Choose a logic level mosfet which can handle more than 200mA.īetter to start with stable 20mA, construct the software driving the LED and when timings are fine then improve the electronics. Then a change of forward voltage +-0.2V results in a current change of 2mA, prox 2%.įrom a stability point of view I'd recommend to operate the two LED in parallel with a separate resistor each. This is +-20%.Ĭhanging the setup to a single LED at 12V, the resistor is 104Ohm. When forward voltages changes +-0.2V then current changes to 0.078A, 0.122A. Ambient Temperature which 'allows' 100mA only up to 25☌ at current high summer temperatures the design should be safe up to 50☌ which limits current to prox 60mA.Ī word of caution when operating LED in series: small series resistors cause quite high current changes when forward voltage of led changes (temperature, part variations):įor the current example when two LED are in series and the resistor is chosen with 18Ohm, operated from 5V, then current is 100mA at 1.6V. The footnotes in the IR led datasheet say that at 100mA additional constraints apply: pulse width <= 100us and duty cycle <=1%. I'd probably use a MOSFET instead of the BC547 as it will need a much lower current from the GPIO, R2 could be omitted and its low on resistance will mean the LED gets more current. The tutorial seems to ignore the voltage drop across the transistor, so the calculations are a bit out. Use a 560 Ohm resistor for R2 to give a slightly higher current to ensure the transistor switches on hard enough. Using the same calculation as that tutorial, that will be 1mA at 3.3V, which gives 660 Ohms. It may get quite warm.Īssuming the transistor has a gain of 200, R2 will need to allow at least 0.5mA into the base of the transistor to let it pass 100mA. The BC547 has a maximum Collector Current of 100mA, so that is running at the limit. If you want to drive the LED at maximum power, that is 100mA, you will need R1 to be 18 Ohms and it will need to be rated for at least 1/3 Watts. In that case R1 would need to drop 5 - 3.2 = 1.8 Volts (actually it would be a little less as the transistor will drop about 0.7 Volts). Podstaw 'rosyjskiego Raspberry Pi 3' jest chiski procesor Allwinner H5. If you put the two LEDs in series that makes 3.2 Volts. Raspberry Pi z AliExpress Wedug schematu pytki referencyjnej przedstawionego na stronie dewelopera, Repka Pi to dokadna kopia Nano Pi K1 Plus, ktry jest w sprzeday od 2018 roku. The Forward Voltage of the LEDs is about 1.6 Volts. Depending how bright you want the LEDs top be, it could be done with a single BC547, but it is pushing the limits of that transistor.
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