BUT FIRST AN IMPORTANT NOTE:
This project uses lethal voltages. If you are not experienced in working with lethal voltages, read this project, but don’t build it. You only have one life, and AC power can take it from you very quickly, or leave you with terrible injuries.
When working on high voltages, remember:
• Keep one hand in you back pocket. Don’t provide a path through your heart.
• Use and insulated mat or proper insulated footwear (bare feet on a tile or concrete floor doesn’t make it.)
• Make sure your equipment is in good repair and is properly grounded.
• Never assume a conductor is safe to touch.
• Don’t work when alone.
• When working with AC line voltages, use a ground fault interrupter.
• Never work on high voltages while under the influence of alcohol or drugs.
Do not build this project if you are not experienced in working with lethal voltages.
One more warning: Failure to use the fuse and AC line rated capacitors used in this project can result in personal injury, death, and the destruction of property. Be sure you know the parts you are using and the role they play in the safety of the circuit’s operation. Any use of this information is at the user’s discretion and risk; in no case will Richard Cappels be responsible for any injury, death, or loss resulting from the reliance of any information provided on this website. If you aren’t competent to work on a particular project, don’t.
Introduction
I made a pair of LED night lights while living in an apartment in Bangkok several years ago. I am not blaming the electrical service in my building, but of the two years that I lived there, I went though several florescent night lights. Finally, I got tired of replacing the lights every few months and decided that since I had a lot of white LED, they could be put to good use. The result was a pair of LED night lights that have served me well for over six years as of this writing.
Each light used LEDs from a different source. The LEDs from Nichea stayed the brightest and whitest while the bargain-basement LEDs from a source in Hong Kong tended to drift toward pink and yellowis tints over time. That’s 50,000 hours, and the lights are still perfectly serviceable, but you can tell from the photo at the top of this page that the plastics have changed color and look their age. Fifty thousand hours and still going strong is testamony to how long conservatively driven LEDs can last, especially given that the Nichea LEDs were manufactured last century -way back in 1999, the early days of modern white LEDs.
There are two strings of LEDs, one conducts on each of the half cycles of the AC power line. It is important to place LEDs operating from opposite half cycles next to each other so as to not have noticeable flicker.
The first thing you should notice about the AC Line Input and Trickle Charging Circuit is the fuse. Not only one fuse, but two, because R1 is a fusible resistor. I’m pretty concerned about personal electrical and fire safety, especially when it comes to AC line operated devices in my home. I’ll come back to the fuse later, but first, a little bit about how the circuit works.
Under normal, operation, a little while after power is first applied to the circuit, the AC voltage from the power line sees R1, the 120 Ohm resistor in series with the 0,47 uf capacitor and the parallel-series circuit of the 24 volt MOV (Metal Oxide Varistor) and the LEDs in series with the 120 ohm resistor. R1, a fusible resistor, limits the maximum current at switch-on, while the 24 volt MOV limits the voltage applied to the LEDs in series with the 120 ohm resistor. The MOV does not conduct in normal operation, but sometimes at the application of power, it saves the LEDs. I tested the circuit by shorting R1 and applying power -the result was one or more damaged LEDs. I repeated the experiment, shorting the 120 ohm resistor in series with the LEDs with the same result. I then removed the MOV and applied power several times and managed to induce failures in the LEDs again. All three of these parts are need to assure that the peak current from the power line will not damage the LEDs.
Read more: A White LED Night Light Design using microcontroller