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Evening!
 

I ordered some small LEDs, for a 12v power supply in red yellow green, pre wired with an appropriate resistor. 

I’m just a bit concerned because I plugged them in to my 12v DC supply via some wago clips, they light up beautiful (a little too bright maybe). But I noticed the resistors were very hot to touch they almost stung my finger tips a bit. 

I’m worried because these will be in card based buildings and the heat was of concern. I have other LEDs which resistors do not get this hot, is everything ok? Is that normal?

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It sounds like you may be loading them too much. You don't them getting too hot.

What power rating are they?

 

If they are 0.25W, then maybe you could try 0.5W?

I've just checked prices on rapidonline: A pack of 100 0.5W 1k resistors are £1.15 as opposed to 78p for 0.25W 1k, so although more expensive, it shouldn't be enough to stop you modelling.

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Resistors will quite happily work at temperatures beyond hot to touch. 100C for carbon film is typical maximum specification higher temps are quoted for metal oxide and wire wound resistors (it's likely your resistors will be carbon film)

 

Typically 70 C is quoted as too hot to touch <1 second

 

 If in doubt - use a resistor with a higher power rating (and increase the resistance to dim your lights) 

 

Incidentally - is your power supply regulated?

12v unregulated supplies can easily be 15-16v on low current draws.

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A quick resolution - connect of the one resistors you have to another and then repeat with another pair, then connect them together in parallel so you have  the power going through either of two sets of resistors that way the current is split whilst the resistance is the same as with one of them - two together double the resistance but then the two in parallel halve it. If they till run hot then they are plainly of too low wattage.

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I would hazard a guess that the resistors you have are 0.125W. For example a standard LED with Vf = 2V and If = 10mA with a 12V supply would require a 1K resistor and the power dissipation would be 100mW(0.100W) so you can see you are very near the ability of the resistor to cope. If you are using white LEDs where the current draw can be as much as 30mA you will make the resistors run hot.  I would uprate your resistors to 0.250W or even 0.5W  to play safe after all you are wiring them inline with the lamps and it is not as if you are worrying about component space on a PCB.

 

Richard

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Running them from 12 volts means you will have to dissipate a fair amount of power in the resistors. If the LEDs are too bright try stringing two or three LEDs in series with one of the resistors. That will reduce their brightness and also let the resistor run cooler.

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The forward voltage of a white LED is usually in the range of 3-5v so four in series should in theory not need any resistor and if they are of the higher forward voltage type will automatically dim due to the limited power supply, assuming all along it is regulated. Have you done an off load reading of the "12V" supply as it could be somewhat higher.

Edited by Butler Henderson
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7 hours ago, Butler Henderson said:

The forward voltage of a white LED is usually in the range of 3-5v so four in series should in theory not need any resistor and if they are of the higher forward voltage type will automatically dim due to the limited power supply, assuming all along it is regulated. Have you done an off load reading of the "12V" supply as it could be somewhat higher.

 

I would not recommend that.

 

https://www.eetimes.com/constant-current-regulation-overcomes-led-quality-variations/

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Hot resistors are a great way to set fire to a card building.  They need to be out in the breeze not tucked in a building.  I expect part of the problem is the LEDs  are intended  for a regulated 12 volt power supply and your power unit is probably a nominal 12 volt probably poking out around 18 or even 22 volts off load like my Duette does.   You can put 4 LEDs in series but chances are they won't be a matched set and one will be brighter than the rest and may fail pretty quickly.

I don't have any white LEDs but I feed my yellow, Orange, Green and Red  ones 3 volts and adjust the individual brightness with resistors as required.  These are lights in model buildings so a variation in brightness is good. I find White LEDs are too bright for pre 1970s pre flourescent light era.

  A 5 volt supply (with a 0.5 or 0.25 amp polyfuse) would be a better starting point for white LEDs than a nominal 12 volt supply. 

 

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1 hour ago, DavidCBroad said:

Hot resistors are a great way to set fire to a card building.  They need to be out in the breeze not tucked in a building.  I expect part of the problem is the LEDs  are intended  for a regulated 12 volt power supply and your power unit is probably a nominal 12 volt probably poking out around 18 or even 22 volts off load like my Duette does.   You can put 4 LEDs in series but chances are they won't be a matched set and one will be brighter than the rest and may fail pretty quickly.

I don't have any white LEDs but I feed my yellow, Orange, Green and Red  ones 3 volts and adjust the individual brightness with resistors as required.  These are lights in model buildings so a variation in brightness is good. I find White LEDs are too bright for pre 1970s pre flourescent light era.

  A 5 volt supply (with a 0.5 or 0.25 amp polyfuse) would be a better starting point for white LEDs than a nominal 12 volt supply. 

 

It doesn't matter what the regulated power supply voltage is 5V, 12V or 27V, as long as the correct value resistor is used to both protect the LEDs and to provide the correct brightness.

There are plenty of on line calculators, to provide the information as to what value to use.

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41 minutes ago, kevinlms said:

It doesn't matter what the regulated power supply voltage is 5V, 12V or 27V, as long as the correct value resistor is used to both protect the LEDs and to provide the correct brightness.

There are plenty of on line calculators, to provide the information as to what value to use.

Most model railway power units are not regulated power supplies.   Its why so many people have trouble with LEDs.  As a rule of thumb if it says "12 volt" its probably best to add 50% and calculate your resistors for 18 volts.  

I advocate using regulated power supplies, however you must use overload protection such as a polyswitch.   Come to think of it most model railway overload protection is inadequate for the tiny resistors and thin wire of a typical LED installation so they could probably use poly switches too.

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5 hours ago, DavidCBroad said:

Most model railway power units are not regulated power supplies.   Its why so many people have trouble with LEDs.  As a rule of thumb if it says "12 volt" its probably best to add 50% and calculate your resistors for 18 volts.  

I advocate using regulated power supplies, however you must use overload protection such as a polyswitch.   Come to think of it most model railway overload protection is inadequate for the tiny resistors and thin wire of a typical LED installation so they could probably use poly switches too.

 

LEDs are diodes. They control their own voltage. You don't. They are not resistors and they don't obey Ohm's Law (they do slightly, but not enough to worry about.)

 

The light output from a LED is determined by the current it is passing. If you overdo the current the LED will be destroyed. The voltage drops specified for LEDs are only an approximation for the particular device and even at that they have a wide range of possible values.

 

If you want to control the light output from a LED you need to control the current it is passing. There are a couple of ways to do that. You can either use a fancy constant current source (not really all that fancy) or you can approximate that by feeding the LED, or a string of LEDs in series, through a resistor from a voltage source that's quite a bit greater than the nominal voltage drop of the LED (or series connected LEDs). The greater the voltage source is the more the circuit approximates a constant current source but the trade-off is the resistor will dissipate more power (heat).

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Why use 12v? After various bad experiences with lighting circuits on standard model railway controller outputs. I started using mobile phone adaptors (5v) with appropriate resistors. Only  limited trials at the moment but no issues. After reading remarks here I might do more research on the the amp's as the adaptors seem to vary from .5 - 2amp

Edited by MyRule1
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7 minutes ago, MyRule1 said:

Why use 12v? After various bad experiences with lighting circuits on standard model railway controller outputs. I started using mobile phone adaptors (5v) with appropriate resistors. Only  limited trials at the moment but no issues. After reading remarks here I might do more research on the the amp's as they seem to vary from .5 - 2amp

 

2 amps??? They must be really big LEDs.

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7 hours ago, DavidCBroad said:

Most model railway power units are not regulated power supplies.   Its why so many people have trouble with LEDs.  As a rule of thumb if it says "12 volt" its probably best to add 50% and calculate your resistors for 18 volts.  

I advocate using regulated power supplies, however you must use overload protection such as a polyswitch.   Come to think of it most model railway overload protection is inadequate for the tiny resistors and thin wire of a typical LED installation so they could probably use poly switches too.

Not quite sure why you need polyswitches, if you use the correct resistor and assemble things in such a way that the resistor can't be accidentally shorted out on a regulated power supply, the resistors will protect the LEDs.

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32 minutes ago, MyRule1 said:

Why use 12v? After various bad experiences with lighting circuits on standard model railway controller outputs. I started using mobile phone adaptors (5v) with appropriate resistors. Only  limited trials at the moment but no issues. After reading remarks here I might do more research on the the amp's as the adaptors seem to vary from .5 - 2amp

Most such power supplies are rated in Watts. At 5 Volts you're unlikely to find many plug packs less than 1 Amp, usually more as it depends on what they were intended for. A mobile phone charger at 5 volts & 1/2 an Amp, is going to take forever to charge.

My present genuine Samsung mobile charger is rated at 9V at 1.67A or 5V at 2A. Why the different ratings, I'm not sure, presumably the 9V is a fast charge?

 

Your problem was related to 'a standard model railway controller', being much more than 12 volts, because it is unregulated. David Broad is 100% correct, on that part of his post.

AFAICS, there is no advantage over using 5V power supplies for LEDs, over 12V ones, just make sure they are regulated AND use the correct resistor value. Do that and no one should have any issues with resistors getting hot or blowing LEDs.

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You could look into using an LM317 regulator IC to provide a dedicated lighting feed and remove the need for resistors altogether (except the voltage setting ones for the IC of course)

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9 minutes ago, Quarryscapes said:

You could look into using an LM317 regulator IC to provide a dedicated lighting feed and remove the need for resistors altogether (except the voltage setting ones for the IC of course)

But what about balancing the brightness or for different colour LEDs, which require different voltages?

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17 hours ago, kevinlms said:

But what about balancing the brightness or for different colour LEDs, which require different voltages?

Depends on the supply voltage, if its up around 20 volts the LEDs will be much the same brightness irrespective of their vf, if its down around 12 you will start to need one for Blue/Clear and one for Red/Yellow/Green

 

Looking up the resistor calculator I find that a typical white LED needs a 390 ohm resistor for 12 volts the forward current is 20ma and the resistor power dissipation is 144mw, well within a 1/4 watt resistor.   Increase the supply voltage by 50% to 18 volts, not unusual, and keep the 390 ohm resistor the figures are  forward current 40ma, and resistor dissipation 467mw.

That is a 100% and 300% + increase respectively from a 50% increase in voltage.

Its enough to severely shorten the life of the LED and make a 1/4 watt resistor very hot indeed.  

My latest LED set up has 56 ohm resistors, 5 volt supply, forward current 20ma, and resistor power dissipation 56mw.  

My resistor chart suggests a 1k ohm resistor for 20 volts for both clear and red LEDs.  

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