ELM711
Example Applications (continued)
resistor (R) will have a voltage across it, and the total
current in the string will be determined by the voltage
across the resistor, and the value of that resistance.
Choosing a resistance value may require a little
experimentation, but to get you started:
a capacitor very close to the IC, much of the current
for these spikes can be supplied by the capacitor
rather than through the (inductive) leads of the supply.
The net result is that there will be less noise generated
at the supply by the ELM711. A value of 0.01µF works
well.
If you plan to use N LEDs in the string, each with a
forward voltage drop of Vf, then choose Vp such that:
We will leave the wiring details to you, but to get
you started, we have found that the wires for a small (2
to 3 foot high) tree should be about 5 feet long. Space
the LEDs about 12 inches apart, beginning at the end,
and install them by cutting a 2 inch piece from one of
the two conductors in the wire. When you bring the two
ends of the cutout together to solder them to the LED,
the other conductor is forced to form a loop, which
allows it to hang from a tree branch.
Vp ³ (N+1) x Vf
Then, given the LED current (I), determine the
value of R from:
R = (Vp - NxVf) ÷ I
For example, if you wish to have 4 green LEDs in
a string, then you need to choose:
Vp ³ (4+1) x 2.0
Vp ³ 10.0 volts
Enjoy the light show!
Since 12V supplies are very common, we will use
that. Then, you need to choose R, which requires a
knowledge of the LED current. A good starting point is
to use 9 or 10 mA for red, yellow and green leds, and
a smaller current (4 or 5mA) for the blue and white
LEDs. As this calculation is for a green LED, we
choose I = 9 mA (or 0.009 A). R is then given by:
Vp
R = (12 - 4x2.0) ÷ 0.009
N LEDs
R = 444W
While 444W is not a standard value, you could
select either 430W or 470W for the resistor and the
current would still be close to the desired 9mA.
Continue in a similar fashion for the other colours. You
may need to try a few resistor values for each colour in
order to make all the LEDs appear to be of the same
brightness, as your eye is more sensitive to some
wavelengths, and some LEDs are simply more
efficient than others.
R
4.7KW
2N3904
In this way, all of the component values can be
chosen and a circuit such as the one in Figure 4 can
be designed. Note that Figure 4 does not show a
transistor driving the white LED, as the wires to it are
short, and the resistor helps to limit currents should
some be induced. If you are considering very long wire
runs, the White LED should also have a transistor
buffer added in the same way as the others.
Figure 3. Driving LED Strings
Figure 4 also shows a supply bypass capacitor
connected across the ELM711 power supply pins. This
is to prevent problems due to the transients, or
‘spikes’, that are inherent in all digital circuit’s current
flow (due to the internal switching process). By adding
ELM711DSA
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