Dcc Lights in 00

Take a look at Express Models (in the UK). They have a number of kits for UK outline locos, and also sell the components for lighting etc.

No c> I would like to add lights to a loco using a dcc chip.

There should be some information with your decoder beware of overloading with too many lamps. LEDs will require a resister to limit current. As TTR mentions you can buy kits for specific models that just require connecting to your decoder, if you go this way check how many decoder functions are needed for a setup so that you get a decoder that can get maximum control of the lights.

Chris

Some decoders have a lighting output at 1.5V for LEDs.

Can one buy LEDs that will operate on 1.5 volts?

LED's normal voltage is 1.5 volts.

Wolf, I'm not convinced that you don't know what you're talking about here, so I'll try again.

LEDs operate by current, not voltage. They do inpractical terms have an operating voltage which varies, depending on colour and various manufacturing effects of between 1.4v and 2.4 or 3.5 volts. The greeny yellow and amber one will light at 1.4-1.5 volts but other more useful colours such as white and red require the higher voltages. Actual brightness is dependant on the controlled current of the output.

You say "1.5 volt decoder output" - is this a 2-3-4 volt current limited output suited to (an) LED or a voltage controlled output? Can one adjust the current output to vary the brightness of the attached LED(s) An uncontrolled current 1.5 volt output is only going to light up a limited variety of amber/yellow LEDs at whateve brightness they happen to achieve.

Regards, Greg.P.

And it will destroy them if it is not current limited.

Jeff

As I understand it, and as I have discovered, LEDs like all diodes have a rated voltage, and will blow if you expose them to much more than that voltage, which for most LEDs is around 1.5 to 2.5 volts. That is, the LED will block up to about the rated voltage. Since directional lighting circuits generally rely on the current blocking function of the LEDs, exceeding the rated voltage is a bad idea.

Also AIUI, the current capacity of an LED depends on its size. At any given voltage, the LED will absorb a certain current. At higher voltages, higher currents are possible, so again, it's a bad idea to exceed an LED's rated (or working) voltage.

Further AIUI the brightness of an LED depends on its composition, which also determines its colour. IOW, the amber LED will have a lower brightness regardless, simply because less of the current is converted in photons than in a green or red LED.

For the above reasons, I understand voltage to be the key rating for LEDs used in model railroading. A lighting engineer would of course be as concerned with current rating.

HTH

And some LEDs have the resistor incorported as well.

Chris

I think he probably means the TCS decoders with 1.5v outputs for those micro lights. As you say its the current that burns the LEDs not the voltage although very high voltage can. The 1.5v is the forward bias voltage of the diodes. But you can buy diodes with the current limiting resister incorporated in it.

Chris

I think there's something I've not understood.

Explanations welcome.

GOR, GOW, mini and micro bulbs generally operate on 1.5 volts. If you overdrive them (excessive voltage) they burn out quickly. If you underdrive them they will last much longer. (basic) LEDs require greater than their forward bias voltage to light up. The current at whatever voltage you apply to them must be controlled. The brightness of each individual LED can be matched by adjusting the value of the resistor placed in series with it. The current will be supply voltage minus forward bias voltage divided by resistance. The data table for each type of LED will tell you the forward bias voltage. Typically red is 1.4-1.5, amber a little higher and white about

Regards, Greg.P.

'Ta!

"> As I understand it, and as I have discovered, LEDs like all diodes have

No you are confusing 2 different parameters, all diodes have a reverse voltage rating, and a forward voltage voltage drop.

The reverse voltage is how high a voltage it will withstand when NOT conducting. This is important for rectifier diodes, but not relevant to LED's unless you are running them from an ac supply or, as you say, reverse biasing them for directional lighting. LED's only light up when they are conducting.

LED's do not have a rated votlage for operating at. The forward voltage drop is just that, how many volts dissapear in the diode. For a rectifier diode it might be 0.7V or even less, LED's vary with colour any may be anywhere from 1V to 3V. It also varies with current. Obviously the applied voltage must be greater than the voltage drop for the led to light.

LED's are when forward biased (ie lit) have effectively a low resistance and will try to take as much current as they can. So unless there is something limiting it they will take as much as the psu will give until they destroy themselves.

Correct

given voltage, the LED will absorb a certain current. At higher voltages,

No things don't work like that. If you have a simple circuit of a psu, a resistor and then an LED to ground what you will see is this.

If you measure the voltage between ground and the junction of the led and resistor you will *always* see just the forward voltage of the led regardless of the current through the led . (The forward voltage will change slightly with current but not by a lot).

The resistor is limiting the current through the LED. Lower the resistor value and the current will go up, the LED will get brighter, but the voltage across the LED will remain much the same. So you see you cannot, as such exceed the forward voltage of the LED it is just a parameter the the led has. Waht you must not do is put too much current through the led and exceed its forward *current* rating.

The situation is different if you reverse bias the LED, (not lit). Since virtually no current is flowing the voltage seen by the led is not limited by the resistor so if the psu voltage exceeeds the *reverse* breakdown voltage then you may damage the led.

More or less correct.

Voltage is of little consequence in the application of led's, current is the key!

You only need consider voltage this way: your supply must be greater than Vf (forward voltage) in order to get it to light. If you are reverse biasing it (directional or ac supply) Vr (reverse voltage) must be greater than the supply

Brightness is dependant on current so the series resistor must limit the current to a safe value, less than Ifmax.

Vf is of some use when you are calculating the value of the series resistor if you wish the set a particular current. Say you have a 12V, and an led with a 2V Vf. Subtract Vf from the psu voltage (12-2=10V). That will give you how many volts need to be dropped across the resistor. Then use Ohms Law to work out the resistor value for the current that you require. Let's say 10mA (0.01A) , so R=V/I 10/0.01 =

Hope this helps Regards Jeff

I forgot to add that the Reverse breakdown voltage Vr for LED's is greater than the Forward Voltage drop, usually in excess of 5V. If you do wish to reverse bias them, for directional lighting for example, then be careful not to exceed this value. If you do have a problem, the solution it to add a rectifier type diode in series with the LED. Rectifier diodes have Vr anywhere up to 1000's of volts!. Don't forget to allow for the additional forward voltage drop of the rectifier diode when calculating resistor values, this can be in the range of about 0.5 to 1V.

If you are running leds from a pulsed supply, (eg DCC) then they may not glow as brightly as you expect because of the pulsed nature of the supply. The max forward current allowed is usually higher for short pulses, so that may be taken into account.

Another way around it may be to add a series diode as above, and a large capacitor across the diode to smooth out the voltage seen by the LED,. This will also help if the led appears to flicker because of the pulsed supply.

Regards Jeff

It does indeed. Thanks.

No it won't. LEDs, just like any other component, have a well defined voltage-current relationship. Just look at the data sheet. 1.5V across an LED will not do any damage at all, regardless of the curremt limit.

MBQ

No it won't. LEDs, just like any other component, have a well defined

LEDs do have a relationship between forward current If and forward voltage Vf (the forward voltage drop increases slightly with current) but it is not a linear relationship. They also have an effective series resistance which also changes slightly with current.

It is the series resistance that limits that max current through the device for any applied voltage, unfortunately the resistance is low enough for the current to be well above the max forward current. In this respect LEDs are no different to any other diode, once they are in conduction unless there is some other limit on the current they will blow up!!! Running an led just in the area where they start to conduct may work, but it is very dangerous, any increase in temperature, spikes or voltage changes will take the led into an area where it will destroy itself. Also that point varies for led to led and colour to colour.

Regards Jeff

I have LEDs for coach lighting connected in parallel to 2xAA alkaline batteries housed in the brake van, with no resistors. They don't blow up, presumably due to the low current output of the batteries.