LED resistors

Basic electrical and electronics, such as DC/Analog control.
brober
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LED resistors

Postby brober » Mon Apr 05, 2021 8:12 pm

I am looking to acquire resistors for 3mm LEDs which are rated at 3.2v. I want to use a 12v DC supply to power them. What value of resistor will be required for each LED?

Many thanks

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RAF96
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Re: LED resistors

Postby RAF96 » Mon Apr 05, 2021 8:15 pm

Using Ohm’s Law and the led spec you can work out the exact resistance need by that voltage to keep the current within spec, or you can just use a 1k ohm resistor like most of us.
If you are using different coloured leds and want to match brightness then you need to do the calcs.
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brober
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Re: LED resistors

Postby brober » Mon Apr 05, 2021 9:19 pm

Not being very electrically minded I have found a calculator for LEDs online that tells me for a supply voyage of 12v, a voltage drop off 8.8v (12- 3.2) at a current of 30mA the resulting R is 180 ohms at 0.25 W. Does that sound right?

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Ironduke
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Re: LED resistors

Postby Ironduke » Mon Apr 05, 2021 10:41 pm

Most LEDs run at 20mA, not 30. Do you have a link to the LEDs you are buying?
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Rob

brober
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Re: LED resistors

Postby brober » Mon Apr 05, 2021 11:34 pm

The LEDs I will be using are from Rapid Electrics:

Kingbright L-7104PWC-A 3mm White LED 1200mcd

Order code: 55-1630

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Ironduke
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Re: LED resistors

Postby Ironduke » Tue Apr 06, 2021 2:04 am

ok so according to the datasheet, 30mA is the absolute maximum forward current. I would aim for 20mA
As you can see from the first graph on page 3, when the forward current is 20mA the forward voltage drop is 3.2V

so 8.8V/0.020A = 440R.
power = 0.176W so a 1/4W resistor will suffice.
but this assumes your power supply is exactly 12V.

You could safely use a resistor of 470R to 1K and the LED will still be visibly bright. Depends what you're using them for.
Regards
Rob

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Flashbang
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Re: LED resistors

Postby Flashbang » Tue Apr 06, 2021 8:45 am

1000 Ohm (1K) 0.25watt will be the ideal Resistor to use on 12 volts DC supply to give a good level of illumination on all LED colours for general use. At 1K per lit LED their current will be around 5 to 10millamps (0.005A - 0.01A) per LED depending on LED colour.
RAF96 has already specified this, and really a 1K resistor on 12volts is a good average one to have in the spares box for any LEDs running on a Regulated 12 volt DC supply :D
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brober
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Re: LED resistors

Postby brober » Tue Apr 06, 2021 9:33 am

Thanks all. 1K/ 0.25W it is. They are for turnout direction indicator lights on my control panel of which there will be 26 lit at any one time... all white.

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Flashbang
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Re: LED resistors

Postby Flashbang » Tue Apr 06, 2021 9:57 am

Just times the lit number of LEDs by their calculated current and you will have their total load placed on the power supply. Always allow headroom for the power supply current wise having it provide a larger current output than is needed.
Usually a 12 volt 1.0 or 1.5Amp Regulated power supply is used for LED illumination panel and on the layout for any buildings or road lams etc.
Note the Regulated... This means the supply voltage is virtually constant regardless of load placed on it. Frequently CCYV or LED power supplies are used. Loads sold on eBay or Amazon etc Most CCTC and LED PSUs will come with a 2.1mm DC moulded Barrel plug and you can obtain in-line connectors that allow easy onward wiring from the plug to the layout or panel. Or alternatively obtain a chassis socket that matches the plug. :D
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Ironduke
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Re: LED resistors

Postby Ironduke » Tue Apr 06, 2021 10:13 am

On a side note, you might find standard brightness red or yellow LEDs easier on your eyes (and wallet) especially on a panel that you are going to look at frequently. White and blue light is a strain to focus on.
https://www.liquidleds.com.au/blogs/new ... r-vision-1
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Rob

brober
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Re: LED resistors

Postby brober » Tue Apr 06, 2021 10:55 am

Thanks for that guys. I am using Guagemaster WM1 as the PSU. It is rated at 1.25 amps. I may be approaching its limit with these extra 26 LEDs as it is also powering station platform lighting and goods yard standards. We shall see when I fire it all up. If necessary I will separate into different circuits with separate PSUs.

brober
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Re: LED resistors

Postby brober » Tue Apr 06, 2021 11:10 am

Follow up query...... which is more appropriate.... metal film or carbon film? Rapid have both listed on their website. Seems 100 would only cost £0.88! Would that be correct?

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Flashbang
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Re: LED resistors

Postby Flashbang » Tue Apr 06, 2021 11:17 am

Either will be fine. 100 for 0.88p is an exceptionally good price Can you provide a link please as its seems far too cheap! You need to add Postage that Rapid slap on!

Ebay sellers in the UK often have 100 1K for around £2.20 including P & P Example... https://www.ebay.co.uk/itm/1-4W-0-25W-Metal-Film-Resistor-1-1K-Ohm-to-4-7M-Ohm/261732761369?hash=item3cf07cff19:g:qNIAAOSw1m1dVTSC
Even cheaper... https://www.ebay.co.uk/itm/110R-to-1K-Carbon-Film-Resistors-0-25W-Pack-of-10-or-100/110871936040?hash=item19d07b7c28:g:srgAAOSwIJlaORZ9
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brober
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Re: LED resistors

Postby brober » Tue Apr 06, 2021 11:54 am


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Flashbang
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Re: LED resistors

Postby Flashbang » Tue Apr 06, 2021 12:24 pm


Hi
Thanks for the link. That is a good price.
Your link goes to a 10K (10,000 Ohm) resistor, it should be 1K (1000 Ohm) is this one, same price though of £1.05 for 100 including VAT. Order Code: 62-0370 https://www.rapidonline.com/truohm-cr-025-1k-carbon-film-resistor-0-25w-pack-of-100-62-0370
Unless you spend £36.00 including VAT on a Rapid order you will have to add £4.79 postal charge (£3.99 plus VAT) So your £1.05 resistors have become much more expensive if you don't reach their £36 order for free P & P limit. Hence why I frequently use eBay sellers as P&P is free and prices are reasonable :D
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