Want to power a single LED from a 12v ac adapter....

Thread Starter

AaronM43

Joined Dec 7, 2019
3
I am working on a Christmas present project for my parents. I have 3d printed a lighthouse for them and I want to put a yellow LED in the top of the tower that they can plug in. My idea is to use one of these LEDs from Amazon. https://www.amazon.com/gp/product/B07L5KWTBT/ref=ox_sc_act_title_1?smid=AELDI63TKIIN9&psc=1 and a 12v AC adapter. My concern is that the LED is rated for 20mA and the lowest 12v AC adapter I can find is 1A.

Can someone smarter than me in electronics help me design what I need to make this work? I am guessing I will need to solder in a resistor or something.

Here is the lighthouse. https://www.thingiverse.com/thing:2196557 You can see in that link that someone put an LED in theirs and I want that same exact thing.

Thank You!
 

paulktreg

Joined Jun 2, 2008
778
The 12v adapter can deliver up to a maximum of 1A. If your LED requires 20mA that's all it will supply.

The resistor is built in so just connect to 12V DC.

With a 1A supply you can install several LED's for better effect.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
669
The photo shows an incandescent light bulb that shines brighter than a little LED and it shines all around.
The cheap Chinese LEDs have a narrow beam angle which points in only one direction. Unless you turn it around and around with a motor then it is not good for a lighthouse.
Why are you ordering 100 LEDs when you need only one LED?
LEDs need DC, not AC. you can probably find a smaller 12VDC at 200mA adapter at less cost.
 

Delta prime

Joined Nov 15, 2019
101
am guessing I will need to solder in a resistor or something.
ITEM Size (mm): 5mm
Wire Length: 21CM
Forward Voltage (V): 12VDC
Forward Current (mA): 20mA
White Color MCD: 16000-20000 MCD
Lifespan: 50,000 Hours
Viewing Angle: 25 Degree
Pre-wired with built in Resistor.
 

djsfantasi

Joined Apr 11, 2010
5,913
As an aside for others who may find this thread, searching for “lighthouse LED circuits” will result in several more complex circuits. Using two LEDs and a circuit that fades them in and out, a rotating lighthouse beacon can be simulated. I used one on a model railroad diorama.
 

Reloadron

Joined Jan 15, 2015
4,868
What you want to note with the LED is what is known as "Viewing Angle" In other words, the viewing angle tells you the space covered by the “cone of light” prior to losing half of its peak illumination level. It is simply a measure of the position where the intensity of the LED light spread reaches 50% of its maximum brightness. You also want to be concerned with the actual brightness of the LED itself, it's about the lumens of the LED and the ones you are looking at have a narrow cone and are not very bright. Again, consider the angle of the cone when thinking about mounting it. You may wish to consider something like this. just as an example without regard to color and the pins would be bent for mounting. The images in your link appear to have a pretty bright LED and not a simple 5mm 20 mA type.

Ron
 

Audioguru again

Joined Oct 21, 2019
669
I suspect that the Chinese rating of 20000mcd of brightness is actually only 200mcd.
Also I suspect that the Chinese beam angle rating of 25 degrees (only 12.5 degrees to each side) is actually only 10 degrees.

Amazon sells clothing, shoes and cheap junk that they know nothing about. I would never buy electronic parts from them.
 

Thread Starter

AaronM43

Joined Dec 7, 2019
3
I spoke to a friend and this is his suggestion for me. Use a 12v 20mA LED, solder in a 750 ohm resistor on the negative leg, and use a 12v 1a AC adapter.
 

bertus

Joined Apr 5, 2008
20,227
Hello,

The leds in the links are prewired for 12 Volts DC use.
Using those leds on 12 Volts AC will break them, as the reverse will be higher as 5 Volts.

Bertus
 

Audioguru again

Joined Oct 21, 2019
669
I spoke to a friend and this is his suggestion for me. Use a 12v 20mA LED, solder in a 750 ohm resistor on the negative leg, and use a 12v 1a AC adapter.
No. An LED operates only on DC, AC will destroy it. A resistor does not convert the AC into AC, a diode does but then the LED will be flashing.
A DC adapter has an AC transformer, four rectifier diodes and a capacitor to smooth the DC output.
The LEDs are designed to be powered from 12VDC and already have a resistor so an additional resistor will simply make it dimmer.

Brightness: A 2.5V yellow LED at 20mA uses 2.5V x 20mA= 0.05 Watts. Your lighthouse probably needs at least 20 of those LEDs in a circle for 1 Watt.
 

Reloadron

Joined Jan 15, 2015
4,868
I spoke to a friend and this is his suggestion for me. Use a 12v 20mA LED, solder in a 750 ohm resistor on the negative leg, and use a 12v 1a AC adapter.
There isn't all that much to this. A 20 mA LED is also going to have a forward voltage drop, that should be known. Standard red, orange, yellow and yellow-green LEDs have a Vf of about 1.8 V, while pure-green, blue, white, and UV LEDs have a Vf of about 3.3 V. So using yellow as an example you take V supply or 12V and subtract V led of 1.8V = 10. 2V and we divide that by the LED forward current of 0.020 Amp = 510 Ohms as a series resistor. Increasing that will decrease the LED forward current. The LEDs in your original link have the series resistors included and that is why they are advertised as 12 V.

Take a good look at a 5mm diameter yellow 20 mA LED and decide if you see what you feel is adequate light since it's your project. A current 0.020 Amp is not all that much current. A 12 Volt 1.0 Amp supply could power about 50 LEDs each drawing 20 mA. Even a 0.5 Amp (500 mA) supply is overkill since at it's voltage the LED will only draw its required current. Anyway, you want DC. They call it an AC Adapter but the voltage out you want is DC. They are common and very inexpensive.

Ron
 

Thread Starter

AaronM43

Joined Dec 7, 2019
3
No. An LED operates only on DC, AC will destroy it. A resistor does not convert the AC into AC, a diode does but then the LED will be flashing.
A DC adapter has an AC transformer, four rectifier diodes and a capacitor to smooth the DC output.
The LEDs are designed to be powered from 12VDC and already have a resistor so an additional resistor will simply make it dimmer.

Brightness: A 2.5V yellow LED at 20mA uses 2.5V x 20mA= 0.05 Watts. Your lighthouse probably needs at least 20 of those LEDs in a circle for 1 Watt.
The ac adapter outputs 12v DC at 1A. The 750 ohm resistor is to bring it down under 20mA.
 

SamR

Joined Mar 19, 2019
1,620
The ac adapter outputs 12v DC at 1A. The 750 ohm resistor is to bring it down under 20mA.
No, it only puts out the amperage needed across the load. Up to 1A max. A 2.5V forward voltage LED only draws ~2V leaving 10V across the resistor. That is only 10V/750Ω =~13mA so close enough. That means the transformer still has ~987mA @ 12V unused capacity. If you want a brighter LED use a smaller value resistor. A ~500Ω resistor will give ~20mA.

EDIT: AG is correct, I missed the "Pre-wired with built in Resistor" part!
 
Last edited:

Audioguru again

Joined Oct 21, 2019
669
The LEDs are advertised at working using a 12V supply, therefore they already have resistors with them to limit the current to 20mA. The 2.5V yellow LED has a resistor that is (12V - 2.5V)/20mA= 475 ohms. If you add another resistor that is 750 ohms then the total resistance is 475 = 750= 1225 ohms. The LED current will be (12V - 2.5V)/1225 ohms= 7.8mA which will be dim.

Please understand that the adapter does not limit the current, instead it is the existing resistors that are already connected to the LEDs that limit the current to 20ma when the power supply voltage is 12V.
 

MisterBill2

Joined Jan 23, 2018
4,842
I am working on a Christmas present project for my parents. I have 3d printed a lighthouse for them and I want to put a yellow LED in the top of the tower that they can plug in. My idea is to use one of these LEDs from Amazon. https://www.amazon.com/gp/product/B07L5KWTBT/ref=ox_sc_act_title_1?smid=AELDI63TKIIN9&psc=1 and a 12v AC adapter. My concern is that the LED is rated for 20mA and the lowest 12v AC adapter I can find is 1A.

Can someone smarter than me in electronics help me design what I need to make this work? I am guessing I will need to solder in a resistor or something.

Here is the lighthouse. https://www.thingiverse.com/thing:2196557 You can see in that link that someone put an LED in theirs and I want that same exact thing.

Thank You!
If the supply is not regulated then the voltage powering the LED will be more than 12 volts, and you will need a higher value resistor. Not a problem if you have a meter and can measure the voltage. I think that you implied an AC powered adapter, not an AC output adapter. Both kinds exist and are sold, and they look similar. So you will need to read the label carefully when it arrives. If the output is AC then you will also need a series diode. The problem is that many LED types can not withstand much reverse voltage, unlike other diode types. But the diode can be almost any kind for this application, a 1N4001 or equivalent is totally adequate and often just a few cents.
 
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