Elektor is all over the globe, starting out from Holland. The same with bicycles, which are even older. All the way from Åland Islands N60 E20 (check that out), Anders Gustafsson wrote 2 us: “I just got so fed up with my bicycle light. I ride the bike to and from work every morning and with fresh batteries the light was acceptable. Problem was that the batteries wore down real fast so I was wondering what a LED could do. The original bulb, an Osram PR2 2.4 V 0.5 A should produce 10 lm. A Cree XP-E should produce 114 lm at 350 mA. I opted for a slightly lower current, or 320 mA, powering the LED from a constant-current switcher which will produce a constant current down to a battery voltage of 1.5 V. To get the output where I wanted, I used a Khatod KLCP 20CR lens with a 6-degree angle.”
Besides a circuit diagram of a simple voltage step-up converter based on a Linear Technology LT1618 chip . Anders kindly included a few photographs of his reworked bicycle light, which are reproduced here mainly as food for thought. The great thing about the LT1618 is that it can operate as a constant-current, constant-voltage source. The fixed-frequency, current mode switcher is rated to operate from an input voltage between 1.6 V to 18 V. Its high switching frequency of 1.4 MHz permits the use of small inductors and capacitors. Here we use constant-current mode and power the converter from two (thick!) 1.5 V batteries to obtain a LED current that’s remarkably steady around 320 mA.
The values of R2 and R3 in the voltage at the output are set up for Vout = 4.64 V using
R2=R3(Vout/1.263 – 1)
which is fairly arbitrary but bearing in mind that some absolute maximum has to be set. For sure the desired constant current of about 320 mA has priority as we aim to power an LED ‘to the max’. With the IADJ pin (4) of the chip tied to ground, the nominal current sensing voltage is 50 mV (appearing between the ISP and ISN pins). Here we have a constant current like
I= 0.05V/0.15Ω = 0.33A
which is just right to push that Cree LED into producing a very bright light beam in darkest Åland Islands and beyond, even if the batteries are juiced. Theoretically! So, Elektor Labs grilled the converter, measured its performance and drew up a table with selected results. In conclusion, the circuit does a good job both when powered from two dry cells (source = 3.0 V) or from two rechargeables (source = 2.4 V). The more mAh’s proudly printed on the batteries, the longer your LED headlight shines. Next we decided to put the start-up response of the converter through an LTSpice simulation, specifically at the lower end of Vin (1.8 V) and the nominal value (3.0 V) as it is often found that these converters fail to start when the batteries are flat. The run-in behavior of the 1.4 MHz oscillator in the LT1618 can be seen in both images. Space being at a premium in the bicycle headlight the choice of SMD parts rather than through hole should be obvious. Hence a very small circuit board was designed and printed here. If you experiment with the circuit on your workbench, do not forget to attach a heatsink to the LED as it is likely to die without one. Even a U-style finned heatsink for TO3 devices does a good job.