Figure 1. Example of a basic SeaSense command.
Vehicle designers have come up with many novel ways of dimming the lights on their vehicles. Dimming control interfaces for underwater lighting include isolated and non-isolated control voltage (05vdc, 010VDC), current loop (420mA), PWM, serial communications, TCP/IP communication, phase control, DMX, variable input voltage and toggle. Many of the above schemes can be used to control a commercial off-the-shelf (COTS) phase control dimmer.
AC wall power in the U.S. is 120V, 60 cycle, single phase. The 120VAC refers to the RMS (root mean square) average value of the sinusoidal voltage waveform. The peak voltage is the RMS value multiplied by the square root of 2 (1.414). Peak voltage of 120 VAC (rms) = 120VAC * 1.414 = 170VAC. This will have important consequence shortly.
Some 120VAC linefed LED lights do not use driver circuits or power supplies. They use the simple, low cost series resistor or linear regulator. These present a risk of flicker using simple dimmer techniques. Other lights, such as DeepSea’s Matrix series, contain integrated drivers and power supplies, so even with a varying AC input, the driver provides constant current and performs dimming with zero noticeable flicker. For the light to be dimmable, the driver must be designed to interpret control signals and produce a range of LED current. The added control circuitry may slightly lower driver efficiency and may cost more, but provides the advantage of a flicker free and broader dimming range. This is important to video imaging systems which are more sensitive to phase blanking and black video frames than film or human eyes.
Phase control dimming is used in household wall dimmers, and is built around a very robust electronic switch called a “triac”. These are electrically very noisy as they delay turnon of the output until a certain phase angle is reached, resulting in very large current transients. These current spikes can get out of phase with the line and cause heating of the transmission lines and wasted energy. These line spikes can also degrade video through cross-talk.
Resistive light sources, such as incandescent bulbs, work well with phase control dimmers as they produce their light by heating an element which makes them react slowly to pulsed power. LEDS, however, are electronic devices and react instantaneously to pulsed power. This will make the LED’s appear to flicker under phase control dimming.
A low cost means of driving LEDs from 120VAC mains is using a bridge rectifier through a current limiting resistor to a series string of LED’s. This has the advantage of making the light somewhat compatible with phase control and variable voltage dimming. The drawback to driving and dimming LEDs in this fashion is the LED will not emit light until the voltage rises to the forward voltage (Vf) of the LED string and will turn off when it drops below that string Vf. Thus, the dimmer is only effective over a small range of control, and the light is susceptible to the “60 cycle” flicker most people are familiar with from fluorescent lights.
The downside of this approach is LEDs will be overstressed if they are driven at too high a voltage, as happens with an AC supply delivering a peak voltage that exceeds the LED limit. For example, a Cree XPG LED is rated with a Vf of 3.3vdc at 1000ma. Wiring 36 Cree XPG’s in series, the total string Vf would be close to 120V rms. As can be seen in the Fig. 4, the LED string won’t turn on until it gets to Vf(Min) of about 2.64V/LED or 95V in the string. Likewise, the LED turns off at this voltage on the backside of the curve. This means the light is not on for about 35% of the half cycle. The peak voltage would be as high as 170V. The manufacturer calls out the Vf(Max) as 3.75V/LED, or 135V with 36 LEDs in series. The LED is therefore over-driven 40% of the time, which is likely to affect its operational life.
A phase control dimmer delays the application of voltage until a point later in the 0180º cycle. When voltage is applied at some phase angle, say 60º as shown in Fig. 5, electrical power transitions from zero to the corresponding peak value instantly, creating inrush current and EMF signal.
One phase control dimmer we tested dimmed an incandescent bulb from off-to-full bright over a rotational range of approximately 300º out of 360º. We then tried an LED light wired with 36 LEDs in a string as described above. The light went from bright to dim in approximately 30º of the 300º, while the remaining 270º was simply “off”. The designer can provide slightly more dimming range by removing some of the LED’s in the string, thus lowering the overall Vf. However, this will result in higher peak currents through the LED’s which may exceed their maximum recommended values. It will also make the light less efficient as the series resistor will have to drop more voltage, creating more heat.
Another alluring approach to 120VAC light design is the use of the Seoul Semiconductor Acriche, an LED claimed by its manufacturer as “the world’s first semiconductor light source that operates directly from AC power without a converter.” Under the LED silicone lens are two parallel, but reverse polarity, strings of 36 tiny LEDs in series acting as a bridge rectifier. The Acriche requires use of an external current limiting resistor similar to the above “low cost” bridge rectifier design, which drops voltage and lowers system efficiency. While use of Acriche may enable direct compatibility with a phase-control dimmer switch or Variac™, the dimming performance will be at best disappointing with all of the weaknesses of the above low cost bridge rectifier design.
This is not to conclude that LEDs can’t efficiently be dimmed using phase control. DeepSea Power & Light has developed an advanced LED driver compatible with a phase control dimmer switch which provides smooth, continuous LED current with a linear dimming ratio in excess of 400:1 at power outputs over 125W and efficiencies over 85%.
With DC driver circuits, LED brightness is commonly controlled using PWM (Pulse Width Modulation), varying forward current, or both. PWM “chops” the effective “ON” time of the LED string at a rate much higher than can be visibly perceived, altering the average “ON” time of the LEDs, thus varying brightness. Changing the LEDs forward current also affects brightness. Used in combination, very high dimming ratios can be achieved.