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 350mA, the power switch of the LT1615 can handle larger currents without problems, but the overall efficiency will suffer. Best results will be obtained when IPEAK is kept well below 700mA 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

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Best Start up Aid for PCs

Since one of the servers owned by the author would not start up by itself after a power failure this little circuit was designed to perform that task. 

The older PC that concerned did have a standby state, but no matching BIOS set-ting that allows it to start up unattended. Although a +5 V standby supply voltage is available, you always have to push a but-ton for a short time to start the computer up again. Modern PCs often do have the option in the BIOS which makes an automatic start after a power outage possible. After building in the accompanying circuit, the PC starts after about a second. Incidentally, the push-button still functions as before.

 

The circuit is built around two golden oldies: a NE555 as single-shot pulse generator and a TL7705 reset generator. The reset generator will generate a pulse of about 1 second after the supply voltage appears. The RC circuit between the TL7705 and the NE555 provides a small trigger pulse during the falling edge of the 1 second pulse. The NE555 reacts to this by generating a nice pulse of 1.1RC. During that time the output transistor bridges the above mentioned pushbutton switch of the PC, so it will start obediently. 

Other applications that require a short duration contact after the power supply returns are of course also possible.

Author : Egbert Jan van den Bussche – Copyright : Elektor

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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

 

 

Streampowers

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Power up down Sequencer

Whether you’re speaking about a residence cinema  or a computer system, it’s very frequently the case  that the more than a few elements of the machine have  to be fliped on or off in a relatively explicit order,  or at least, routinely. Constructing this  sort of automation system is neatly inside the  functionality of any electronics enthusiast invaluable of the identify, however on this ‘all-digital’ age,  many of the circuits of this kind to be found  in newbie electronics magazines or web-sites use a microcontroller. Even although that  is indeed a logical solution (in  extra meanss than one!), and you  would probably even say the easiest one, it  does pose issues for all those  people who don’t (yet) have the  amenities for programming these  kinds  of  IC.  So  we  decided  to  offer you now an method that’s  very different, because it only makes use of a  simple, low cost, commonly-avail-able analogue integrated circuit,  which in fact doesn’t have to  be programmed. Our project in  reality makes use of as it’s ‘brain’ an LM3914,  a familiar IC from National Semiconductors,  usually  used  for  using  LED  VU  (volume  unit)  meters. 

Circuit diagram :

Power-up/down Sequencer Circuit Diagram

Before taking a look on the circuit  for  our  mission,  let ’s  just  remind ourselves that the IC has  one analogue enter and ten out-puts supposed for riding LEDs.  It can function in ‘point’ mode,  where the LEDs light up in flip,  from first to last, depending on  the enter voltage, but just one LED is lit at  any supplyn time. Alternatively  it will most probably function  in ‘bar’ mode (this is the mode usually used  for VU meters), and on this case, the LEDs light  up one after the other, in any such means as to create a strip of light (bar) that is longer or  shorter according to the enter voltage. This is  the mode selected for the LM3914 in the circuit described in some detail under. 

So as in an effort to control the AC energyed equipment  our  sequencer  is  meant  to manage, we're using solid-state relays — four, in our example, although which you can reduce or elevate this number, as so much as a most of ten. Since the enter instruments in solid-state relays are LEDs, they can be driven right away with the help of the LM3914 outputs, for the reason that that’s precisely what they’re designed for. As best 4 relays  are on hand, these are unfold across out-puts L2, L4, L6, and L8, however you can choose  any arrangement you prefer to go nicely with the quantity  of relays you want to have to make use of. 

Resistor R7 connected to pin 7 of the LM3914  units the current fed to the LEDs by means of the LM3914  outputs. Here, it’s been set to 20 mA, due to the fact  that's the price anticipated by the solid-state  relays chosen. The input voltage utilized to  pin 5 of the LM3914 is none instead of the  voltage present throughout capacitor C1 — and  that is the place the circuit is ingenious. When  the change is set to ‘on’, C1 charges slowly  thru R5, and the LEDs of the solid-state  relays on the outputs gentle one after another  as this voltage raises; in this approach, the gadgets  being keep watch overled are energyed up within the order you’ve chosen. To energy-down, all you have  to do is flip the change so that C1 discharges  through  R5,  and  the  LEDs  go  out  in  the  reverse order to that wherein they had been lit,  in turn energying down the devices connected to the solid-state relays. Easy, isn’t it? If you’re not pleased with the sequence velocity,  all you need do is increase or reduce the  value of R5 to be in a place to alter the pace one  manner or the different.

The circuit needs to be energyed from a volt-age of around 9 to 12 V, which doesn’t even  wish to be stabilized. A easy ‘plug-top’,  ‘wall wart’ or ‘battery eliminator’ unit will be  good, just so long as it is able to supply-ing enough current to power all the LEDs. As  the LED present is set by R7 to 20 mA per LED,  it’ll be simple for you to work out the current  required, according to the collection of solid-state relays you’re the use of. 

In our prototype the kind S216S02 relays  from Sharp have been used, primarily as a consequence of they  proved comfortably on hand via mail order. They even have the good thing about being compact,  and their switching capacity of sixteen A means  which you can dispense with a heatsink if you’re  the use of them for a computer or residence cinema  system, the place the present drawn via the vari-ous gadgets will more than likely be expected to stay under  1 A. These solid-state relays should be safe by a fuse, the score of which wants to  be chosen according to the current drawn  through the tools being powered. 

Also no longere the presence across the relay time periodinals of a VDR, sometimes called a GeMOV or  SiOV, intended to protect them from any spurious voltage spikes. You can use any sort  that ’s intended for operation on 250 VAC  with none problem. The prices of fuses F1  to F4 are after all going to rely upon the  load being secure. 

Construction of the circuit shouldn’t present any particular problem, however as the solid-state relays are connected right away to AC  power, it's very important to install it in a fully-insulated case; the case can additionally be used to  mount the energy outlet sockets keep watch overled  with the aid of the circuit. Note that sockets are female  elements.

Let’s simply finish this description with the sole  restrict imposed with the help of our circuit — but it’s  very simple to agree to, providen the intended  use. In order to remain brought about, the solid-state relays must elevate a minimal holding  present, which is 50 mA within the case of the  instruments we’ve selected. In sensible terms,  this just means that each and every of the devices powered by means of our sequencer should draw as a minimal  50 mA, or in different phrases roughly 12 VA at  230 VAC, or 25 VA at a hundred and twenty VAC.

Author :Christian Tavernier

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