piclist 2002\10\19\141702a >
Thread: NPN LED current drive
www.piclist.com/techref/displays.htm?key=led
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face BY : Wagner Lipnharski email (remove spam text)



Dave Tweed wrote:
{Quote hidden}

Just to add some old/new words;

As a LED switcher, a small transistor should not to be seen as a current
limiter or extra power heatsink, for this job a much lower cost resistor is
recommended, except of course if the supplied voltage is not known, so the
transistor should be used also as a current limit device.

As even unregulated power supply voltages can not change so much, few volts
as expected, the resulting current change will be in the same order, or
almost non perceptible by the user.  I would say that even when the
supplied voltage changes from 14 to 17V you still can use a simple current
limiting resistor in series with the LED and forgeting about constant
current controlling.

In this case, a small NPN transistor setup as a simple switcher, emitter to
ground, series resistor at base, led and resistor at collector will be most
than enough.

The good point of such simple and very common setup, is that you can
substitute the transistor by ANY logic gate (74HC00 family) without change
nothing, except if the supplied voltage is higher than the logic VCC, in
this case, a 7406/7 gate is in order.

The limiting current resistor calculation is easy as  (VPower - 2 / 0.01),
14V-2V/0.01 = 1kohms 1/4 or 1/8W is enough.

W46NER

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