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DafyddElvy

DafyddElvy

1:22.5 & 15mm Scale Trams, , NG Steam Railways
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It has been more years than I care to remember since I did electronics at college, HELP, PLEASE (shouting)

I have decide to try LED strips for the interior lighting in my trams rather than simply replacing the grain of wheat bulbs with single LED's as I have been doing.

The LED strips have arrived and they are a wee bitty too bright, not entirely unexpected. A quick search on the www and it suggests I simply need to use two equal value resistors to half the voltage, I have 22, 47 and 100 ohm resistors in stock, Q1. can I just use two of these or should I use 10k resistors as the sketch suggests?

The www sites various give resistor values in ohm, Q2. how the heck does a novice like me make sure I am getting the correct resistor when one site quotes things in a k value and the sales sites quote things in ohm values?

I have rigged up two LED strips with a volt meter connected, to measure the light out put at different voltages when connected to my old H&M controller, 5-6v will be more than sufficient for the Lux (impressed myself that I know what the measurement lighting intensity is :rofl: ) level suitable for the trams, so Q3. as resistors normally go to the positive leg of an LED do I simple take any two of the resistors I already have, feed + and - to each resistor and take the feed, per the diagram, to the positive leg of the LED strip?

I hope that all makes sense, thanks.
 

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Q1, LEDs are current based not voltage, so a single resistor will work.

Q2, 1000 ohm = 1k ohm, so that is quite simple.

Q3, Reference resistor, what is your input value, what are the LEDs rated at, there any resistors in the LED strips, how many LEDs are there. Add the correct value resistor to either leg.
 
JimmyB said:
Q1, LEDs are current based not voltage, so a single resistor will work.

Q2, 1000 ohm = 1k ohm, so that is quite simple.

Q3, Reference resistor, what is your input value, what are the LEDs rated at, there any resistors in the LED strips, how many LEDs are there. Add the correct value resistor to either leg.
Click to expand...
Jimmy,

This is everything I know about the LED strips I've purchased is detailed below, the plan is to use 2 of these in each tram

  • Brand:evemodel.
  • Size: LxBxH: 100 x 8 x 2.5mm,with cable 20 cm long.
  • Forward voltage:12V~18V, AC or DC Compatible.
  • Forward current (typ/max): 20mA/30mA.
  • Light Color: Warm White.

In put voltage will be 8 x rechargeable 2300 mAh AA batteries, which will also be powering 2 traction motors via a locoremote or R/C receiver, thanks.
 
8 X 2300 batteries = 9. 6 volts (nominal), I would try around a 360 ohm resistor, or use 3 or 4 of your 100 ohm resistors ( in series), and add or take one away as required till you are happy with the brightness.
 
What Jimmy says, LEDs work on current, so if you knew what milliamps you wanted, you calculate that simply with a "LED resistor calculator"

LED output brightness is NOT linear with voltage or current... so you will have to experiment to get the brightness you want.

Be sure to get the right wattage you need, or your resistor might get way too hot. (Again sites with calculators will often tell you the wattage necessary).

You need to experiment with resistors to get what you want, then double check the wattage.

There are MANY sites on this, and it really is worth a read so you "get it" to avoid mistakes such as blown LEDs or melted plastic car bodies.

Greg
 
This is going to make you mad, i'll bet.

Do you just want the answer or do you want to understand? There's really very little middle ground.

Learn Ohms law, under electronics, or get an LED calculator, really.

You could read this: LED Lighting Basics

Which included links to LED resistor calculators.

Learning this halfway is a waste of everyone's time, because you will have exactly the same question next time.

  • I and others have given you a simple way - experiment
  • You could "learn" by googling
  • I've given you a link to my site where you are given the basics
  • You've gotten a link for a LED resistor calculator (on my site), and there are many other sites to learn.

You will not hurt anything by adding resistors and experimenting, except possibly overheating the resistor.

Greg
 
Greg Elmassian said:
This is going to make you mad, i'll bet.

Do you just want the answer or do you want to understand? There's really very little middle ground.

Learn Ohms law, under electronics, or get an LED calculator, really.

You could read this: LED Lighting Basics

Which included links to LED resistor calculators.

Learning this halfway is a waste of everyone's time, because you will have exactly the same question next time.

  • I and others have given you a simple way - experiment
  • You could "learn" by googling
  • I've given you a link to my site where you are given the basics
  • You've gotten a link for a LED resistor calculator (on my site), and there are many other sites to learn.

You will not hurt anything by adding resistors and experimenting, except possibly overheating the resistor.

Greg
Click to expand...
Greg,

More years ago than I like to remember I studied electronic engineering as an add on to the mechanical engineering degree, not having used either in my career has led to me forgetting most of the things I sat exams for, all I regularly calculate these days relates to forecasting and paying contractors, and sometimes checking how much concrete someone has ordered.

Thank you for the LED basics web link, a question I suppose I need to ask myself is do I really need to relearn this information for my own modelling requirements.
Strangely I still remember Ohm's law, 746 is also a number that for some reason stays in my mind.

I suspect my inquisitiveness is going to get the better of me now their is a challenge to my remory, thanks.
 
It's really more straightforward than it seems.

But in your case, you do not know what resistor to add, because we have no idea what current the system needs for your desire.

(in this case we can sidestep the voltage vs current discussion on LEDs)

In actuality, you are reducing BOTH the voltage and current in the LEDs, but we just do not know the "target" current needed. (or the voltage).

So just try it, with resistors, and watch for heat buildup in the resistors as you add them to get to your desired brightness.

After you get that set, we can help by double checking the wattage necessary on the resistor, just to avoid overheating of the resistor.

I think this is the simplest and most straightforward path now, again I emphasize that by ADDING resistors, you cannot hurt anything.

Maybe get an assortment of 1/2 watt resistors and then you can experiment easily.

Regards, Greg, and thank you for taking this as sincerely trying to help you!
 
Another reason we are all making 'best guesses' is your '12V' LED strips, will already have a series resistor in their makeup, so we are already adding to what is there..

I would suggest using the 100 ohm resistors, to get an appreciable effect.

PhilP
 
I use a simple way to figure out the resistor needed is by knowing that 10 ma is 1 volt for each 100 ohms dropped for 10ma leds at full brightness and 20ma leds it is 50 ohms per volt dropped by the resistor for full brightness. Now increase the resistor ohms to make the led dimmer.

Better yet is I use the cl2n3 which always limits the current to 20 ma and I use 2 identical 10ma leds in parallel sometimes. Great for headlights and also for DC users as one does not need resistors and full brightness occurs at low voltage up to the max track voltage. To dim the lights, add a resistor in parallel to the led!!
 
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