Mains voltage operates at high voltages and with alternating current. In the USA mains is typically 120V and 60 Hz, and in Europe it’s 230V and 50Hz.
Unfortunately LEDs are low voltage devices and lack inherent current regulation. Even more unfortunate, LEDs need precise current control for reliable operation. Any current spikes or reverse current can damage an LED system.
Many different types of AC LED systems exist to provide suitable LED drive currents. Systems have evolved over many years and take many different forms.
Conventional AC LED systems
One of the most straightforward AC LEDs designs consists of two anti-parallel strings of LEDs. The number of LEDs in each string is chosen so that the total voltage adds to an appropriate turn-on voltage and the polarity of the strings are opposite to protect against reverse current. Current will flow through a string of LEDs as the voltage rises beyond the turn-on voltage and will stop flowing when the voltage falls below it. A series power resistor, or similar, is used with each string to regulate the current.
A slightly more advanced design rectifies the AC source so it’s always positive (or negative) and oscillates at 120Hz instead of 60. This design can use a single string of LEDs and have them turned on much more frequently.
While these simple AC LED designs can work quite well in applications such as Christmas lights and night lights, they have a number shortcomings that make them unsuitable for general lighting. These include:
- Low efficiencies and very high THD, resulting in poor power factor
- Electrolytic capacitors are almost always required for current smoothing, giving poor reliability
- High power designs can be unreliable
ASIC-based LED systems
In recent years a new type of AC LED system has been employed by a number of component manufacturers. An Application Specific Integrated Circuit, or ASIC, is used along with simple circuitry to connect an AC source directly to a string of LEDs. ASICs can range from very simple devices to components sophisticated enough to control cell phones. In an AC circuit, an ASIC is used to logically control output current and voltage.
The major advantages with ASIC based designs are efficiency and power output. High power designs and compact form factors are possible. With careful attention to design efficiencies can be relatively high (> 85%) and power factors can be passible (> 0.7). Electrolytic capacitors are frequently required with the resulting impact on reliability
There are, however, a number of significant disadvantages, including:
- Dimming is possible but deep dimming is difficult. Often individual banks of LEDs turn off while dimming, which lowers the total light level but can be visually distracting
- Safety issues – the high voltage running through the system means it should not be touched. Generally the AC system is designed to be totally enclosed by plastic or glass which addresses safety concerns but at the expense of form factor and optical control
- Very few of these systems would pass UL or ETL
- Few systems are commercially available outside Europe
- Varying designs and light output can be difficult. The ASICs and supporting circuitry can be quite rigid making it difficult to adjust light output or form factor without creating a completely new design.
Key words:How Do AC-LEDs Work?
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