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.
The Circuit
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.
I also applied power with the 0.47 uf capacitor shorted. R1, the
fusible resistor opened as it should have, to prevent fire. I added the
separate 800 ma fuse because I didn't know the current rating of R1 and
wanted to have at least one fuse in the circuit for which I knew the
current rating.
The RMS current through the input circuit is approximately equal to the
input voltage divided by the series impedance. Impedance is equal to
the square root of resistance squared plus the reactance of C1
squared. The reactance of C1 is
1/(6.28 x 50 x .000,000,47) = 6,778 ohms. This is very large compared
to the resistance, which is dominated by the 120 Ohm R1, so as a
practical matter, we can say that current is equal to the input voltage
divided by the reactance (6.8k) of C1. In the calculation above, I
neglected the voltage drop of the LEDs to simplify the calculation; the
more fastidious among us will want to take that additional drop into
account.
The RMS current, in amps, is equal to 240 VRMS / 6.8 k = 35 milliamps
RMS. The average current is 89% of the RMS value, or 31 ma.
Given that any given LED is only on during one half of the AC power
cycle, the average current through the LEDs will be approximately 15.5
ma. When designing an LED device to operate for many years, its a good
idea to go easy on the current so that the light output doesn't drop
off ot an unusable level too soon.
A few critical words on the election of components for this part of the
circuit. The key safety components are C1, R1, and F1. C1 is an X type
capacitor. X type capacitors meet certain safety specifications and are
designed to be safely used across the AC power line. Other types of
capacitors may fail from the constant AC power line voltage across it,
and when they fail, the results could be deadly. Don't use other types
of capacitors. The reason that C1 is a 0.47 uf capacitor is that this
is the largest value X capacitor that I could get my hands on.
A review of safety capacitors can be found at the URL below (you should
be able to click on URL -just remember to use your browser's back
buttom to come back here!):
http://www.justradios.com/safetytips.html
F1 is there just in case C1 does short out. I feel that, although the
likelihood of C1 failing as a short are extremely remote, that to me,
its worth the small cost of the fuse to provide a little additional
peace of mind.
Relevant to the 0.47 uf capacitors, two 470k resistors are across the
capacitor to bleed off the charge on C1 so that I won't get shocked if
I touch the power failure light's AC line contacts. The time constant
is only 940k Ohms x 0.47 uf = 0.4 second, so the voltage would bleed
down to less than 5 volts in about two a seconds. The reason I
used two resistors in series instead of just using a single 1 Meg Ohm
resistor, was to assure that there was sufficient margin in the
resistor's voltage rating. The resistors I used were nonflammable 1/4
carbon film type.
R1 is there to limit the maximum current when the power failure light
is first plugged into the AC line. Since the RMS voltage is 240 volts,
the maximum peak voltage across the input is 240 volts x 1.414 = 340
volts peak. If the power plug makes contact with the AC line at the
instant the power line is at 340 volts. Since C1 is discharged
when the power night light is first plugged into the AC line, and the
MOV clamps voltage at about 24 volts, the maximum current through R1
and the MOV is approximately
(340V - 24V) / 20 Ohms = 16 amps. Without R1 the current would be very
high, and a small fraction of that current could damage one of the
semiconductors in the circuit if, by chance of a ground loop, some of
that current were to get into other parts of the circuit. It also keeps
the sparks down when you plug the power failure light into the wall.
The 120 ohm resistor in series with the LEDs serves the important
purpose of limiting the current through the LEDs. Since the maximum
voltage across V1 is around 24 volts peak, the maximum
current into the LEDs is (24 volts- 20 volt LED forward
voltage/120 Ohms = 33 milliamps (not a coincidence that that this is
close to the normal peak current calculated above. V1 does not
conduct often during normal operation.
Construction
And Testing
A piece of pre punched fiberglass board was cut to fit inside a
florescent night light enclosure and the components placed on the
board. Care was taken to keep the creepage path between opposite phase
conductors to the left of the MOV in the schematic at least 10
millimeters.
I used the plastics for a florescent night light that I bought at a
department store, so I felt confident that the enclosure design was
safe to use for this application. The circuit draws considerably less
power than the florescent lamp did, so heat was not a concern.
After putting the night light together, double-check each connection
against the schematic and check the DC resistance at the AC plug. If
you don't read close to 1 meg ohm, something is wrong; recheck the
wiring and the components. Don't skip the inspection and electrical
test.
Before applying power, carefully inspect the exterior to assure
yourself that no electrical conductors other than the AC plug are
exposed.
When applying power for the first time, its a good idea to put the
night light at the end of a 1 or 2 meter extension cord (even longer is
better!), then plug the extension cord into the wall. If there are any
flying parts, an that happens sometimes, the distance should help
protect you.
I made two of these. One has eight LEDs and the other has ten LEDs, and
both have been operating almost continuously for over six years as of
this writing. The provide plenty of light to a hallway and a bathroom.
Each fixture costs me about US$1.60 a year to operate. While making
small corrections in November. 2015, I note that now, after about 9 1/2
years of continuous operation they lights are still operating.
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