If the purpose of a resistor is to limit current, then it must be in series to a LED. Otherwise any single LED could thermal runaway and toast.

Here is an analogy....

Suppose you had three kids and you wanted give each $10 for the movies, would you put $30 on a table and trust them to take only their share? lol, no!

Yes, supplying each LED with its own resistor is going serial. Combining parallel AND serial is very common. My table has segments of 8 LEDs and a resistor all in series, but fourteen of these segments are in parallel.

There are many cases where you can get away with no resistors at all. Truth be told, resistors really do not control current (they only limit it for a given voltage). For real current control, something like a mosfet with a feedback loop is a way to go (they really are very cheap). But of course, the most efficient is connecting directly to a power source.

Its my opinion that resistor tutorials are for beginners, and as such should steer towards the safest design and most fool-proof (series, not parallel). This particular tutorial has many inaccuracies! For example, in series, there is no guarantee that voltage will be divided equally, only that each device will have the same current.

Honestly, it is not worth its ascii.

I highly advise either following a in-series resistor calculator, or learn basic electronics.

@ rojerj: Feel free to update the tutorial as you see fit, it is open for the community to enhance and expand. Obviously I am still learning and, on a side note, have no formal education in electronics. I recommend advising on the tutorial rather than here to start at the source.

O’Malley

This calculator is a great tool if you want different possible solutions to your setup

I have used that cal on my project. So far I have no problems with my LEDs.

I use that calculator, it saved my bacon.

Mine says to use 6 1-ohm 1/4v resistors. I already have a bunch of 100-ohm 1/4v resistors. This is probably a stupid question, but could I use those instead? Would the extra resistance make the circuits not work?

I think the most important point missed is that any circuit with LEDs must always contain at least 1 resistor, even if you have the correct voltage. Even when you have a “perfect” solution of a 12V supply for 6 LEDs of 2V, you should still include a resistor. This is because an LED does not limit the current running through it as a resistor does - and is to some extend what rogerj is talking about. For more detailed info, read here:
http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/1/

I won’t go as far as to tell people to avoid parallel at all but I would highly suggest doing this:

1. Make “LED-strips"(modules) of some number of LEDs in series with a resistor. For instance, if you have a power supply of 12V and 1.5V LEDs, put 7 LEDs(10.5V) in series with a resistor that cuts off the last 1.5V. This limits the current to some extend.
2. Put these strips in parallel on your power supply. Having 7 LEDs in series will most likely average out the small production differences and create pretty equal strips.

To me, this seems like the safest solution we can manage for this. As far as I can see, this is the method rogerj is also using. Correct me if I’m mistaken.

And no, it does not matter if you place your resistor before or after your LED. Think of current as a garden hose. If you slow down or stop the current in just one part of the hose, it affects all the water in the hose.

ALJ

that’s the best combination: to have 7 leds in series per strip (with one 18 Ohm resitor) and paraleled stips as many as needed

ironclownfish - 24 May 2008 09:48 PM

Mine says to use 6 1-ohm 1/4v resistors. I already have a bunch of 100-ohm 1/4v resistors. This is probably a stupid question, but could I use those instead? Would the extra resistance make the circuits not work?

Technically you could parallel 16 100-Ohm resistors for an equivalent of 6.25-Ohms, although this is pretty impractical.

It should be possible to chop the supply to the LED’s using a pair of 555’s and a power FET. Then you could use a pot. in the circuit to select change the duty cycle going to the FET. You would need to be well above the video frame rate to avoid aliasing. Does anyone have a circuit like this?

this might be something obsolete for most, but none the less… someone may benefit from it… i have my led’s powered through a usb cable… so no external or excessive wiring is needed… the process and materials needed are as follows : (x) leds, wire, solder, scrap usb cable, (x) resistors ( resistance dependent on your specific setup ), wire snips, wire stripper, and a few minutes…

process: cut one end off of a usb cable to expose the wires on that end leaving the other end untouched…
once wires are exposed you will need to locate the red and black wires, trim all others as needed ( you only need red and black for this purpose, the others are data )
strip the ends of the red and black wires, red=+ black=- if you didn’t know....
the standard for usb is 5v.  not alot, but eneough to work with for a few led’s, not sure how high you can go off of one, but i suggest if you are going to fun several usb cables, get a hub…

and as always, BE CAREFUL… you are messing with a direct line to your motherboard… i take no responsibility if you damage your rig, this is intended to be a rough guide, NOT A BIBLE, you will need to crunch out your own numbers if you plan to use this approach since there are so many different types of leds and what not…

good luck and hope this helps someone