7/28/2019 Led Lamp Circuit
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Circuit board installed in an automotive lamp holder, attached
to a fluorescent lamp base
Introduction
This is a minimal parts lamp made with four white LEDs. It
features regulated light output from10V to around 20V and works
well as a flashlight.
Specifications
Nominal Operating Voltage: 12V DC
Operating Current: 40ma
Theory
The LM317L and resistor act as a current regulator set to 40ma.
Current flows from the battery
through one pair of LEDs, through the regulator, through the
other pair of LEDs, and back to the
battery. The capacitor filters out noise on the power supply
lines. The LED pairs must be
7/28/2019 Led Lamp Circuit
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matched so that the current through them is roughly equivalent.
Small resistors could be placed
in series with each of the four LEDs to improve the balance, but
the parts count would go way
up.
Construction
There is one trick with this circuit, matched pairs of LEDs must
be used. Normally, a batch of
LEDs from the same manufacturer will be matched close enough for
this application. If
unmatched LEDs are used, one LED per pair will be bright and the
other one will be dim. It is
best to first build the circuit on a plug-in proto board to
verify that the LEDs light evenly.
A small circuit board was made using press-n-peel blue film, the
board was cut into a circularshape using a nibbling tool. The parts
were soldered in, and the board was mounted inside of thebottle
cap. The bottle cap protects the LEDs and prevents bright light
from coming out of the
side of the assembly. The cap came from a 1 Liter "Aqua Fina"
brand water bottle. The single
screw can be used to mount the assembly to an external
bracket.
The completed circuit board and cap assembly was mounted in an
old automobile turn signal.
The lamp head was mounted onto the end of an old fluorescent
lamp base with a goose neckadjustable arm. The resulting lamp is
quite effective for night reading, and it's not too ugly.
Alignment
If you have a variable power supply, it is best to bring the
voltage up slowly the first time power
is applied to the circuit. If any of the LEDs don't light, turn
off the power and fix the problem. If
all of the LEDs light up evenly, the circuit should work (for
many years) on 12V.
Use
Connect this circuit to a 12V battery or power supply, be sure
to observe the correct polarity. The
LEDs should put out a bright white light. You can read by this
light, and it is useful for
emergency illumination. The low current draw allows it to run
for many hours on a battery.
Parts
4x white LEDs (matched), T1-3/4 size
1x 30 ohm 1/4 W resistor
1x 0.1uF capacitor
1x LM317L adjustable voltage regulator
1x plastic bottle top from a water bottle (1 liter Aqua Fina
brand)
1x 4-40 3/8" screw
2x 4-40 nuts
