Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: tt1 on November 20, 2011, 09:41:52 PM

Title: Led problem
Post by: tt1 on November 20, 2011, 09:41:52 PM
Hello folks, have purchased a set of led nav lights, holders, and resistors as a kit.  The instructions state that the leds only require about 2.8 volts each therefore only one resistor is supplied to power 2 leds.  The resistors are shown as wired in the positive (longer leg) of the leds via a switch to the + battery terminal, they do not work vice versa.  The problem I have is by doing this, only the red lamp works and not the green, have tried swapping the wiring on the green with no joy, the red still works. When wired to a resistor on its own the green works ok, (doesn't seem as bright as the red but that may be just a visionary thing due to colour difference, not sure)

         Any ideas what may be wrong?  please keep it simple as I'm a bit of a dummy within the black art.  Oh, the resistors are all the same and the colour bands, are brown, black, red and gold - no idea which way round they're supposed to be read.  Any advice appreciated as always.

                                Regards, Tony. :-))
Title: Re: Led problem
Post by: wartsilaone on November 20, 2011, 09:50:10 PM
Green Led's do tend to be dimmer, not sure why. something to do with how they work. If you can get hold of some more resistors you could fit two to the green led to increase the current flow a little bit. This should balance the light level out.

Title: Re: Led problem
Post by: Tug-Kenny on November 20, 2011, 10:40:44 PM
Hi Tony

You wanted it simple so here I am    :}

1    LED's only work around 2.8 volts
2    resistors work either way around
3    Connect one short lead of the red to earth
3    told it was simple.  Connect the short green lead to earth
4    Connect the long Red lead to one resistor
4    Connect the long green lead to the other resistor
5    connect the two loose resistor wires together
5    connect this joined end to the positive of the battery

7   Watch them glow

ps   did I tell you I couldn't count     {-)
pps  the resistors are 100 ohms   but you didn't want to know that


Title: Re: Led problem
Post by: gwa84 on November 21, 2011, 12:24:02 AM
you could make it real simple and get one of thease   :-))

Title: Re: Led problem
Post by: tt1 on November 21, 2011, 04:46:19 PM
Thank you chaps, problem is Ken I'd still be using 2 resistors and they supply only 1 resistor per 2 leds. Very strange how the red one works either on its own or on a shared resistor, whereas the green will only work by itself on one resistor. No worries I'll either get another resistor or use G O W bulbs!  {-)

                          Thanks again and regards, Tony. :-))
Title: Re: Led problem
Post by: malcolmfrary on November 21, 2011, 05:33:55 PM
Last time I looked in Maplin, resistors were about 5p each. 
It is possible to wire the LEDs in series, and just use the one resistor.  Both get the same current that way, the green might not be just as bright, and this might not matter anyway.  If the two LEDs are wired in parallel off one resistor, bearing in mind that their voltages will probably differ, the one that wants the lowest voltage will "win", and glow.  Probably the red.  The other probably won't, or will be much fainter than its mate.  If the value of the resistor was calculated for one LED at whatever the su[[ly voltage was, it will not be able to offer the correct current to two of them.
Title: Re: Led problem
Post by: tt1 on November 21, 2011, 05:46:55 PM
Thanks for the explanation Malcolm, makes a lot of sense to me, cheers, Tony.  :-))
Title: Re: Led problem
Post by: Umi_Ryuzuki on November 22, 2011, 04:08:36 AM
Actually, yellow and red LED work on 1.8 - 2.2v, so they need a higher resistor at a given
voltage, than a green, or white LED, which require 3.2 - 3.4 volts.

That is why the green LED is dimmer when run on the same resistor as the red LED.
Because the higher resistance used to keep the yellow and red LED from burning out is just
a tad high to properly light a green LED, and sometimes is too high to allow a white led to run at all.

Title: Re: Led problem
Post by: ACTion on November 22, 2011, 08:55:42 AM
You're going to have to admit to yourself that the kit you bought is inadequate to the tune of one resistor. As for the E-Bay "kit", what a nice little earner! Three LEDs, three resistors and a Large Letter stamp for a fiver return - I wish I'd thought of that one.
The correct way is to use one resistor per colour at the very least, to give the right forward voltage for the LED. This value is R Ohms in the following simple equation:

               R = (S-F) x 50, where S is the supply voltage and F is the Forward Voltage for the LED.

For UltraBright types, F is 2.0 for Red and 3.3 for Green. On a 12v supply then, R for Red would be 500 and for Green would be 435. If the calculated value isn't listed then go to the next higher value. If the supplier of LEDs doesn't list the forward voltage then buy them from someone who does e.g. Component Shop.

For multiple LEDs I'd advise connecting them all in parallel with the power supply and using one resistor of the appropriate value for each LED. Multiple LEDs on one resistor can bring power considerations into play (bigger resistors).

BTW Doc, Maplin are currently charging 24p per 0.6W metal film resistor  :o now that's what I call a very  nice little earner!!

Dave M
Title: Re: Led problem
Post by: tt1 on November 22, 2011, 09:43:44 AM
 Thank you very much for the information Umi and Dave,  :-)) great tutorial as ever Dave, learning all the time  O0 {-)

                                                 Regards, Tony. 
Title: Re: Led problem
Post by: Tug-Kenny on November 22, 2011, 04:27:51 PM

I remember when they were a penny each.  cough, cough.


Title: Re: Led problem
Post by: Netleyned on November 22, 2011, 04:40:03 PM
3p from Component Shop
even less for bulk buys

Title: Re: Led problem
Post by: Subculture on November 22, 2011, 04:52:08 PM
There are two kinds of green LED, and they have different forward voltages.

The older type of green LED, has a lower forward voltage of about 2 volts. The later green Led's often written up as 'pure green' are the ones with the higher forward voltage quoted.
Title: Re: Led problem
Post by: ACTion on November 22, 2011, 10:27:54 PM
3p from Component Shop
even less for bulk buys
Yep - 3.30 for 1000....................  8)