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 connection, other than as a satisfied customer etc.
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.
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
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.
<> 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.
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.
The low current output of the batteries is due to their internal
If you ever have to replace the batteries you had better be sure of
their internal resistance, as the efficiency of batteries is continually
being improved. (ie lowering internal resistance)
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.
"> 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
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
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
You only need consider voltage this way:
your supply must be greater than Vf (forward voltage) in order to get it to
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 (12-2V). 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 1000ohms or, 1kohm.
Hope this helps
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.
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.
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
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
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
A 1.5v output would probably be intended for micro bulbs.
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