The growers now more and more realized that the very little light we call it “glow” in this article can impact the yield of plant greatly as the famous professor Dr. Bruce Bugbee indicated in his course. So understand why the “glow” happens and how to avoid the “glow” is important for luminaire designers.
Why the “glow” happens
There are two ways to turn off the lights, one is to cut AC input off and the other is to “dim off” using the dimming control signals. Simply cutting off the AC input is surely able to prevent the light from glow because there is not energy path to the LED at all, but it is not in our discussion range since this is not a “smart” or “modernized” way to deal with grow light industry.
Today we only talk about the glow in the “dim off” status with AC input connected to the luminaires. There are two reasons as below:
- Output voltage of the LED driver is not low enough during the “dim off”. There are a lot of LED driver using lower output voltage to low level to realize the dim-off.
- The LED chips and the capacitors including the Y capacitor inside driver and parasitic capacitors between LED and earth form a current path to make the glow happen. As the figure shows below, the line input AC voltage gives the energy through CY1, Cp1 and Cp2 to the LED. CY1 is the Y capacitor and Cp1/Cp2 is the parasitic capacitor between LED and LED board heatsink which is connected to earth. For isolated drivers, CY1 is much bigger than Cp1 and Cp2, so the issue is not serious in most of the cases. But for non-isolated drivers, CY1 is almost zero, thus the common mode voltage is actually directly applied to Cp1 and Cp2, and when Cp1 is not equal to Cp2, there can be voltage difference between Vout+ and Vout- thus makes the LED glow.
How to avoid the “glow”
Knowing the reason of the glow, we are able to solve the problem by the following ways.
- Do NOT use the way of lowering output voltage to dim off because different LED behaves differently and we don’t know by how much voltage, the LED may conduct. So the eliminate the possibility, we need to totally shutdown the LED driver rather than just lowering down the output voltage. Also there is another benefit that the standby power is much lower because the driver is in “sleep mode”. But the other result is that the cost of the driver is higher by adding such kind of circuitry inside the driver.
- To avoid the common mode leakage current through the parasitic capacitors, we have two ways, either disconnect the earth or disconnect the input/output during the dim-off status. Obviously we are not allowed to disconnect the earth by safety standards with AC line connected. So the only way to have to do is to disconnect the input or output to LED by an internal relay. Also the drawback is the higher cost.
Making sure of the darkness for cannabis and vegetable grow is so important that the LED driver manufacturers like uPowerTek is paying great efforts to the designs to avoid “glow” in off status. For the luminaire designers, you have to make sure that the luminaire is fully tested in the dark room before finalizing the product. Please feel free to let us know if you have any questions.