Help me refresh my electronics knowledge

I want to put together a small lighting system for my parkflier. I bought a pack of LED's from Miniatronics Corp. It comes with 18 LED's and 18 470 Ohm resistors. The recommended voltage is 3 volts for the LED. It says you can use the resistors to prevent the LED from burning out higher voltages. I forgot how to figure out how to use resistors to limit voltage. If the receiver outputs 5 volts, and the LED needs 3 volts how many 470Ohm resistors do I need to limit the voltage to 3 volts. The only equation I remember is I=V/R but that doesn't seem to be enough to figure this out.

Reply to
Normen Strobel
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R=V/I I= V/R V=IR

Jack

Reply to
++++Jack++++

One should be plenty, and probably too much. I=V/R works, but you need to figure in a voltage drop of .7 volts across the diode. So you'd figure 4.3 instead of 5 for V. See what you get, but you may end up having to parallel two of the 470's, or use 220 or less, to get a good brightness level. One 470 is typical for 12V operation (about 25-30mA in this case). Do they give a mA rating for the LED's?

Reply to
John Alt

No they don't provide a mA rating for the LEDs. I wouldn't doubt if each color is different. From my tests at 3volts the LED's aren't very bright. Would they be brighter if I ran them at 5volts, or would they just die.

Reply to
Normen Strobel

Might die, but you can put 3 in series and try them that way. The voltage each would see would be just a little more than the 3 volt supply on one.

These are very low output units. If you are going to try night flying, you need to get something better. I've seen several guys using very bright white led's on the nose and tail and using red and green for the wings. Works real well. Try to find something in a wide angle.

I haven't used these, but here's a source

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Reply to
John Alt

You are on the right track, however you need to know the current requirement of the LED. Common or garden variety LEDs (ie. not high brightness ones) like 15-20mA. In determining V, you need to account for the voltage required at the LED, 3v in your case.

So to run a 20mA LED from 5v would be: R = (V-3)/I = (5-3)/.02 = 100R

If you only have 470R resistors, using 5 in parallel will get you 94 ohms. Could be kinda bulky though :)

Russ.

Reply to
Russ

Norm,

The resistors are current limiters. You should be safe using one resistor in series with the LED on receiver voltage.

Reply to
Paul McIntosh

Here is another good source for bright LEDs

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Reply to
Paul McIntosh

Not really for an LED. depends on the color. Up to 2V for some.

So you'd

Reply to
The Natural Philosopher

Run em at between 10mA and 50mA max. Use the resistors to limit the current when run from 5V.

If you have been sticking them across 3V and they haven;t blown up, my guess is they have resistors in already.

Reply to
The Natural Philosopher

Reply to
S. Boucher

Thanks this helps. I think I'm going to end up going to Radio Shack and find some higher output LEDs, and hopefully they will list the mA.

Reply to
Normen Strobel

With all do respect Paul, that is an incorrect statement.

LED's are current devices. Let us assume that the LED requires 10ma to obtain normal brilliance at 3 volts.

At 5 volts, 2 volts must be dropped across the resistor at 10 ma R = E/I or

2/.010

At 6 volts it would be 3/.010 ergo the resistor size varies with the supply voltage.

Reply to
w4jle

I understand what you say but you have to vary the resistance if you vary the voltage. Current changes with voltage.

Reply to
Paul McIntosh

you can run high output leds in serries like two and one resistor and have a more eficient system (less current draw (smaller batteries needed))

if they are 3volt leds two can be run off of a 6volt battery in serries and have great output (if they are truly "bright" )

also a note leds have a verry small angle of light pattern (like 15 degrees!!)

so a small prism or the like will help with dispurtion

Reply to
ed

Where do I get a small prism? I found Radio Shack had one wide angle LED but all the rest were narrow.

Reply to
Normen Strobel

Get a 1K pot and put it in series with the LED. then connect the battery and wind down the pot until the voltage across the LED is the required 3 volts. Measure the resistanace of the pot and get the nearest fixed resistor. Simple, ai'nt it! Andy from Oz

Reply to
Andrew Richard Jary

And after you cook the LED using this method, use a current meter and set the desired current flow for the LED you have, if you what maximum brilliance. Folks, the "D" in LED is diode and in the above scenerio once the forward drop of 1.x volts is reached it will remain basically constant until you destroy the junction. The 1.x forward voltage depends on the color of the LED as they are different.

Dan Thompson (AMA 32873, EAA 60974, WB4GUK, GROL) remove POST in address for email

Reply to
Dan Thompson

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