Model Railway Forum

Hi All,

I'm starting to build a control panel for my layout. A lot of the points are wired up in pairs so I'll only need a handful of LEDs I think. I see other peoples wiring diagrams where it says you need a 12V supply and then a resistor, the explanation always says that the resistor you need depends on other factors.

But I can't find an "idiots guide" on how to work out what I need.

If it matters, I will be using SEEP PM1 Point Motors to power the LEDs. That way I only need 1 PM1 and 1 PM2 per pair of points.

Thanks in advance

Hi
You can use any DC voltage but whatever voltage is chosen do try and obtain a power supply that's regulated. That means regardless of load the output is constant (up to the maximum rating of the power supply) A 5 volt phone charger can be used where not too many LEDs are to be lit all at once.
12v DC is perhaps more common and power supplies here are often listed as for use with CCTV etc Example 12v power supply

You should always fit a series current limiting series resistor. So...
To calculate the series resistor needed use this web sites calculator http://www.hebeiltd.com.cn/calculator/v5/led.php You will need to know the supply volts, the LED rated forward volts, the LEDs rated current in millamps and the number of LEDs to be lit by the one resistor (usually one LED). As a rule of thumb work on red/green/yellow LEDs as needing 2.2volts and an operating current of 10ma. White LEDs increase their volts to 3.5 at 10ma.

I find on 12v DC a series resistor of between 1000 (1K) and 2500 (2K5) Ohm at 0.25 watt is ideal. Note the higher the resistor value the dimmer the LED will become, up to a limit where the LED will eventually not light at all. I have used 10000 (10K) at 12v and still had the LED lit!
Note If the LED is sold with a rated working voltage i.e. 12volts, then no resistor is needed, as one is factory built in.

Andrew_ECML wrote: ↑Thu Aug 10, 2023 10:04 am <SNIP>
If it matters, I will be using SEEP PM1 Point Motors to power the LEDs. That way I only need 1 PM1 and 1 PM2 per pair of points.

If you have one PM1, which has the built in change over contacts, and a PM2 with no contact on a crossover pair of points you will only indicate that the one point has moved correctly! The PM2 may not move or only partly move and you would still get a lit LED showing the route is set via the PM1. Its better to have both motors/points proved with separate LEDs then you will get true indications.

The info you need is on page 3 of the electrical section of this link.

https://www.brian-lambert.co.uk/Electrical-Page-3.html

Thanks guys. You're brilliant.

This calculator especially was the best. http://www.hebeiltd.com.cn/calculator/v5/led.php