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Circuits designed by David Johnson, P.E.
Last Updated on:
Saturday, December 23, 2017 03:21 PM
List of Dave's Circuit Designs
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More
Flashers & Blinkers
LED's |
Line Powered White LED Array
February 27, 2010
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There has been a lot of
excitement lately in the lighting industry. Some companies are beginning to
manufacture general lighting fixtures, which replace incandescent and fluorescent
lamps, using LED lights. When proper heat management is designed into the light
fixtures, these LED lights can produce more light with less electricity and last much
longer than other devices. They can also operate well in cold temperatures.
Some of these new lighting fixture designs use a large number of white LEDs, wired
into long series wired string arrays. The circuit below is designed to provide a
regulated DC voltage for these strings. |
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Examples of LED Lights Using an Array
of Small Devices |
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The
circuit begins operation by first rectifying the AC voltage with a bridge rectifier. The
pulsating DC output of the rectifier is fed to a switch circuit. This circuit operates as
an intelligent voltage switch. The NPN transistor Q1 in concert with the n-channel
MOSFET Q2, routes a specific peak voltage to the storage capacitor C2. The voltage
is selected to be 5 to 7 volts higher then the total forward voltage of the LED series
string. The capacitor C2 should be sized based on the rule of thumb of 4000uF per
Amp of current. If the LED string requires 20ma of current, the capacitor should be
at least 80uF. Select the voltage rating of the capacitor to be higher than the peak
voltage. I would also suggest using a quality capacitor with a 105C temperature
rating. |
A 15v
zener diode limits the voltage to the gate of the n-channel FET Q2 and also limits the
voltage across the transistor Q1. This circuit will also operate with 240vac power
lines, however, the transistor Q2 should be one with a 400v rating. The circuit
works best when the LED array voltage is above 30v and the total current does not exceed
1/2 Amp. |
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Click
on Drawing Below to view PDF version of Schematic |
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