Resistors for multiple led's

[Enndee]

When wiring 6 or 8 led's to a bus can I use just one resistor in the feed or does each led require its' own resistor? Sorry if this is a stupid question but I am a total novice when it comes to electrical matters!

Edited April 21 by Enndee

[Jeremy Cumberland]

If, whenever the LEDs are turned on, it is always the same number of LEDs that are lit, then you could have just one resistor, but you have to size the resistor for the total current, not for the current of a single LED. If the number of LEDs lit at any one time varies, then give each LED its own resistor.

 

Personally, the only times I'd choose one resistor for several LEDs are:

• with a single multi-aspect signal where only one LED will be lit at a time, when the resistor (sized for a single LED) will be in the common wire. Signal manufacturers often recommend this and tell you the size of resistor to use (or supply the resistor with the signal) and say where in the circuit to put it.
• where several LEDs are all switched together, such as a series of street lamps. The resistor needs to be sized for the current of all the LEDs operated by that switch.

If in doubt, use a resistor for each LED.

[Enndee]

Thank you, a very clear explanation.

[MikeB]

I agree with Jeremy, with one caveat.  If it is a series of identical street lights, for example, one resistor is OK.  If colour light signals, the different colour LEDs have different properties and having individual resistors may help each colour to be of similar intensity.  I'm sure somebody more knowledgeable can explain this in terms of the typical current /resistance of different colour LEDs.

[ikcdab]

21 minutes ago, MikeB said:

I agree with Jeremy, with one caveat.  If it is a series of identical street lights, for example, one resistor is OK.  If colour light signals, the different colour LEDs have different properties and having individual resistors may help each colour to be of similar intensity.  I'm sure somebody more knowledgeable can explain this in terms of the typical current /resistance of different colour LEDs.

i think that, bearing in mind the "rounding" that goes into choosing resistors from the standard values, the differences are so little that the same resistor fits all for our white/red/green/yellow purposes. For a 12v supply, start with a 1k resistor. You can tweek this as much as you like, but I dont think that it will make much difference to individual colours.

[57xx]

On 21/04/2024 at 21:52, MikeB said:

I'm sure somebody more knowledgeable can explain this in terms of the typical current /resistance of different colour LEDs.

 

It's not resistance, it's the forward voltage drop and the forward current that are used to determine the resistor required for a specific supply voltage.

 

On 21/04/2024 at 16:31, Enndee said:

When wiring 6 or 8 led's to a bus can I use just one resistor in the feed or does each led require its' own resistor? Sorry if this is a stupid question but I am a total novice when it comes to electrical matters!

 

One thing no one has mentioned is that if you want to run the LEDs off one resistor, you need to run them in series. If you run them in parallel, you will run the risk of a rogue LED with slightly different  characteristics hogging all the current and the others won't light up. The amount of LEDs you can run in series will depend on the forward voltage of the ones you use and the supply current available.

 

This page has info on LEDs to enlighten you further.

[DCB]

In 2024 with 5 volt power supply from USBs an industry standard it makes no sense to power LEDs from a "12 volt" supply Many "12 volt" supplies deliver well over 12 volts off load over 20 in many situations.  For anything but blue and white I use 3 volt power supplies and resistors to adjust the brightness. of signal lamps,  That way the resistors run cool. dissipating maybe 20% of the load not 90%. I have seen badly scorched card buildings where resistors have run very hot inside them.   Heritage White and Blue will not light on 3 volts.   Heritage red need about 30% of the current of Heritage Green for similar brightness.
If you must use 12 volts step the voltage down through a number of 1amp diodes in series rather than by resistors as it gives a much more stable voltage much less dependent on the number of LEDs in circuit.  

[PatB]

I tend to agree with DCB. As USB wall warts are now essentially free, and will happily power pretty much anything other than traction current and solenoid point motors, with far less waste heat to worry about, I cant think of many good reasons to use anything else for accessory power on a new build.

[MikeB]

17 hours ago, PatB said:

I tend to agree with DCB. As USB wall warts are now essentially free, and will happily power pretty much anything other than traction current and solenoid point motors, with far less waste heat to worry about, I cant think of many good reasons to use anything else for accessory power on a new build.

I can think of two reasons.

1. You are not using  simple switches but a lighting or signal controller which requires an approx. 12V input and outputs approx. 12V to the LEDs.

2. The manufacturer of the signal / lights etc has "helpfully" incorporated a resistor in the signal / light to avoid users destroying them by applying a 12V supply, so if the input voltage is  5V there is little or no light emitted.

 

Edited April 28 by MikeB

[kevinlms]

On 28/04/2024 at 10:49, DCB said:

In 2024 with 5 volt power supply from USBs an industry standard it makes no sense to power LEDs from a "12 volt" supply Many "12 volt" supplies deliver well over 12 volts off load over 20 in many situations.  For anything but blue and white I use 3 volt power supplies and resistors to adjust the brightness. of signal lamps,  That way the resistors run cool. dissipating maybe 20% of the load not 90%. I have seen badly scorched card buildings where resistors have run very hot inside them.   Heritage White and Blue will not light on 3 volts.   Heritage red need about 30% of the current of Heritage Green for similar brightness.
If you must use 12 volts step the voltage down through a number of 1amp diodes in series rather than by resistors as it gives a much more stable voltage much less dependent on the number of LEDs in circuit.  

If ANY resistor runs hot enough to burn cardboard, the person who did that has no idea of what their doing!

 

Resistors come in different Watt ratings, using a 'standard' 1/4 Watt is being lazy and possibly risking fires. Carbon resistors are commonly available as 1 Watt as well and wire wound ones at 5 or 10 Watt, so no excuse.

[briang]

That famous Chinese supplier has 3 V power supplies for pence. I am just getting back into the modelling at the young age of 77 and I thought that apart from my DCC supply I would knock up a power supply in 3, 5 and 12 volts. the 5V supply from the same source as the 3 V and the 12 V from a now disused battery charger. I can put them into their own wooden case and thus keep fingers away from 230V and run busses around the model for different purposes. That would seem to do away with the need for resistors though so far I have only tried my LEDs on battery power.. Capacitors are cheap enough to add one if we think that the supply needs to be smooth, that would certainly encourage the longevity of the LEDs.

[Il Grifone]

The resistor serves to limit the current through the LED and should not be omitted and each LED should have its own resistor.

About 10 mA is usually OK as a LED current, though different colours may need adjustment to this figure.

Edited May 6 by Il Grifone

[57xx]

On 29/04/2024 at 05:16, kevinlms said:

If ANY resistor runs hot enough to burn cardboard, the person who did that has no idea of what their doing!

 

Resistors come in different Watt ratings, using a 'standard' 1/4 Watt is being lazy and possibly risking fires. Carbon resistors are commonly available as 1 Watt as well and wire wound ones at 5 or 10 Watt, so no excuse.

 

Have to say that was my first thought when I read the bit about scorching cardboard. Even a 1/4w resistor will be well within it's limits dropping ~10V at a 10mA current for a single LED. 

 

23 hours ago, Il Grifone said:

The resistor serves to limit the current through the LED and should not be omitted and each LED should have its own resistor.

About 10 mA is usually OK as a LED current, though different colours may need adjustment to this figure.

 

Agreed about never leaving the current limiting resistor out, although I'd caveat that by saying each LED or set of LEDs in series.  You can't just slap a lower voltage across them and expect everything to work ok.