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# 30mA LED Dimmer(LM358N)

2014-12-09 04:50
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This article briefly describes the 30mA LED Dimmer (LM358N). This principle is easy to understand, but also very practical. Depth understanding of circuit elements, can be more effective to grasp this principle. Do you know the circuit, for example, can understand and buy these components: LM358N .

If you’ve ever tried dimming aLEDwith a simple potentiometer, you know that the approach does not work very well. Just as with ordinary diodes, the voltage-current characteristic ofLEDs is far from linear. The result – depending on the potentiometer setting theLEDbrightness will hardly change most of the time as the pot is turned and a sudden variation at the end. The best method to tackle this problem is to power theLEDfrom a current source with zero to 100% adjustment range. The circuit shown here is an example. A low-currentLED(D1) is used to generate a reference voltage that’s first buffered by one half of an LM358. The actual current source that powers theLEDis built around the second opamp in the chip.

#### Figure 1 30-mALEDDimmer Circuit Diagram

The potentiometer allows the output current to be adjusted, with R2 acting as a current sense, the resistor dropping the same voltage as the one obtained from the pot. Using Ohm’s law we find that the maximum current through R2 amounts to about 29 mA (ILED= 1.6 V / 56 Ω). If necessary, the current may be adapted to suit otherLEDtypes, for example, 20 mA is obtained with R2 = 82 Ω and 10 mA at R2 = 150 Ω. It is also possible to connect severalLEDs in series.

The total voltage available for theLEDs is determined by the voltage drop across series resistor and the opamp, and, of course, the supply voltage. In this way, the highest number ofLEDs may be found fromULED, total = Ubatt – 5.1 V. In principle, it is possible to increase the supply voltage to 30 V in order to connect even moreLEDs in series. This does, however, call for the value of series resistor R1 to be increased to prevent overloading the low-currentLEDused in the voltage reference. If you intend to experiment with larger numbers ofLEDs (say, in arrays) then the maximum loading of the opamps becomes an issue. TheDIPversion of the LM358 may dissipate up to 830 mW. The power, P, is calculated from P = Ubatt – 1.6 –ULED,total × ILED,max.