Re: LEDs in SERIES

From: John Popelish (jpopelish_at_rica.net)
Date: 06/23/04 

Date: Tue, 22 Jun 2004 20:09:04 -0400

C M Baker wrote:
>
> As I sit here at my computer, I have 6 LEDs, specification of which
> is: Forward Voltage 1.7v, Max If 100ma, running from a 13.7v supply,
> in series. According to calculations, current limiting resistor should
> be, (13.7 - (6 x 1.7)) = 3.5v using V=IR, 3.5/.1 = 35 ohms. To limit
> the current to approx 100ma, I have had to put a 100 ohm & 120 ohm in
> parallel,(to limit heat buildup in the resistors) giving a calculated
> resistance of 54.5 ohms. At switch on, this gave a current of 96.5ma
> which has gradually risen to 100.5ma during the last hour.
> My 'research' and my friendly man at the local MAPLINS store say that
> as LEDs are not straight forward resistive components like light
> bulbs, they don't follow Ohms Law directly, and all my testing on a
> bread board also indicates the same. There appears to be two 'camps'
> concerning series LEDs, one saying yes they do follow Ohms Law and one
> saying no they don't. Someone out there must surely have the
> definitive answer or formula. I am not trying to be rude to those
> emminent people who say they do follow Ohms Law, but why do they do it
> for you and not for me?

Ohmic resistance has a voltage drop proportional to the current
through the resistance, and that factor of proportionality (volts per
ampere) is called ohms. Nothing is precisely ohmic (perfectly fixed
proportion between voltage and current) regardless of the current.
Some materials are pretty good over a temperature range that is
useful, and they make resistors out of these materials. Any PN
junction (silicon, germanium or exotic materials like used in LEDs)
have a completely different relation between the voltage drop and the
current. Current through these devices is more accurately described
as an exponential function of voltage drop (or conversely, the voltage
drop is related to the logarithm of current) but this effect is also
dependent on temperature (as you found as your LEDs warmed up from the
I*V power they consumed minus the energy they emitted).

Here is a data *** for an LED that is perhaps similar to the ones
you are using:
http://www.vishay.com/docs/81009/81009.pdf

Figure 4 describes the typical voltage current relationship at room
temperature. Note that one of the scales is logarithmic and one is
linear. If a simple logarithmic relation between current and voltage
drop existed, this graph would be a straight line on this kind of
graph, and it is pretty close for very small currents up to about
rated current of 10^2 milliamps. For higher currents, the series
resistive drops of the wire bonds and the material on each side of the
junction starts to show itself, and the graph heads off toward a more
resistive line.

Figure 5 shows how the voltage drop associated with one point on the
line from figure 4 (the 10 milliamp point) varies with temperature.
You can assume that this variation is reflected all along the current
line (with small error). 

-- 
John Popelish
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