Although LED lights are now sold in large quantities, the performance of such solid-state lighting (SSL) products, especially in areas such as dimming, is still lower than conventional bulbs. Indeed, conventional drivers designed for LED replacement lamps are difficult to provide truly satisfactory dimming performance, energy efficiency, and reliability at an affordable price. However, digital control in the drive electronics now removes unnecessary bleed resistors (usually used to enhance the operation of thyristor dimmers) while reducing flicker-free dimming to low illumination levels.
The use of dimmers in home lighting equipment has been extensive, saving energy, reducing electricity bills, and providing comfort and convenience. With energy-efficient LED lighting, dimming is not very important from an efficiency standpoint. However, at home and in restaurants, entertainment venues or conference halls, lighting can easily affect mood. This requires smooth, flicker-free dimming that covers a wide range of light levels.
In many applications, the ideal scenario for deploying solid state lighting is simply to upgrade by replacing incandescent lamps with LEDs of the same form factor. This is because users expect new luminaires to work flawlessly with their existing dimmers, but the actual results may vary depending on the type and quality of the dimmer.
Dimmer operation
Standard triac-based phase-cut dimmers are designed to drive resistive loads provided by incandescent bulbs. The thyristor dimmer is also known as the leading edge dimmer because dimming is achieved by changing the sinusoidal waveform by adjusting the conduction angle of each half wave of the alternating current using triac (Fig. 1). The conduction angle of the delayed triac makes the lamp dimming.
figure 1. Start by the leading edge dimming delay of the TRIAC dimmer until the end of the cycle.
When a trigger pulse is applied, if the current remains above the device's specified holding current, the triac turns on and remains on for the remainder of the cycle. If the load is an incandescent bulb, the current is easily maintained above the hold current threshold, so the triac remains on until the current decreases at the end of the cycle.
However, two factors can limit the dimming range, and even incandescent bulbs can have some effect. Dimming circuits typically include an electromagnetic interference (EMI) filter that includes an inductor and capacitor assembly that can introduce ripple noise into the current waveform. If the inductor is a low quality component, this pulsating noise can be sufficient to cause the current to flow from on to surge immediately below the holding current of the triac, causing the device to turn off, causing visible flicker in the lamp. Likewise, if a low cost triac with relatively high holding current is used, the dimmer will not be able to maintain the dimming level.
Figure 2 compares the minimum load current required for five triac-based dimmers on the market today. The peak value indicates the minimum current that the LED driver needs to sink from the triac to ensure proper operation.
figure 2. The holding current of five different dimmers.
LED load current
Running a dimmer at low load currents is a challenge, and the challenge is exacerbated when the load is an LED replacement bulb. Unlike incandescent lamps, LEDs are not pure resistors. The impedance is reactive, which prevents the current from rising above the holding current threshold of the triac so that the device remains conductive after the trigger pulse is removed. Low levels of dimming can also be difficult to achieve because LEDs typically consume less power than incandescent bulbs. Therefore, the current developed by the triac may be very low at the phase angle required for maximum dimming. The current can be reduced below the minimum holding current of the triac, causing the lamp to flicker or suddenly stop working.
In practice, dimmers need to achieve more dimming levels than people perceive. Since the human eye compensates for low light levels by enlarging the iris, the relationship between the electronic dimmer setting and the perceived dimming effect is not linear. If the dimmer is adjusted to reduce the light to 10% of its maximum measurement level, the human eye will only see a reduction of about 30%. In order to achieve a 10% perceived dimming level, the dimmer must be able to reduce the measured light level to only 1%. The tenth edition of the Lighting Manual, published by the Institute of Lighting Engineering (IES), records the relationship between perceived light and measured light:
Perceived light = 100 × √ (measurement pupil 100)
Manufacturers of LED replacement lamps need to be able to ensure that triacs of different quality and cost are interoperable and that flicker-free lighting is reduced to low dimming levels to maximize customer satisfaction.
The network-managed industrial switch literally means a switch that can be managed by the network. There are three management methods, which can be managed through the serial port, through the Web, and through the network management software. It provides a terminal-based control port (Console) and a Web-based page. And support multiple network management methods such as Telnet remote login network. Therefore, network administrators can perform local or remote real-time monitoring of the switch's working status and network running status, and manage the working status and working mode of all switch ports in a global view.
The managed industrial Ethernet Switches all follow the SNMP protocol (Simple Network Management Protocol), which is a set of network equipment management specifications that comply with international standards. All devices that follow the SNMP protocol can be managed through network management software. You only need to install a set of SNMP network management software on a network management workstation, and you can easily manage the industrial Ethernet switches, routers, servers, etc. on the network through the local area network. It is also an in-band management method through the interface of SNMP network management software.
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