What value resistor to use.

If you ask your Club Guru what value resistor to use to ballast an LED, you will get an answer like, "It's simple, use Ohms Law to work it out. R=V/IV." etc. The Guru will not notice your eyes glazing over and so you depart, none the wiser.

Rejoice, Bretheren! I have the answer.

LEDs come in various brightness ratings (MCD) stands for milli candelabras or such like.  What we need to know is what size resistor fits what.  If we want outside lighting, we can use 600 MCD upwards.  If we want signals, we will use the common garden, say 3mm at perhaps 40 MCD - it doesn't matter.

It doesn't matter what voltage you have in your power supply.  It will be 12 to 14 or so.

First we need to know if there is still life in that tiny plastic body.  On the market is the Acme (road runner) LED tester.  Here is one I prepared earlier.



Simply bung the LED's legs into the holes and if it lights up - we're away.

The next piece of equipment is a rotary resistor, or thingamyjig.  It is about the size of a lady's powder compact, but so people who see you with one of these won't think you are a Nancy, the manufacturer has attached a black and a red wire with alligator clips.



These also have another more sinister (left handed) purpose . . .  If you clip one alligator to one leg of the LED, and the other leg of the LED to the appropriate polarity of your power source, things will begin to happen.

You will notice that the arrow is pointing to the highest value (1M).  Rotate the dial in clicks until the LED is glowing at the level you desire.  You can go up or down, comparing brightness(es) until you are happy.

Read the value that the arrow head is pointing to, and that is the value for that LED at that voltage.

You will note that the lowest value on the hooziewatsit is 5 ohms.  Practice turning the gizmo so you NEVER turn that way at first.  You may find that the special white smoke inserted in the LED by the Maker at the time of manufacture comes wisping out - and it's the very Devil to get it back in!

The thingamajug and the watsicallit should be available at your local electronics store.

Hope this helps. Max

Max you are a pal, i have been struggleing for ages trying to work out resister values
guess who is off to the shops right now!
:cheers;-):cool:

My pleasure. Owen

Any chance of links where we can buy these items Max?

That will take some work from here in Oz, Bob. I'll let you know. In the meantime, maybe Owen will find them.

Their trade names should do Max as I can then google them

OK I have found the LED tester here at £4.95  :

http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Test-Equipment/General-Testers-Meters/LED-Tester/32606/kw/

Bob is right Max,maplins & rapid do the led tester £6.99 & £ 4.95 respectively
but no one has the resister thingy it may be german.so a name or model would help.
lol::cool:

OK, so far so good, so is the standard LED 10ma ? (meaning that we should use this "hole" in the tester to test our LEDs?

Jeff, looking at the data contained in some Electronic firm catalogues, if we consider Diffused 3 & 5 mm LEDs, red is 15mA, while yellow is 20, orange & green are 30. That is why using a common resistor for both red & green in a colour light signal, Red seems very bright. I would use the 20mA socket.

The Ohms Law method - I would use the next highest value of resistor = for instance if the formulae came out at 560 ohms, I would use 620  & err on the side of caution.

Jeff. Because there are so many varieties of LEDs, it's possible you may need a different value resistor for red and green. It doesn't matter which holes you put the legs in to test for signs of life as none of them has the power to blow the LED, and because you are hooked to a separate little circuit within the tester, you don't get a sensible idea of how the LED will actually look. To decide which resistor to use, I use the rotary, connected to my layout power source. Mine is 14v and 1k resistors do it for me. They are a good balance between brightness and longevity. Hope this helps. Max

well i have trawled the web (incuding oz )and no luck with the
rotary job.

Jeff, an easy way to test led lights is use a 9vlt battery with a 1k
resister on the pos+ side thats how i do mine.
:cool wink:cool:

Hi
The box is called a Resistance Decade box.
They are often quite an expensive bit of testing kit!  
Rapid do a multi knob version for around £42GBP
http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Test-Equipment/Substitution-Boxes/Resistance-substitution-box/71538/kw/Decade%20box
Lots of other supliers though. Try a Google search for 'Decade Boxes' or 'Resistance Decade Box' etc :-)

Nice one Flashbang :exclam:exclam The Rapid version is quite expensive, but with your information we may be able to come up with a cheaper alternative.

This will give a ball park value.

 

http://led.linear1.org/1led.wiz

My LED tester is a rotary switch with each terminal on switch connected to resistors  of various values from 330 ohm to 4.7k ohm.

 The tester is perminantly connected to my DCC test track 14.5 volts. My main layout is also 14.5 V at track

 No matter colour or type the Led is plugged in, DCC system turned on, rotary switch is tuned to each possition untill Led is glowing at desired brightness.

 Cheers

  Ian