Due to significant growth in the LED lighting market, how to choose a right dimmer for LED lighting fixtures is critical. Compatibility between LED fixtures, drivers, and controls can be confusing, and if they are specified improperly, performance will suffer. Before choosing the dimming control, these are some factors that need to be considered including lighting applications, required dimming performance, and control preferences. The first thing we need to consider is the lighting applications. Is the LED fixture for new construction or retrofit? You can have a variety of control options for new construction, but only have limited control options for retrofit. For retrofit application, the dimming option for LED lighting fixture is adapted to the existing dimming system.
Next is to know the desired dimming range. The dimming range of a lighting fixture is based on the performance of the driver. Choosing the right dimming control can reduce flicker and affect the ability to achieve the desired dimming range. However, the dimmability, low-end light level, and performance of the [product are determined by the driver.
Once we determine the dimming range, then the most important step is to choose the type of dimming control. There are many types of controls and control systems, depending on the dimmability of drivers. It is either for primary side (high voltage), secondary side (low voltage) of the driver, or an additional device for embedded wireless connectivity. Most common types of dimming methods for drivers are:
Forward phase(leading-edge) dimming:
Reversed phase (Tailing-edge) dimming
The Power Factor(PF) is the measurement of the relationship between the AC source voltage and current. Power factor can range from 0 to 1.0, with 1.0 being ideal. In a purely resistive AC circuit, voltage and current waveforms are in phase, changing polarity at the same instant in each cycle, and all the power entering the load is consumed. However, when reactive loads are present in the circuit such as capacitors or inductors, energy storage in the loads results in a phase difference between the current and voltage waveforms. During each cycle of the AC voltage, extra energy, in addition to any energy consumed in the load, is temporarily stored in the load in electric or magnetic fields, and then returned to the power grid a fraction of the period later. The non-productive power increases the current in the line, and cause lower power factor. Thus, a circuit with a lower power factor will use higher current to transfer a given quantity of real power than a circuit with a higher power factor.
Besides Power Factor, we need to know what the Apparent Power is. When inductors and capacitors in the circuit, they dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually consume power, which is called “reactive power.” The actual amount of power being used, or dissipated, in a circuit is called “True Power.” The combination of reactive power and true power is called apparent power, which is measured in the unit of Volt-Amps(VA).
Here is the relationship between Power Factor, True Power and Apparent Power:
Power Factor(PF)= True Power(W)/Apparent Power(VA) = W/VA
Power factor can be an important aspect to consider in an AC circuit, because any power factor less than 1 means that the circuit’s wiring has to carry more current than what would be necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load. However, poor power factor can be corrected, which is called power factor correction(PFC). PFC can be done by adding another load to the circuit drawing an equal and opposite amount of reactive power, to cancel out the effects of the load’s inductive reactance. Usually, the power factor of transformers on the market is 0.7-0.85, and 0.9-0.95 for electronic drivers with PFC function.
In the lighting industry, Color Rendering Index (CRI) has been widely used for measuring the quality of the light source. CRI values range from 1-100, with 100 rendering colors the best to natural sunlight. CRI is calculated from the differences in the chromaticity of eight CIT standard color samples (R1-R8), the smaller the average difference in chromaticity, the higher the CRI. However, new research indicates that the CRI value may not accurately describe how well LEDs render colors. For example, an LED can have a relatively high CRI but render red poorly.
The R9 value is one of the 14 pigment colors scientists have established to measure color rendition, and it produces strong, vibrant reds. It is important because strong reds are prevalent in skin tones, meat, fruit, and artwork. LED with higher R9 values produce the most vivid colors. However, R9 is not measured in CRI. Sometimes LED with high CRI value might have negative R9 value, which is not good for a more accurate representation of the actual color. Whether in an art gallery, hospital, restaurant, or residential home, the color red is critical in order for the human eye to interpret pure colors.
Title 24 and JA8 is California’s energy standard for new homes and commercial buildings, and it requires a 90+ CRI and R9 value of 50 or more as high quality, high efficacy light sources. So next time, do not forget to ask about R9 value when you need high quality LEDs.
Voltage drop describes the energy(voltage) that a component uses when current flows in a circuit. Ideally, wires, connections and switches should never drop voltage. If one of these uses voltage, there won’t be enough energy left to operate the real load. In general, most of the served loads, such as light fixtures, have much greater power consumption than wires (conductors) serving them. However, given enough distance and enough current, resistance found in wires will cause the voltage to drop a large amount over the length of wires. In the lighting industry, voltage drop of wires is critical and it will affect the performance of the light source.
How does voltage drop play a role in the design of fixture?
Typically, 3% or less is the recommended level for efficient operation of light fixtures. If the amount of voltage drop exceeds a certain level, it will result in poor performance, and decreased lifespan.
This is most commonly seen as dim light output from lights, and it is noticeable when many lights are wired in series. You will observe the brightness decreases as lights get further from the power source.
Some factors of voltage drops are:
How does voltage drop behave with different mediums or materials?
To calculate voltage drop, you need to know the wire resistance. Different material of conductor has its own resistance.
The resistance of wire depends upon four elements:
The formula of voltage drop is:
Voltage drop (Vd)= current in the run (amps) x conductor resistance (ohms per 1000 ft.) x length of the run (feet)
If current in the run is 0.5A, the wire resistance (24 AWG “copper” wire @20°C) is 0.02567 (ohms/ft) and the wire length is 10 ft, then the voltage drop (Vd) is= 0.5(A) x 0.02567(ohms/ft) x 2 x 10(ft) = 0.2567 (V).
If the input voltage from the power supply is 12V, then the voltage drop at the end of wire is 2.14%, which is under 3%.
However, if the material of wire is changed to aluminum (the wire resistance of “aluminum wire” is 0.03865 (ohms/ft)@20°C), and all other conditions are the same, then the voltage drop will be 0.3865(V). It is 3.22% of 12V input voltage, which is higher than using copper wire.
In conclusion, given the same conditions of ambient temperature, the wire length, wire size, and the resistance of different materials plays an important role in causing voltage drop.
Class 2 – is a classification referring to the NEC – National Electric Code. To avoid potential cable overheating due to excessive currents and electric shock, the output of the power supply is limited to 60VDC or 100VA, (100W when used with an AC-DC power supply). You will often see 24V output DIN rail power supplies or LED drivers rated at 91W rather than 100W because if the power supply is overloaded, a tolerance in the over current protection has to be accounted for.
Class II – Refers to power supplies with either a double or reinforced insulation barrier between the input and the output. Class II supplies do not rely on an earth connection to protect against shock hazard. Many cell phone chargers and laptop power supplies are Class II. TDK-Lambda’s DSP series also are Class II, having just a Line and Neutral AC input without a ground connection.
One advantage of Class II is better surge protection between input and ground and usually a lower earth leakage current.