I will be using LEDs on my control panel for turnout indication. I’ve asembled a one LED test circuit - I’m using Peco turnouts with momentary switches, and latched relays with separate 12VDC for the LEDs - and it’s working. I’ve got a 1k resistor for the LED
The questions: There will eventually be 30-40 LED indicators - will I be able to run this many in parallel off the 12VDC bus? Will the LEDs get dimmer as I add more? Shall I remove the resistor?
I appreciate there have been a few posts about LED lighting but I can’t seem to find the specific answer for this.
the amount of led’s is only dependant on the AMPERAGE of the power supply, not the voltage. a 10 amp supply will power twice as many as a 5 amp supply.
The average LED generally pulls somewhere in the range of 20-30ma. It all depends upon the configuration and specific LED but this is a good number for planning purposes. So for a 1A supply you are looking at 30-40 LEDs. For a 2A 60-80. Etc…
The LED is wired in series and only drops the voltage (by limiting the current) to keep from burning out the LED. The current is the same through both the LED and the resistor. With a 1k resistor and you assume the forward votlage drop of the LED is .7v, then the current should roughly be 11.3ma per LED. You’ll want to ensure that this will give you adequate brightness. For 12V supplies I often use 560 ohm resistors which provides approximately 20ma. If you are wiring this in series with a stall controller, like a Tortoise, the resistance of the coil in the Tortoise will drop the current to around 15ma.
No, never remove the resistor. An LED is not a lightbulb and does not follow the same electrical laws. Once an diode begins to pass current it will pass as much as possible. The resistor is not to “drop the voltage” but to “limit the current”. Without the resistor the diode will try to pass every amp available from the power supply and consequently buring it out.
LEDs aren’t like light bulbs. When a bulb says 30ma, that means the bulb draws 30ma. When an LED says 25ma, that’s the MAXIMUM current it can pass without damage. Depending on your LEDs, with a 1K resistor you are probably around 10-15ma per LED. A 1.5 amp power supply will drive 100 of them.
Depends on the amount of power your supply can deliver.
Since LEDs are wired in parallel, you need a current output greater than the sum of all the currents flowing through them.
For example, 50 LEDs drawing 20mA will need at least 1A of current. So, for best results, the PS should be able to deliver at least 1A, but 1.5A won’t hurt. (Assuming all can be lit at the same time.)
If only half will be lit at any one time, a 1A supply would be adequate.
OK, so this is all starting to make some kind of sense! My only question now is about the resistor(s). Cacole says to use one with each LED. I was going to use one only, on the negative rail of the 12VDC bus, then hang the LEDS in parallel off that bus (switches and relays not shown for simplicity):
That won’t work like that, at least not with more than a couple LEDs. And even if it did - currents in parallel add - so if you turn off some of the parallel LEDs witht he switches and relays you haven’t drawn, the current across the remainder will increase and potentially destroy them.
Each LED should have its own resistor. The ONLY time it is safe to use 1 resistor with more than one LED is when it is absolutely impossible for both LEDs to be on at the same time - for example a bicolor LED or two LEDs wired back to back. Depending on the polarity of the DC supply, only 1 LED can be on, so a single resistor works. Don’t skimp to save 2 cents. You can buy 100 packs of resistors from Mouser or Di