For the sake of my education let me ask. Doesn't an LED have a
predictable voltage drop across the junction? I seem to recall 1.5
volts is that correct? If it is I only need as many LEDs in series as
will equal the voltage and they will need no external resistance. Is my
The voltage-current characteristic of an LED has a pretty sharp "knee"
characteristic of most diodes. Very little current flows as voltage
increases up to the turnon voltage, and then current rises very rapidly
with very little change in voltage. The actual voltage depends on the
type of LED (different colours use different chemistry and have
different voltage) and temperature.
LEDs are easy to drive from a constant-current power source. You set
the current you want, and the power supply adjusts its voltage to
whatever is needed to make the LED draw that amount of current. Put a
bunch in series, and all get the same current.
But most power sources are closer to constant-voltage instead of
constant-current. It's nearly impossible to set the voltage needed for
a particular current, since a small voltage change results in a large
current change, and the voltage needed depends on temperature (which
depends on current among other things!).
So you generally use a resistor in series with the LED, or string of
LEDs. The power supply voltage is equal to the sum of the voltages of
the LEDs in the string, plus a few extra volts. The resistor is sized
to give the desired LED current with the "extra" voltage applied across
it. This gives a system where small changes in LED turnon voltage
produce small changes in the voltage across the resistor, which produce
small changes in current through the LEDs. Without the resistor, you'd
get large and unwanted changes in current.
As others pointed out, it is possible to build a system using only a LED
and a battery *if* the battery's own internal resistance is enough to
stabilize the system.
A LED acts as a voltage regulator (regulating the voltage across it to
about 2V). It will draw as much current as needed to do this. If the
LED is across a 12V battery it will "happily" destroy itself trying to
maintain 2V across that 12V battery. I made that mistake once.
Immediately, there was a POP and half the LED disappeared.
If the supply voltage is below that, you don't get any light at all.
The flashlights that use LEDs without resistors use batteries that
cannot supply more current than about 20mA. The nearly-infinite load
from the LED lowers the battery output voltage until it's below that
which the LED will conduct. Then the battery recovers and the cycle
That is, you need a battery with limited current capacity. You're
using the battery's internal resistance (Rth, if you care) as the LED
LEDs in parallel with each other? It's unlikely that all would light,
since the threshold voltages would be slightly different, and the one
that's lowest would prevent the others from lighting.
As someone said earlier, your LEDs probably are actually LED modules,
and come with built-in resistors. You have a separate resistor in
series with each LED.
Modern LEDs can appear very bright. I noticed that with the holiday
lights I had this year (yes, I know that's "last year", but it is
still less than 2 months ago). Those LEDs look brighter than the
If they're all from the same batch, they're probably pretty well matched
in forward voltage, and it only takes a little bit of series resistance
within each LED to approximately balance the current between LEDs.
This isn't a *good* way of connecting multiple LEDs, but it's not
automatically doomed to failure.
The little 1 W and 3 W LEDs appearing in flashlights are now brighter
than anything I ever saw from any incandescent bulb in the same size of
flashlight (2 AA battery).
These are "3V LEDs" provided for a school project. These would
seem very likely to be LEDs designed to operate easily off of
3 volts, since LEDs are rarely described as "X volt" in any other
circumstance. Certainly not at the "consumer" level.
Nothing wrong with that as long as they are in series.
No. a good mix of math and science.
Go back to Ohms law. 2X resistance = 1/2 current
Aahhh... Remember that we've been talking incandescent light bulbs.
A LED is a diode and not a bulb even though it gives off light.
Although the same principles apply since LED's use a dropping resistor
to limit current, similar to items in parallel.
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