Led lights

D

Dinas Bryn

Registered
Morning all, over the years I have put many LEDs in Building, street lamps etc., most of them outside and most have gone through winters here in North Wales, with a new extension to the layout in the garage I used all my stock and bought another 20 sets from Maplins over the last 2 weeks, ref N27FN 5m LED and ref M43R Min Res 43R, I soldered up as normal testing as I went along all failed, I took most of then back and 3 chaps checked them, I was found to have been soldered wrong and would have blown the rest, they loosely put a set together ( out of the remainder I hadn't used) and it worked, I came home and did the same, I then put the lamp together and like the rest blow after 2mins WHY any ideas Les
 
Insufficient information really Les..
?Single LED's and series resistor??
What voltage are you using??
Is each resistor + LED across the supply??

My guess is your resistor is way to low a value.. Would have thought with the voltages we tend to have about the layout, you would need 430 ohm minimum.

Phil Partridge.
 
PhilP said:
Insufficient information really Les..
?Single LED's and series resistor??
What voltage are you using??
Is each resistor + LED across the supply??

My guess is your resistor is way to low a value.. Would have thought with the voltages we tend to have about the layout, you would need 430 ohm minimum.

Phil Partridge.
Click to expand...
Hi Phil, I am using 12v DC meter shows just over 10v, 1 resistor per LED, all other LEDs are on the same wiring with no problems Les
 
Resistors are too small

led you using has max forward current of 100ma, with a max forward Voltage of 4v

Calculation at 12v dc would give resistor Value of 82 ohms

Would suggest 100 ohms for safety
 
Sparky I am using 560 ohms with a 5% tolerance for a single 12 vlt led. Now from that statement, it might sound like I know what I am talking about! But, I don,t. So, from what you have said previously, am I using too big a resistor?
DB no trying to hijack your thread mate. Only I appear to have the same probs. as you from time to time.
 
resistor values depend on the forward current and forward voltage of the LED

Both you can get from Manufacture specs

Then you can put them in something like this http://led.linear1.org/1led.wiz

Different colours have different specs, Whites for example usally need more current
 
Yeah, ordinarily you'd assume something like 15mA and 2V forward voltage for a typical LED, so my initial reaction on reading the original post was that 43 ohms is 10x too low for a 12V supply, no wonder they blew!

As these are hi-brightness LEDs you can run more current if you do want full brightness. 100mA still seems a lot though (more like a bulb rather than an LED), and soon adds up to a lot of load on the power supply. I'd probably go a bit higher than sparky and try 180 ohms resistor, which would allow about 50mA through the LED at 12V. If that's not bright enough then try lower values down to the 100 ohms as suggested.

Here's the online resistor calcuator I've used in the past: http://led.linear1.org/1led.wiz <edit> ah - the same as sparky! </edit>

It's worth buying a stash of various resistors and trying values a little higher than the usual calculation, so you can limit brightness when you only need a subtle glow.

@Rod: 560 ohms is reasonable, that works out at around 20mA for a typical LED at 12V.
 
Does a simple idiot style guide exist on LED's and on various ways we can use them on stock or buildings?
 
Mij said:
Does a simple idiot style guide exist on LED's and on various ways we can use them on stock or buildings?
Click to expand...

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

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

You may click on any question mark on these links to obtain more info. if needed.

;D
 
For those who want a bit of maths:
Ohms law V=IR
V = voltage (in er volts)
I = current (in amps)
R = resistance (in ohms)

So 2V across the LED, supply of 12V: 12-2 =10 (volts across the resistor).
For 20mA forward current (0.02 Amps)..

Re-arrange the above formula to give the resistor value:

R=V/I

V(across the resistor) =10 Volts, divide by I=0.02 Amps gives a value of 500 ohms..
Nearest standard values are 470 ohms (brighter) or 560 ohms (slightly less bright).

Knowing how Power Supplies tend to give higher than rated outputs, I would use 560 (or even 680) ohm resistors.
 
PhilP said:
Knowing how Power Supplies tend to give higher than rated outputs, I would use 560 (or even 680) ohm resistors.
Click to expand...

Only too true. Never ever for example jump to the conclusion that a 12v source such as a standard power supply or battery pack built to general or specified "hobby" standards of any type, actually supplies a maximum of 12v. The best advice is to use 13.8 volt as the basis for all so called 12 volt non laboratory standard supplies. Even if your trusty Dmm or Amm actually show a reading of 12v when the probes are put across the terminals of a live power supply. Most are built to a price and not a standard. So quite simply on the basis that 15% headroom is the industry standard for calculating voltage peak fluctuation of the generic 12V power supply, it is a very good idea to use that as an addition to any known low voltage application for the purpose of in circuit resistance calculation. Also do bear in mind that resistors have a standard error stated by manufacturers that is an average figure of limited batch samples taken and therefore theoretical. Even if deadly accurate a 5% resistor tolerance on 680 ohm is a figure anywhere between 714 -646 ohms. Interesting where much higher resistances are involved like 50k.

Finally just to make life interesting warm up a standard type resistor to "hot to touch" temperature while under load and measure the resistance drop off across the circuit and consequent increase in voltage drawn by a variable load when compared to cold. Resistors of course are designed to get warm as their way of dumping energy. Running higher power LED arrays on an aquarium using ally finned heatsink section, it can get hot enough to blister skin.
 
There are apps you can get for android (smart phone or tablets) or calculators to download on line. I use this one as it allows for all parameters and connection types;

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

I have found by way of experimentation that 10 mA current works well especially for white leds, it imparts a nice "tungsten colour" and gives a better "scale illumination". You do not want a G scale headlamp illuminating 3 G scale KM in front of it do you? I also bought a LED tester on eBay for $2 it think it was, this is a good little bit of kit it lets you plug in any led any current and WYSIWYG... Using as low current as you can get away with will also extend the life of the led which is a consideration if they are encapsulated or hard to replace.

http://www.ebay.com.au/itm/UN3F-Mini-Handy-LED-Tester-Test-Box-2-150mA-for-Light-emitting-Diode-Bulb-Lamp-/281208792656?pt=AU_B_I_Electrical_Test_Equipment&hash=item417959a650&_uhb=1
 
Another thing that I have found most helpful, particularly before soldering up. I have a Test Rig that I got from Maplin. This consists of Circuit runs with lots of holes that you can press Resistors, Diodes and LED`s in. You can also get pre made up wires that can be plugged in. I used this to sort out LED Lighting Circuits in one of my Locomotives and it has been used many times since. Searched for it on the Maplin Site, but not sure what they call it. Perhaps this pic will help, sure similar things will be available Worldwide.

image.jpg

JonD
 
Try searching for "breadboard" - hours of fun have been had with that stuff in the past - and saved a few £'s by avoiding mistakes.

Dave
 
DaveB2 said:
Try searching for "breadboard" - hours of fun have been had with that stuff in the past - and saved a few £'s by avoiding mistakes.

Dave
Click to expand...

Thanks Dave, thats the stuff. Cheep enough for us impoverished G Scalers.
JonD
 
Many thanks for your help but at 81 isn't there a simple answer I could go back to Maplins with, you mentioned heat, I cover both leads from the LED with sleeves and everything is very hot when I take the light out Les
 
I've done some experiments this morning as I had some white LEDS the same part number from maplin

I used a 13,8 Volt bench supply, a 100 ohm resistor gives just under 4 volts forward voltage and a very bright LED, in my mind too bright

200 ohms gives a nice bright lights with a forward voltage of just over 3 volts

300 ohms yellows the LED and the Brightness considerably

250 Ohms gives a Moderate Light still white with a forward voltage of 2,7 Volts

Hope that helps

I only had 100 ohm resistors in hand so i used them in series or parallel for the trials
 
BUT, we perceive different colours as different levels of 'brightness'..
So, you can compare the 'MCD' ratings of red LED's against each other, but a green LED of the same MCD rating will not appear to the eye as the same level of brightness.

Do NOT get me started on colour temperatures and (supposedly) 'warm-white' LED's! ;) :D
 
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