the opposite direction, and back to zero again. This cycle is also known as Hertz (Hz), named after the man who discovered it, Heinrich Hertz. In the United States, our systems operate at 60Hz, whereas other places in the world, such as the UK and Australia, operate at 50Hz. If we want to convert an AC voltage into a DC voltage, we need to understand this basic principle. In AC, the sine wave travels in both a positive and negative direction, whereas in DC voltages, there is no sine wave that changes polarity 60 times per second. Instead DC provides a constant surge or flow of current in one direction. A cool side note – we can simulate an AC waveform with DC by changing polarity (direction of current flow) 60 times per second, resulting in a similar thing to a sine wave – but it’s actually a square wave. A square wave is a different kind of thing – yet in some ways similar to a sine wave. In DC a square wave starts at zero, rises to a maximum voltage instantaneously, returns to zero, then returns to a maximum voltage of reverse polarity instantaneously, then immediately back to zero. There is no curvature. If we think of the theory behind these voltage forms, “direct” current is a direct pulse; it does not fluctuate between positive and negative. “Alternating” current alternates, or shifts, between a positive and negative. To accomplish the transformation, a lot of drivers employ a voltage transformer and a rectifier. The transformer bucks the voltage from a high voltage of 120V AC, for example, to 12V AC. The 12V AC is then passed across a device called a rectifier, which levels the AC sine wave and only pulsates the positive wave, and converts the incoming 12V AC into 12V DC.
Now we will touch on ballasts, their purpose and how they function. Historically speaking, there are 2 different types of ballasts: magnetic and electronic. Magnetic ballasts, also called “chokes,” that run T12 fluorescent tubes are discontinued in the United States. This is because of the energy consumption due to the design of these models. These magnetic ballasts work on the concept of electromagnetism. The simple design of a copper coil allows for incremental flow of electricity through the light tubes. In addition to fluorescent tubes and two-pin CFLs (compact fluorescent lamps), magnetic ballasts also power HID lamps such as Metal-Halide and High-Pressure-Sodium, as well as HPF (high power factor) lamps. Many of these types of ballasts are being phased out in favor of more energy efficient types. When a ballast is energized, there is an initial arc that occurs to “start up” the light. The voltage is then lowered and regulated to maintain a steady burn of the lamp. You could think of a ballast as a start/run capacitor. When called for to turn on, there is a sudden inrush of current to sort of jump-start the device, but after that initial peak power, the current simmers down to allow for a smooth operation. This is why many times the light, when initially turned on, is much brighter than seconds later when it hits the “run capacitor” stage.
As we discussed earlier, the sine wave for AC voltage is 60Hz, which can be measured with devices but rather hard to see with the naked eye. This is not the case with tube lamps. Older models perform at a more primitive level as to where you can see the sine wave produced as flickering of the lights. As our technology became smarter, we could “cut out” the high points of the sine wave and produce a more even flow of electricity across the tubes, resulting in a more steady source of light, as opposed to the stuttering nature of earlier devices.
There are typically two methods used for starting lighting the lamps: pre-heat method and rapid start method. In the pre-heat method, small cylinders which contain gas (usually neon or argon) sit behind the lamps and begin to heat up as current is passed through it. As this cylinder, or the starter, is heated, it allows the current to flow into the filament containing mercury gas, thus, lighting the lamp. The rapid start method operates a little differently. The ballasts maintain an electrical charge through the filament, enough to iodise the mercury gas, almost acting as a capacitor. When called to turn on, it pushes the full amount of current through the gas, causing it to light to lamp.
This brings us to the second, much newer type, electronic ballasts. These ballasts act in a very similar way drivers do by converting AC voltage into DC voltage. The DC voltage is then fed into a high frequency stage between 20kHz and 80kHz. Once the switch turns on and calls for lighting, the ballast will discharge around 1000 V which lights the tube instantaneously. After this inrush, the ballasts lower the voltage to between 230V and 125V, and then even lower to a limited current.
What about Dimming?
How about having the ability to dim these fixtures? With advanced technology, we can absolutely dim these fluorescent tubes, CFLs, and LEDs. CFLs and LEDs have the easier transition into the ability to dim. The only requirement for these kinds of lights are that both the dimmer switch and bulbs are compatible and the fixtures of bulbs have the ability to dim. Many times if a bulb is not of the dimming technology, it will strain on the system and cause the ballast to die prematurely, or the light may cut out at low levels of dimming. This is not the case in fluorescent lighting. T8 tubes require a special dimming ballast as well as the dimmer switch to be installed in order for them to operate. A dimming ballast differs from a normal ballast by means of exciting the gas in the tubes at a much lower frequency, while still maintaining electrode heat. This is kind of similar to the method LED drivers use to dim lights. They exhibit a technique known as pulse-width modulation, or PWM for short, in which the output current is switched on and off at a constant frequency with a variable duty cycle. They can also be dimmed by reducing the DC current, but using this method typically results in color change in the LEDs and makes dimming at low levels increasingly difficult.
Today we covered a history of ballasts and their more primitive technology in comparison to more modern ballasts and how they have saved on energy. We discussed LED drivers and their simplistic design. We learned a little history of the Hertz and who it was named after, as well as sine waves for both AC and DC currents. We learned about dimming for both ballasts and drivers, the similarities between the two and specific devices required for certain lamps.