[mike31154]

With respect to running LEDs on AC or DC. Note that the article I linked to in an earlier post says that LEDs are 'usually' considered to be DC devices. LEDs are nothing more than diodes which happen to emit light. Diodes are commonly used in any number of circuits whether AC or DC. They will conduct when voltage goes positive & block current when it goes negative. They're used in a bridge circuit configuration in most power supplies to rectify an AC signal to DC. A bridge circuit consists of 4 diodes. By hooking them together in this manner the result is a DC output with some 'ripple', so not entirely pure DC voltage. The ripple or noise is minimized by further filtering of the signal using resistors, coils & capacitors.

Therefore, you can in theory run almost any LED on an AC signal, it does not necessarily have to be specifically designed for AC. However, the same way you need to limit the voltage & current when using a DC supply or driver, you need to ensure any AC signal used does not exceed the diode's or LED's maximum current, or it won't live very long. The diode must also be able to handle the reverse voltage while blocking the signal when it goes negative. Depending on the frequency of the AC signal (60Hz in the case of our homes), you will not even perceive that the LED is actually turning off & on 60 times a second. The average human eye can not perceive a flicker at that frequency. That's one of the reasons 60 Hz is used to supply our homes with power. Should that number ever drop into the 40 Hz or lower range, you will start to see a flicker in most lighting, including incandescent & some flourescents.

There are plenty of applications where AC, or for better efficiency, PWM (pulse width modulated) signals are used to drive LEDs. This can save power, reduce heat & increase the life of LEDs driven that way since the power to the LED is actually switched on and off at a high frequency. The main difference between AC & PWM is that the AC is a nice curved sine wave going from positive to negative through a zero reference, whereas a PWM signal is more analogous to a square wave going form zero to whatever the nominal positive voltage is, then back to zero. Our house AC goes from zero to 120, back through zero to -120 and so on. Clearly 120 volts either way would quickly fry any LED in short order. This is why we use some sort of transformer/rectifier or driver to bring that voltage/current down for most LED applications.

Using an AC or PWM signal to power our tank LEDs makes little sense, since we want our livestock, mainly coral to get maximum exposure to the light source and switching to LEDs from MH or T5 is already saving us a bundle in power. We don't want or need to turn the LEDs ON/OFF in rapid succession for our purpose. It does however make sense in small battery powered devices such as headlamps or flashlights, where we want to maximize battery life. Using a PWM in such a device turns the LED off & on rapidly, inperceptible to the eye, but saving power since the LED is turned off for half the time.

Here's an interesting tidbit on the concept. A couple of years ago during a backcountry ski trip, one our ski companions came back inside after dark saying he perceived some sort of strobe effect while watching the snow fall with his Black Diamond LED headlamp. Needless to say we ridiculed him, getting old, too much alcohol, you get the idea. Bruce insisted that he was sober & after some coaxing got a few of us to go out & verify that he was not going bonkers. Sure enough, there was the strobe effect! Back inside we went & started wondering whether we were all seeing things. After some discussion, more beer, wine, single malt scotch, we finally came to the conclusion that BD is using some sort of oscillator circuit (or micro inverter) in the headlamp to turn the LEDs on & off rapidly. Too rapid for the eye to see normally, but the falling snow somehow enabled us to percieve the flicker or strobe, sort of like a helicopter blade when filmed. Personally I found it difficult to believe BD would go to those lengths to extend battery life in something like a headlamp. But it is a competitive market and out in the boonies, where a headlamp could get you back home, you want to get the most out of those 3 AAA batteries. Bruce actually contacted BD after his return & they confirmed that there was indeed a PWM circuit in the headlamp.

Ha, what a saga. If you read all of that, thanks, it certainly took a while to put down.