Should you use a diode when using LED

[Buzzbomb]

Ok folks, I have started to tinker with LED for the engines of my 1/72 Viper.

I have a usefull book on this sort of thing but the author always uses a diode.

I can understand why (with my simpleton electonics knowledge) to ensure you protect your components.

But it seems to add unecessary complications with calcultion of resistors and the like. The Diode I have bought does not actually give you stuff like forward voltages etc etc.. I know that is my issue for not chasing this down, so some of the excellent online calculators are not much good to me.

So to explain.. I am putting in 3 x 5mm White LED in parallel (I hope) with a rating of 3.2v
Power supply is 6V (4 x AA batteries) I have calculated that the resistor value should be 27.7 ohms..
Using R=(6v-3.5v)/0.09A

Now putting a meter over this gives me something like 5.6 Volts... that is waaaaay too much for the LED. Now this is with the DIODE in the circuit.

My confusion is based on are my calculations right with a Diode in the circuit or am I a total klutz with my calculations in the first place.

Brian T.. the electronics first time dabbler.

[USS Atlantis]

Ok folks, I have started to tinker with LED for the engines of my 1/72 Viper.

So to explain.. I am putting in 3 x 5mm White LED in parallel (I hope) with a rating of 3.2v
Power supply is 6V (4 x AA batteries) I have calculated that the resistor value should be 27.7 ohms..
Using R=(6v-3.5v)/0.09A

Now putting a meter over this gives me something like 5.6 Volts... that is waaaaay too much for the LED. Now this is with the DIODE in the circuit.

My confusion is based on are my calculations right with a Diode in the circuit or am I a total klutz with my calculations in the first place.

Brian T.. the electronics first time dabbler.

.09A is 90MA - I don't know of any LED's that run that 'hot'. Every LED I deal with only runs 20MA (.02A) so re-calcing for that

(6-3.2)/.02 = 140 Ohms

Try that instead - or the next higher standard value.

BTW: The only time I use a Diode in an LED circuit is to modify a blinking pulse rate/load on a 555. Only adds complexity for a plain LED circuit - all you need is

Power supply
LED
Resistor

Remember that LED stands for Light Emitting Diode - so you already have a 'diode' in the circuit

Atlantis

[en'til Zog]

*blink* "diode"?

Hi Buzzbomb!

I've never used a diode - other than an LED - in my circuits. I do use polarized power connectors (fancy talk for a 9 volt battery connector). As long as you are sure of which connection is PLUS and which isn't you shouldn't need an extra diode. Unless you're a "belt and suspenders" sorta builder and want that extra polarity protection (which is not wrong at all, just a bit of overkill in most cases).

HTH!

[Buzzbomb]

See.. ask the question and you get the answers. That is why this community is so great.

Fantastic guys.. you have helped out this dummy beautifully

I will redo my practice circuit using your suggestions.

Thanks

Brian T

BTW that would perhaps exlain why one of my LED went Supernova

[jwrjr]

A led IS a diode. Is it possible that this is what you are seeing? "Common" leds don't run hot. However, very high power leds do normally run hot. Especially the "Luxeon" or equivalent types run very hot to the point of needing a heat sink. Figure that if a led uses 100ma or more, a heat sink will be required.

[en'til Zog]

Ah, the dreaded "supernova" effect.

That's usually caused by accidentally applying more voltage than the LED can stand, like the full power from a 9 volt battery. Diodes may not be required, but resistors often are, and occasionally the wrong end of the resistor hits the wrong leg of the LED and... POOF!

Ask me how I know!

[Buzzbomb]

Uh ohh.. what do you mean wrong leg of the resistor ??

I was under the impression that they were not polarized ?

I tried out a simple wire circuit with a what I thought was a 100 Ohm resistor (brown, black, black) and still got 5 + volts through to the LED end

So what I intend to do today is back to Jaycar (local electronics dudes) to get a 3 v battery pack to experiment with (plus get this Viper puppy on the road quicker) so that I will not "lose" any more LED doing experiments.

Anyway.. thanks for your help so far.

[USS Atlantis]

Uh ohh.. what do you mean wrong leg of the resistor ??

I was under the impression that they were not polarized ?

Just like any semi-conductor, LED's are polarized with a positive and negative lead. Standard now-a-days is that the longer leg goes to the positive side of your circuit and the shorter leg goes to the negative. If you hook them up in reverse, that could be why you're blowing them.

I tried out a simple wire circuit with a what I thought was a 100 Ohm resistor (brown, black, black) and still got 5 + volts through to the LED end

Ok, lets plug some stuff in the formula and check it

9v power supply
3.2v LED load
20 ma current draw at the LED

(9-3.2)/.02=290

Whoops - I miscalculated earlier - you need a 290 ohm resistor to drop the voltage to the 3.2 of your single LED - 130 for a pair of them in series

So what I intend to do today is back to Jaycar (local electronics dudes) to get a 3 v battery pack to experiment with (plus get this Viper puppy on the road quicker) so that I will not "lose" any more LED doing experiments.

Anyway.. thanks for your help so far.

Sorry 'bout that.

Place I go to for LED calculations is http://led.linear1.org/led.wiz, though that one says 330 ohms for that LED

I'd go with the higher value - the only thing it'll do if it's higher than needed is drop the brightness of the LED a little bit.

Atlantis

[en'til Zog]

"Wrong leg"... Well, usually you have "Battery - wire - resistor - LED - more wire - back to the battery."

If the leg of the resistor nearest to the battery touches the leg of the LED leg that is normally protected by that resistor, allowing the full battery power to flow into and through that LED instead of through the resistor and THEN the LED... That's the "wrong leg".

Or... "POOF"

Again....

That's one reason I like to use CMOS IC chips to drive LEDs - they inherently limit the current to the LED. Well, except for the 555 timer chip.

[Buzzbomb]

Thanks Clyde/Atlantis

That about sorts it out on the resistor stuff... it was more of a sanity check for the way I understood stuff.

Atlantis.. thanks again for checking this out and bearing with me.. I have never done any electronics before so I have been learning heaps. I do use the LED calc sites.. but now I have all the jargon sorted out.. I think I should now be better off, the whole power diode thing confused me a bit.

I think I will go the 330 ohms as well.. to be sure, to be sure

Come hell or high water I WILL get this working.. it has now become a challenge

[Madman Lighting]

This is why I invented my own widget for driving LEDs. I got tired of picking resistors, soldering em up, etc.

FYI folks, LEDs come in a huge variety of sizes, shapes, colors and current ratings these days but most of the "hobby" ones are still 20mA current rating.

I got to rig up a string of Luxeon K2 LEDs at work little while ago with a full range dimmer controller for one of our customers. Neat project! Those suckers pulled 750mA of current and I could PWM dim them down to 1/2000th of full intensity. They also put out about 16 watts of heat at full load! Nice handwarmer.

Of course I was using a Linear Tech controller to do it so it was lots of fun.

-JC.

[USS Atlantis]

FYI folks, LEDs come in a huge variety of sizes, shapes, colors and current ratings these days but most of the "hobby" ones are still 20mA current rating.

-JC.

True, oh wondrous lighting one, but when someone talks LED's and is asking basic questions, I'll assume (yes, dangerous, but oh well) that they're not talking Luxeon, Bi or Tri state, but plain, ordinary 2-3v 20ma single color.

BTW: Buzzbomb, there are several sources for LED's

My preference is http://www.unique-leds.com/index.php, they have a wide variety of high-output LEDs, all in 20ma ratings at reasonable prices

Resistors - local Radio Shack - get the 500 pack of mixed, that'll hold you for a long time

Atlantis

[Madman Lighting]

Atlantis Wrote
True, oh wondrous lighting one, but when someone talks LED's and is asking basic questions, I'll assume (yes, dangerous, but oh well) that they're not talking Luxeon, Bi or Tri state, but plain, ordinary 2-3v 20ma single color.

Sorry, sometimes I forget how it was when I was new and learning all this stuff.

Atlantis is right, there are lots of sources for plain, ordinary LEDs that run on 2-3V and 20mA. Just hook positive volts to the long pin, negative to the other and use Ohm's law to figure 20mA (or a little less) thru the part.

-JC.

[Buzzbomb]

Well folks.. thanks to all of you.. I have lights

No more supernova's yet anyway.

Just to confirm I am using ordinary old, garden variety, cheap as chips LED's.

Man I was happy to see lights.

Although now I have been let loose...................

Thanks

Brian T

[USS Atlantis]

Good to know you're "lighting the world" now

If you want a look at a more complex circuit, I have a schematic and an AVI file of the result on my Enterprise Build page Here .

Links to the schematic and AVI file are about halfway down the page

BTW: This was my first e-design after over 25 years of being away from both modeling and electronics.

Atlantis

[Madman Lighting]

Good to hear Buzzbomb has joined us "enlightened" ones

Atlantis: I checked out your TOS Enterprise page, neat bussard collector. Thanks for sharing.

-JC

[en'til Zog]

IT... IS... ALIVE!


YAYAYAYAY!


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