Showing posts with label white. Show all posts
Showing posts with label white. Show all posts
Wednesday, June 12, 2013
White Flame Hanger Lamp
After experienceing the earthquake and tsunami in Japan earlier this year, people began to realize the terrible nature, while forcing us to learn how to adapt to it. T & O Studio Japanese designer Toshihito Okura believes that we re-examine the use of daily necessities every day before after disaster, and find some new features and a new way to use these products as much as possible to reduce the impact on the natural environment.
He designed a unique lighting named the "white flame" that was created by one-off plastic coat hanger, fashion magazines and the strings of 3mm diameter wire together. Lampshade is made with hanger was hanged by strings wire, wire droop naturally by the weight of hanger to a appropriate curvature. The magazine becomes a stabilizing role in the base of the wick and light lines are wrapped beneath it.
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He designed a unique lighting named the "white flame" that was created by one-off plastic coat hanger, fashion magazines and the strings of 3mm diameter wire together. Lampshade is made with hanger was hanged by strings wire, wire droop naturally by the weight of hanger to a appropriate curvature. The magazine becomes a stabilizing role in the base of the wick and light lines are wrapped beneath it.
Saturday, April 13, 2013
Step Up Booster Powers Eight White LEDs
Tiny white LEDs are capable of delivering ample white light without the fragility problems and costs associated with fluorescent backlights. They do pose a problem however in that their forward voltage can be as high as 4 V, precluding them being from powered directly from a single Li-Ion cell. Applications requiring more white LEDs or higher efficiency can use an LT1615 boost converter to drive a series connected array of LEDs. The high efficiency circuit (about 80%) shown here can provide a constant-current drive for up to eight LEDs. Driving eight white LEDs in series requires at least 29 V at the output and this is possible thanks to the internal 36-V, 350-mA switch in the LT1615.
The constant-current design of the circuit guarantees a steady current through all LEDs, regardless of the forward voltage differences between them. Although this circuit was designed to operate from a single Li-Ion battery (2.5V to 4.5V), the LT1615 is also capable of operating from inputs as low as 1 V with relevant output power reductions. The Motorola MBR0520 surface mount Schottky diode (0.5 A 20 V) is a good choice for D1 if the output voltage does not exceed 20 V. In this application however, it is better to use a diode that can withstand higher voltages like the MBR0540 (0.5 A, 40 V). Schottky diodes, with their low forward voltage drop and fast switching speed, are the best match.
Many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35 A. Inductor L1, a 4.7-µH choke, is available from Murata, Sumida, Coilcraft, etc. In order to maintain the constant off-time (0.4 ms) control scheme of the LT1615, the on-chip power switch is turned off only after the 350-mA (or 100-mA for the LT1615-1) current limit is reached. There is a 100-ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. This current overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values.
This will be the peak current passed by the inductor (and the diode) during normal operation. Although it is internally current-limited to 350 mA, the power switch of the LT1615 can handle larger currents without problems, but the overall efficiency will suffer. Best results will be o btained when IPEAK is kept well below 700 mA for the LT1615.The LT1615 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. The LT1615 also contains circuitry to provide protection during start-up and under short-circuit conditions.
When the FB pin voltage is at less than approximately 600 mV, the switch off-time is increased to 1.5 ms and the current limit is reduced to around 250 mA (i.e., 70% of its normal value). This reduces the average inductor current and helps minimize the power dissipation in the LT1615 power switch and in the external inductor L1 and diode D1. The output current is determined by Vref/R1, in this case, 1.23V/68 = 18 mA). Further information on the LT1615 may be found in the device datasheets which may be downloaded from www.linear-tech.com/pdf/16151fa.pdf
Continue reading...
The constant-current design of the circuit guarantees a steady current through all LEDs, regardless of the forward voltage differences between them. Although this circuit was designed to operate from a single Li-Ion battery (2.5V to 4.5V), the LT1615 is also capable of operating from inputs as low as 1 V with relevant output power reductions. The Motorola MBR0520 surface mount Schottky diode (0.5 A 20 V) is a good choice for D1 if the output voltage does not exceed 20 V. In this application however, it is better to use a diode that can withstand higher voltages like the MBR0540 (0.5 A, 40 V). Schottky diodes, with their low forward voltage drop and fast switching speed, are the best match.
Many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35 A. Inductor L1, a 4.7-µH choke, is available from Murata, Sumida, Coilcraft, etc. In order to maintain the constant off-time (0.4 ms) control scheme of the LT1615, the on-chip power switch is turned off only after the 350-mA (or 100-mA for the LT1615-1) current limit is reached. There is a 100-ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. This current overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values.
This will be the peak current passed by the inductor (and the diode) during normal operation. Although it is internally current-limited to 350 mA, the power switch of the LT1615 can handle larger currents without problems, but the overall efficiency will suffer. Best results will be o btained when IPEAK is kept well below 700 mA for the LT1615.The LT1615 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. The LT1615 also contains circuitry to provide protection during start-up and under short-circuit conditions.
When the FB pin voltage is at less than approximately 600 mV, the switch off-time is increased to 1.5 ms and the current limit is reduced to around 250 mA (i.e., 70% of its normal value). This reduces the average inductor current and helps minimize the power dissipation in the LT1615 power switch and in the external inductor L1 and diode D1. The output current is determined by Vref/R1, in this case, 1.23V/68 = 18 mA). Further information on the LT1615 may be found in the device datasheets which may be downloaded from www.linear-tech.com/pdf/16151fa.pdf
Author: D. Prabakaran
Copyright: Elektor Electronics
Copyright: Elektor Electronics
Thursday, March 28, 2013
Buck Converter 1 Watt White LED Driver
This is an example of efficiently driving a 1 watt white LED from a 12 volt
battery using a buck converter. The LED could simply be connected with a
series resistor to get the desired current, but the efficiency would be
only 25% since the resistor would drop 9 volts while the LED only requires
3. The buck converter provides about 90% efficiency.
The idea is to establish a circulating current through the inductor, diode
and load, while the switch replenishes the lost load energy on each cycle.
The duty cycle of the switch will be the output voltage divided by the input
voltage, or about 3/12 (25%) in this case. Its actually a little greater
since there is a small (2.2 ohm) resistor in series with the LED that drops
about 0.5 volt, so the total load is about 3.7 volts and the duty cycle is
around 31%. The circuit could also be used to charge AA batteries from a
12 volt source with adjustment to the duty cycle.
The driver section uses a CMOS hex inverter (CD4069) where two of the
inverters form an oscillator with 31% duty cycle at about 11.5 Khz,
or 66us off time, and 21uS on time for the MOSFET switch. The remaining 4
inverters are used in parallel to provide additional drive current to
the gate of the MOSFET. The duty cycle can be adjusted with either the
15K or 20K resistors.
The minimum inductor value was worked out from E = L * di/dt and a LED
current of 250mA. The minimum value is where the current falls to 0 during
the switch off time, or 66uS. The peak inductor current would then be twice
the average or 500mA and the inductor will charge from 0 to 500mA in 21uS.
So, di/dt is 0.5 /.000021 = 23810 amps per second. The inductor voltage
(E) will be 12 minus the load voltage 3.7 or 8.3 volts and the minimum
inductor value L will be 8.3 / 23810 = 0.35 mH. The actual value used should
be somewhat higher to avoid the current falling to zero and to avoid large
peak currents and possible saturation. The example here uses a approximate
2 mH inductor so the change in current is about 100mA and the peak current
is lower at about 300mA. The current waveform is shown in the LTspice
picture below. Notice the current ramps from about 50mA below the average
current to about 50mA above the average or about 100mA total change. The
15 ohm resistor in the LTspice picture represents the LED plus a 2.2 ohm
resistor. The MOSFET is represented by the SW (switch) component, and the
drive circuit by the V3 symbol.
The inductor (pictured below) should be rated for saturation current of more than the peak current, or maybe 300mA in this case. The toroid inductor used is fairly large for the task measuring about 1.5 inches diameter with 20 turns of #18 wire. The core is conductive so it probably should be taped in case the wire insulation fails. The picture shows the naked core for illustration. A smaller core with an air gap could be used to avoid saturation, but would require more wire which would add to the losses due to the wire resistance. Another approach is to use a higher frequency so smaller inductors can be used. But this will add to losses since there would be more switching transitions per unit of time, which adds to the loss. The diode is a VSK330 schottky 3 amp variety for low loss, but most any 1 amp rectifier could be used with somewhat less efficiency. The IRFZ44 MOSFET is also an overkill rated at 50 amps max but very low on-resistance of only 28 milliohms. A much smaller device could be used, but I dont have the numbers. Note the circuit has no regulation, so the 12 volt input should be stable. If the battery voltage varies, the duty cycle and LED current should be set using the highest expected supply voltage.
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