OK, I have been mucking about with electronics and stuf for years and
there is not much that stumbles me... but...
LEDs in series, OK? say I want to run 6 LEDs in series from a 12v
power source. if each LED has a 2V drop, that amounts to the 12v
supply... how do I limit the current to whatever I like?
bog standard question, driving me mad for a week or more.
LEDs, like all diodes, don't exactly have one standard voltage drop.
The forward voltage drop is dependent on the forward current, and your
2V nominal forward voltage drop at a nominal 20mA will be more like
1.7V at 1mA and 2.4V at 60mA. This is similar to a 1N4001, which will
have a forward drop of a little less than 0.6V at 1mA, 0.7V or so at
100mA, and more than 0.8V at 1 amp of forward current.
Let's assume you put your 6 LEDs in series across a power supply of
exactly 12V. Rather than being limited by a resistor, the forward
current will actually be limited by the increase in forward voltage of
the six LEDs in series. This might look like you can save a componet
here, except that the forward voltage of LEDs is somewhat chancy. It
changes with changes in temp and also changes over time. There is
also quite a bit of variation between forward voltage drops of LEDs,
even within the same manufacturing lot.
This means you're risking either to little light from the LEDs, or
excessive current (which can lead to early failure). Either way, not
the best solution -- even if you hand pick teh LEDs for an individual
As Mr. Popelish says, it's obviously best to use two strings of 3
LEDs, and then use a series resistor which will limit the current
through each string to the desired amount. Let's say you want 20mA
through each string, and you know the nominal forward voltage of each
LED is around 2V. Take your 12V supply, and subtract the 6V across
the three LEDs, Then use Ohms Law to select a resistor which will
take up the remaining 6V when 20mA is passing through it. That would
6V / .02A = 300 ohms, a standard 5% value.
If you absolutely have to have the six in series, you might want to
use a 555 voltage doubler to increase your nominal source voltage
across the LEDs to 23V or so, and then select a series resistor which
will do the job. Here's a good link to help with the voltage doubler: