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Hot resistors


Matti
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30 minutes ago, DavidCBroad said:

You need two, or one for each LED.

 

Exactly, but my reply was to Quarryscapes who advocated a voltage regulator to eliminate resistors.

 

Don't know why anyone would go to the trouble of eliminating resistors, when they are the cheapest product, in the electronics industry.

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46 minutes ago, DavidCBroad said:

You need two, or one for each LED.

 

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 a 56 ohm resistor, 5 volt supply, forward current 20ma, and resistor power dissipation 56mw.  

 

Something is slightly off. 20mA through 390 ohms dissipates 156mW. Assuming it really is 20mA the correct resistor for 20mA from an 18 volt supply is 690 ohms.

 

Obviously it's a good idea to know what the actual power supply voltage is. It doesn't need to be regulated. All you need to do is measure it with a multi-meter and there really is no excuse for not owning one considering how inexpensive they are.

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There is no reason to run an led at its max spec values. Most will run at half that value.

 

I use a standard 1k resistor on all my leds colour legs with an extra one in the main common power feed if they are a bit too bright.

 

I normally use common anode 3-legged bi-colour leds for DCC purposes, hence the extra dumb down resistor where I have them fed from track voltage.

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On 24/03/2020 at 08:47, 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)

Only if you are using the LM317 as a constant current source. Otherwise, you should ALWAYS be using resistors.

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