Project
by Mark Dobrosielski
Calculating Current
Limiting Resistor Values for LED Circuits
An LED is one of those product components that
just has to work. If I look at my computer from
across the room and don't see its LED winking
back at me, I assume it's turned off; I never expect that
the LED might have burned out. There's good reason for
that: When operated within specs, an LED has a lifetime
of 100,000 hours or more.
The key to maximizing LED life is limiting the current
that runs through it. This is frequently done with a
simple resistor whose value is calculated using Ohm's
Law. This article reviews how to apply Ohm's Law to
single and clustered LED circuits. I have also provided
an Excel spreadsheet to simplify — and speed up — the
process.
Single LEDs
When computing the value of a current limiting resistor for a single LED, the basic form of Ohm's Law — V =
IR — becomes:
Vbatt - Vled
R = Iled
where:
Vbatt is the voltage across the resistor and the LED.
Vled is the forward voltage of the LED.
Iled is the forward current of the LED.
resistor, and (Iled)2R is the power dissipated by the
resistor. Calculating the power dissipation is a step that
many people — hobbyists and professionals alike — tend
to skip. So, what do you call a 1/8 W resistor that needs
to dissipate 1/2 W? Charcoal.
LEDs in Series
The equation above gets only slightly more complicated when you connect multiple LEDs in series. The voltage drop across the LEDs increases, reducing the voltage
drop across the resistor. The current through the resistor
(and the LEDs) remains the same:
Vbatt - nVled
R = Iled
where n is the number of LEDs in series. Figure 1(b)
shows an example with three LEDs connected in series.
The voltage drop across the LEDs is three times the
voltage drop of a single LED.
LEDs in Parallel
If you connect multiple LEDs in parallel, the current
through the resistor increases (though the current
through each LED remains the same). The voltage drop
across the LEDs is unaffected, as is the voltage drop
across the resistor:
Figure 1(a) shows an example of a single LED circuit.
Incidentally, Vbatt - Vled is the voltage drop across the
Vbatt - Vled
R=
mIled
Figure 1. Simple LED circuits. (a) Single LED circuit. (b) LEDs in series. (c) LEDs in parallel.
F
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NUTS & VOLTS
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c
where m is the number
of LEDs in parallel.
Figure 1(c) shows an
example with three
LEDs connected in parallel. The current
through the circuit is
three times the current
of a single LED.
LED Arrays
50
If you connect multiple LEDs in an array,
JANUARY 2005
