What voltage are most hobbyist LED lamps rated for (is there a range
of voltages)? When wiring up an LED lamp I seem to remember something
about "of course, adding a (resistor? capacitor?) to the circuit" in a
discussion somewhere. Which, what values, and in parallel, series? Is
there a website out there somewhere with information about simple LED
circuits?

Hi there, Carl5219!

For a basic LED circuit, you will need a resistor to limit the amount
of current flowing through to prevent a short circuit.

I found an excellent web page describing this very well which contains
the following quotes:

"It turns out that normal wire has very low resistance. Often, its in
the neighborhood of .01 ohms, or even less. If you use a 6 volt
battery, and connect a plain wire across its terminals, then the
amount of theoretical current flow is Current = Voltage / Resistance,
or Current = 6 / .01 which equals 600 amps. Now, it turns out that
most batteries will only hold around 2 or 3 amps. This means the
battery will drain almost immediately, which is not good.
(...) you should never allow electricity to flow from one end of the
battery to the other without using some sort of resistance. If this
occurs, its called a short circuit, and will usually start to burn or
at least smoke badly."

So basically, the resistor is there to prevent the huge current
flowing straight from the positive terminal (+) on the battery to the
negative (-) which will cause a short circuit.

"(...) resistance is extremely important in our circuit. First, it
insures that we haven't created an electrical short. Second, it turns
out that the LED has some really small wires inside it, and cannot be
allowed to carry very much current. In fact, most LED's will burn up
if you allow more than 30mA to pass through them."

So the resistor will have two roles here.
One, to limit the current to prevent a short circuit and two, to
prevent the LED from overheating due to excessive current.
The LED has very little in the way of resistance by itself and because
the wire inside is so small, it will overheat in the same way as a
lightbulb and it will not lower the current going back to the negative
terminal on the battery.
Resistors are usually wired in series.

All quotes taken from:
http://www.nwlink.com/~kevinro/guide/electronics.html

A basic circuit can be seen here, taken from the above page:
http://www.nwlink.com/~kevinro/guide/images/elect2.gif

===========

There are several ways of calculating resistance and current (amps) in
a circuit.

Voltage = Current * Resistance
Current = Voltage / Resistance
Resistance = Voltage / Current

(For more on these, visit Ohmslaw.com: http://ohmslaw.com/ohmslaw.htm
)

So, for a 9 volt battery we can work out the following:

Current = 9 / 330 ohms which is .027 amps or 27mA.
So knowing that a typical LED will not handle more than 30mA, a 330
ohm resistor would be almost at the limit of a typical LED.

I also found a very handy resistor calculator for LEDs:
http://colourclassicfaq.com/general/led.html

===========

For example, have a look at the LED here:
http://www.allelectronics.com/spec/LED-94.pdf (PDF document)
It has a 2.4v max voltage with an average 30mA current rating (max
50mA).
If we use the resistor calculator I mentioned above, we need a 120 ohm
resistor.

But what if you only have a 1.5v battery?
Simple, using the Resistance method: 1.5v / .050 amps (50mA) = 30 ohms
resistor needed.

In general, most LEDs will be rated for a max of 5v but you should
check first, if possible.


I hope this helps!
Kind regards,
errol-ga.


Related Google searches:

"led voltage"
://www.google.co.uk/search?q=led+voltage

"led guide"
://www.google.co.uk/search?q=led+guide

"led circuit guide"
://www.google.co.uk/search?q=led+circuit+guide

---

Request for Answer Clarification by carl5219-ga on 25 May 2003 19:47 PDT

Great answer... if I might ask something for clarification... with
such a low/almost no, resistance it sounds as if the LEDs could be
wired in series just as well as parallel (in a multi-LED circuit) with
no decrease in output, unlike incandescent bulbs. Is this right?

---

Clarification of Answer by errol-ga on 25 May 2003 21:22 PDT

Hi!

There will be a slight decrease in output due to the fact that each
LED will consume *some* power but it won't really be noticeable unless
it's a huge array of them.
They only consume a tiny amount of milliamps each and have virtually
no resistance properties.

Try wiring up ten or more in series to see the effects.

errol-ga.

Great, just what I needed!

Many thanks for your generosity, Carl5219!

Answering this question was a nice refresher course for my electronics
skills so I am very grateful for your question.
errol-ga.