LED Resistors

DafyddElvy

DafyddElvy

1:22.5 & 15mm Scale Trams, , NG Steam Railways
Country flag
I have a few Bachmann trams which I am converting to battery power, and the lights to LEDs.
The feed to the lights in the roof are feed from each end of the model.

My question, can I install LEDs in series with a single resistor at one end or do I need to provide each LED with it's own resistor?
 
resistor per LED....

Think about it a bit.... what drives the calculation for the resistor value?

The voltage available and the load (current)..... one LED lit is a certain current, two LEDs lit is double the current, thus needing a different resistor.

So, a single resistor cannot be used for multiple LEDs except if:
all LEDs are either on or off at the same time (then you total the current needed)
OR
only the same number of LEDs are on together.


Hopefully now you see not a good idea.

I left out the part that more LEDs together will normally need a higher wattage resistor too, bigger and more expensive and more heat.

Greg
 
Greg Elmassian said:
resistor per LED....

Think about it a bit.... what drives the calculation for the resistor value?

The voltage available and the load (current)..... one LED lit is a certain current, two LEDs lit is double the current, thus needing a different resistor.

So, a single resistor cannot be used for multiple LEDs except if:
all LEDs are either on or off at the same time (then you total the current needed)
OR
only the same number of LEDs are on together.


Hopefully now you see not a good idea.

I left out the part that more LEDs together will normally need a higher wattage resistor too, bigger and more expensive and more heat.

Greg
Click to expand...
I suspected that was the answer but thought I'd ask, thanks.
 
DafyddElvy said:
I have a few Bachmann trams which I am converting to battery power, and the lights to LEDs.
The feed to the lights in the roof are feed from each end of the model.

My question, can I install LEDs in series with a single resistor at one end or do I need to provide each LED with it's own resistor?
Click to expand...
Yes, you can with the correct resistor and similar LED's. In series they should all be on at the same time. I've had no problems doing that. It doesn't matter too much which end you put the resistor on. Also if you are using bidirectional lighting to change the color of LED in forwards or reverse (with one LED in reverse polarity to the other), the two polar opposite LED's can share the same resistor and it doesn't matter which side you put the resistor on. The best thing to do would be to get a breadboard and experiment with your circuit design before you install it.
 
If identical leds are in series then they only need one resistor. I have placed 3 leds in series with one resistor. However, you need to be aware of the supply voltage as 2 of the 12 volt leds in series need at least 24 volts or more plus the resistor to light up. 2.2 volt leds with 2 in series would be great with a resistor for DCC users as track voltage would be constant. If track voltage is not constant then I would use 2 or more leds with 20ma current in series with the CL2N3 IC chip, or 2 10 milliamp leds in parallel with a cl2N3 IC chip. Note this CL2N3 device like the leds need a bridge rectifier if voltage changes polarity.
 
phils2um said:
I could be wrong but I believe "12V" leds already have a resistor built-in.
Click to expand...
Not sure if this is entirely correct for all 12v led’s, but I have certainly had 12v resisters fitted led’s when purchased as you say.
 
dunnyrail said:
Not sure if this is entirely correct for all 12v led’s, but I have certainly had 12v resisters fitted led’s when purchased as you say.
Click to expand...
No,you usually by LEDs, then have to work out the resistance required according to the voltage used and the forward or backward voltage of the LED. One of the electronic wizards will explain properly
 
Paul M said:
No,you usually by LEDs, then have to work out the resistance required according to the voltage used and the forward or backward voltage of the LED. One of the electronic wizards will explain properly
Click to expand...
I reckon I bought some 12v rated LED’s that were thus as the resisters were pre fitted for 12v DC, something that the 4mm guys that are still not DCC would want.
 
Usually when you buy a "12V" LED it's a standard LED with a resistor pre-wired in one of the leads, hence you can remove that and fit an alternative if necessary - or just add a second resistor if your supply voltage is higher.
However I do believe there are a few LEDs out there with a resistor as part of the moulded package itself.
 
You must log in or register to reply here.
Back
Top Bottom