Model Railway Forum

I am just wondering if anyone can help me I'm going to put some led lights on my layout im going to power them from a regulated power supply which you can set the voltage on from 3v up to 12 v i was wondering if i need to use a resisters i have ordered some power distribution board things on the internet to connect them to i wanted something like the woodland scenics just plug system which is a bit expensive oh the led's i bought are 3v thanks lorraine

Always fit a suitable resistor to each LED. I am familiar with the distribution boards you describe and even though some can set the output voltage, the resistor will prevent any damage should the voltage be exceeded.
This calculator may help you work out what value you need to use. In practice you may find that the recommended value will leave the LEDs too bright so some experimentation may be required to get the right result.

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

The accepted practice is to use a higher voltage and reduce the voltage to the LED with a resistor. The function of the resistor is to limit the current without it LEDs can experience what's called thermal runaway which will limit their life even with the correct voltage applied. In many cases on layouts most modellers find LEDs working in their normal operating range are too bright and the use of higher value resistors than recommended or calculated are often used.

The next consideration is how many resistors? Typically one per LED is recommended, this does have the effect of more current drawn in a circuit with multiple LEDs but the advantage of one failure does not affect the others. LEDs can be linked in series and use one resistor to control the current, strips of LEDs are a good example this tends to draw less current so more can be run off a given PSU but one failure will affect the others in the circuit.

A useful calculator can be found on;

www.electronics2000.co.uk

Richard

Thanks for your quick reply i wil use them then
Lorraine

Tricky Dicky wrote: ↑Tue Apr 30, 2019 1:53 pm The function of the resistor is to limit the current without it LEDs can experience what's called thermal runaway which will limit their life even with the correct voltage applied.

The function of the resistor is ONLY to limit the current. The notion of simply applying the 'correct voltage' is false and should never be employed as a technique. The voltage quoted is a consequence of current passing through the LED, not the other way round. Its value depends on the internal physics and chemistry of the LED itself, which is a non-linear semiconductor device.

Gordon H wrote: ↑Thu May 02, 2019 6:21 am

Tricky Dicky wrote: ↑Tue Apr 30, 2019 1:53 pm The function of the resistor is to limit the current without it LEDs can experience what's called thermal runaway which will limit their life even with the correct voltage applied.

The function of the resistor is ONLY to limit the current. The notion of simply applying the 'correct voltage' is false and should never be employed as a technique. The voltage quoted is a consequence of current passing through the LED, not the other way round. Its value depends on the internal physics and chemistry of the LED itself, which is a non-linear semiconductor device.

Exactly the point I was trying to make but keeping it simple. The OP mentioned 3V LEDs presumably referring to Vf and mentioned a PSU capable of providing 3V the implication being could she simply connect her LEDs directly to the PSU and as you say and I said a resistor is required to limit the current. Simply providing Vf or as I put it "correct voltage" to keep it in simple terms was not enough.

Richard