Author:N.S. HARISANKAR VU3NSH
Source: EFY
This
ultra-bright white LED lamp works on 230V AC with minimal power
consumption. It can be used to illuminate VU meters, SWR meters, etc.
Ultra-bright LEDs available in the market cost Rs 8 to 15.These LEDs
emit a 1000-6000mCd bright white light like welding arc and work on 3
volts, 10 mA. Their maximum voltage is 3.6 volts and the current is 25
mA. Anti-static precautions should be taken when handling the LEDs.
Fig.1: The circuit of ultra-bright white LED lamp
The
LEDs in water-clear plastic package emit spotlight, while diffused type
LEDs have a wide-angle radiation pattern. This circuit (Fig. 1) employs
capacitive reactance for limiting the current flow through the LEDs on
application of mains voltage to the circuit. If we use only a series
resistor for limiting the current with mains operation, the limiting
resistor itself will dissipate around 2 to 3 watts of power,whereas no
power is dissipated in a capacitor. The value of capacitor is calculated
by using the following relationships:
XC = 1/(2fC) ohms —————(a)
XC = VRMS /I ohms ———— (b)
where XC is capacitive reactance in ohms, C is capacitance in farads, I is the current through the LED in amperes, f is the mains frequency in Hz, and Vrms is the input mains voltage.
The 100-ohm, 2W series resistor avoids heavy ‘inrush’ current during transients. MOV at the input prevents surges or spikes, protecting the circuit. The 390-kilo-ohm,½-watt resistor acts as a bleeder to provide discharge path for capacitor Cx when mains supply is disconnected. The zener diode at the output section prevents excess reverse voltage levels appearing acrossthe LEDs during negative half cycles. During positive half cycle, the voltage across LEDs is limited to zener voltage.
XC = 1/(2fC) ohms —————(a)
XC = VRMS /I ohms ———— (b)
where XC is capacitive reactance in ohms, C is capacitance in farads, I is the current through the LED in amperes, f is the mains frequency in Hz, and Vrms is the input mains voltage.
The 100-ohm, 2W series resistor avoids heavy ‘inrush’ current during transients. MOV at the input prevents surges or spikes, protecting the circuit. The 390-kilo-ohm,½-watt resistor acts as a bleeder to provide discharge path for capacitor Cx when mains supply is disconnected. The zener diode at the output section prevents excess reverse voltage levels appearing acrossthe LEDs during negative half cycles. During positive half cycle, the voltage across LEDs is limited to zener voltage.
Fig.2: 16-LED combination
The
100-ohm, 2W series resistor avoids heavy ‘inrush’ current during
transients. MOV at the input prevents surges or spikes, protecting the
circuit. The 390-kilo-ohm, ½-watt resistor acts as a bleeder to provide
discharge path for capacitor Cx when mains supply is disconnected. The
zener diode at the output section prevents excess reverse voltage levels
appearing across the LEDs during negative half cycles. During positive
half cycle, the voltage across LEDs is limited to zener voltage.
Fig.3: 46-LED combination
Use
AC capacitors for Cx. Filter capacitor C1 across the output provides
flickerfree light. The circuit can be enclosed in a CFL round case, and
thus it can be connected directly to AC bulb holder socket. A series
combination of 16 LEDs (Fig. 2) gives a luminance (lux) equivalent of a
12W bulb. But if you have two series combinations of 23 LEDs in parallel
(total 46 LEDs as shown in Fig. 3), it gives light equal to a 35W bulb.
15 LEDs are suitable for a table lamp light. Diode D1 (1N4007) and
capacitor C1 act as rectifying and smoothing elements to provide DC
voltage to the row of LEDs. For a 16-LED row,use Cx of 0.22 μF, 630V; C1
of 22 μF, 100V; and zener of 48V, 1W. Similarly, for 23+23 LED
combination use Cx of 0.47 mF, 630V; C1 of 33 μF, 150V; and zener of
69V, 1W.