A stupid question about LEDs

[Martino]

Hi everyone.

I need some help because although I have a basic understanding of stuff electrical , LEDs are a complete mystery to me.

I run the South Bucks Light Railway on track power DCC, so I guess about 20-24 volts DC is in the track at all times. When I have illuminated buildings, I just put a (LGB, because I bought a load cheap some years back) bulb inside and wire it to the track. No problem, and it's nice and simple.

I've just got a couple of Roundhouse gas lamps and they come with 12 volt warm LEDs, with a resistor. Now, I'm pretty sure I can't wire those direct to the track without blowing them.

What to do? Can I get an out door transformer and hide it in a building? Can I get a widget that will allow them to plug into the track ? Can I buy 24 volt LEDs that I can just plug in?

See, told you it was a stupid question!

 

[GAP]

This what I am considering to illuminate my layout.
http://www.jaycar.com.au/productView.asp?ID=MB3501

Basically I will use a 12V sealed lead acid battery for my power source, hidden in a building, and this to keep it charged.
A workmate is using one of these to keep his ride on lawn mower battery charged and he is impressed with it.

I live in Australia so similar climate and amount of sunshine as Florida

As for calculating LED resistor values I use this site, it does everything for me I use 20mA as my current value (generic value).
http://led.linear1.org/1led.wiz

I'm not 100% sure about DCC but I think it is effectively ac so if you tap of and rectify the voltage then you will have DC.
Others will have abetter understanding.
If it is ac and you full wave rectify you will have to multiply the voltage by 1.414 to get the equivalent DC voltage and use this voltage in the calculator to get the resistance value.

 

[mike]

a old car battiery charger sat in a dry shed with a 12 volt cable running to the buildings and street lamps is what i use.. i leave the complicated stuff to the experts as i just like mine to work. and theres no such thing as a stupied question..

 

[tramcar trev]

No such thing as a stupid question...
Somewhere in here I posted some details of a constant lighting circuit using an LM317??? voltage controller. Very simple to make can be made just soldering the component leads together and it can be set using a couple of resistors to set the output voltage and it can be run of ac/dc up to 48V from memory. I used this to run 2.2v LEDs ( 9 of them in my tramcars) from the 12 v supply and this would work for your situation and supply 12 v from the 24 v with little loss and no stress about keeping batteries charged etc.
Ok found what I was looking for. Have a look here at this calculator; http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
It shows how to set this up the 3 legged chip and 2 resistors and thats it. The LM338 may be better for high current you can buy 10 here they handle 5 amps which would mean to draw say 500ma from them would let you get away without heat sinking them http://www.ebay.com.au/itm/10-pcs-L...ultDomain_0&hash=item3f1146243d#ht_3470wt_905

Its not as complex as it looks and it would be best in the long run for a permanent instalation.

 

[Neil Robinson]

Just to add to any confusion here's another option.

The circuit diagram for the as supplied 12V DC version will be as shown in the the top image. The resistor may be in the other, negative, lead it doesn't matter.

To change it to work on 24V AC or DC make either of the changes shown. One is if you can't easily access and change the provided resistor, the other allows for replacement of this resistor if it is accessible.

The 1N4148 fitted in the direction shown allows for AC operation and also protects against accidentally reversed DC connection.

Edit, I've just read Nick's subsequent post. I agree with his comments on higher value resistors. The values I quoted are, in my opinion the lowest that may be safely used. For less bright (but probably not noticeably so) leds use higher values.

 

[ntpntpntp]

Are the resistors that come with your LEDs separate or pre-wired? Do you know the supplied resistor value?

If they're separate, simply replace with larger value resistors. Use one of the online resistor calculators are suggested in the previous posts. I reckon you're looking at at least 1200 ohms, maybe as 1500 ohms.

If they're prewired you can simply add another resistor in series to handle the extra 12 volts, say at least 680 ohms, maybe 820 ohms or 1000 ohms.

Resistors are so cheap you might as well get a few different ones around those suggested values and experiment - always start with the highest value and if too dim try the next lower value. Use the highest value you can get away with.

 

[Martino]

Very many thanks everyone. I'll digest that lot and try, probably, everything!

I'm really grateful. Just look out for the large blue flash just south of Alabama!

M