Voltage supply question regarding 12V LED and USB port

Thread Starter

LLAmp08

Joined Oct 14, 2019
4
Hello,
I am working on a project with LED lights in lamp, I sourced the correct sized power adapter already for what I planned to use and then I realized any lamp without a USB charger in this generation is basically trash and makes me cry on the inside. Background set here's the question.


My known information is here- LED load is soaking up ~30 watts @12V's 3A. I want to add a USB charging port into the circuit.
From what I understand this is going to consume around 15w and I need to change my voltage to 5v 1.5A. My outlet adapter 12v 3A ~50w range

QUESTION- Whats the best way for me to accomplish this supply change cheaply? From what I have read and assumed its called BUCK converter but the terms seems broad vague (magical) and too general until I see the actual working solution. Am I on the right track? Any specific hardware links to clear up the mystery and save me from purchasing the wrong items would be greatly appreciated.
 

Tonyr1084

Joined Sep 24, 2015
3,674
Welcome to AAC.

Something new people often do here is describe their situation without posting any pictures or schematics. At best I'm unsure of what you're asking. You mention USB and 12 volts. USB is typically 5 volts; so how that's going to give you 12 volts - or how you are thinking you can get that from USB is going to be a difficult process if possible at all.

USB also doesn't run very high amperage. But using a BUCK converter you CAN change 5 volts into 12 volts. Just as an example 5 volts at 12 amps is going to turn into 12 volts at 5 amps. In either situation your wattage is going to be the same (in theory).

Before we can offer any meaningful advice we're going to need a clearer picture of what you have, what you want and what your goal is.
 

Thread Starter

LLAmp08

Joined Oct 14, 2019
4
Thank you! Maybe I can clear it up a little? My supply is a 12v 3a outlet adapter my LEDs are running on 12v 3a. I want to add a USB charging port into the circuit before my LED load- These typically run on 5V 1.5A -so google tells me. Whats the best way to change my voltage and current?
 

Tonyr1084

Joined Sep 24, 2015
3,674
OK; do you want to run both the LED's at 12 volts and the USB charger at 5 volts? If that's the case you're probably better off just going with a USB charger. They're cheap and everywhere. However, if you want the LED's to indicate when the charger is on then dropping your voltage to 5 volts is the first step. Then take the LED you want to use as an indicator. Subtract the Vf from the 5 volts. Assuming a typical red LED with a common Vf of 2.2 volts, 5 - 2.2 = 2.8 volts. That's enough to light your LED. Assuming you want to power it at 15 mA (a good amperage for most LED's) then 2.8 ÷ 0.015 = 186.67Ω. That will give you plenty of brightness on the LED and it should last a good long time. Probably outlast the charger.

Now, if you're using the LED's as a reading lamp - Home Depot, Lowe's and many other hardware stores (probably even WalMart) will have LED reading lamp replacement bulbs already designed to operate at 12 volts. Sometimes we like to build even though buying can be easier. But why build when you can buy? But if you want to build - - - .
 

ElectricSpidey

Joined Dec 2, 2017
739
Best is a subjective term.

Did you want to build the charger circuit yourself?

A circuit based on the 7805 is about as simple as you can get. (linear)

Also there are simple switcher chips available. (SMPS)

There are also plenty of 12v to 5v DC to DC buck modules available. (pre built)

And then there is always something like this...
https://www.amazon.com/Ginsco-Charger-Socket-Outlet-2-1A(4-2A)/dp/B01N6S2SNM/ref=sr_1_5?keywords=12+volt+to+USB+charger&qid=1571068144&sr=8-5

You could also bypass the 12 volt supply altogether and connect any run of the mill USB charger to the input for the 12volt supply.
 
Last edited:

Alec_t

Joined Sep 17, 2013
10,458
My known information is here- LED load is soaking up ~30 watts @12V's 3A. I want to add a USB charging port into the circuit.
From what I understand this is going to consume around 15w and I need to change my voltage to 5v 1.5A. My outlet adapter 12v 3A ~50w range.
A 12V 3A adapter can provide a maximum of 36W in theory, but it is never a good idea to run any device at its rated maximum. If your LEDs require 30W then there is no safety margin for driving the USB charger port.
 

Reloadron

Joined Jan 15, 2015
4,591
If you want to supply a 30W lamp plus a 15W charging port that is 45W.

Your 12V 3A supply can output 36W.

Do you see the problem there?

Bob
That is what it all comes down to. You can never get more than you started with. Additionally as mentioned, your current load is right on the line. Normally I like to see my available supply provide about 20% over my load demands, sort of a small comfort zone.

You can increase your supply to 12 volts at 5 amps and from there the simple turn key solution would just be a 12 V to 5 V USB charger which is easier to buy than make.

Ron
 

Thread Starter

LLAmp08

Joined Oct 14, 2019
4
Additionally as mentioned, your current load is right on the line. Normally I like to see my available supply provide about 20% over my load demands, sort of a small comfort zone.

Ron
Indeed rough numbers

Thank all of the attention and commenting. I see some errors I was missing already. Im gonna need a bigger AC adapter for starters. Then from what I am finding a Linear Regulator probably AMS1117-5?
 

TeeKay6

Joined Apr 20, 2019
457
Indeed rough numbers

Thank all of the attention and commenting. I see some errors I was missing already. Im gonna need a bigger AC adapter for starters. Then from what I am finding a Linear Regulator probably AMS1117-5?
@LLAmp08
If you use a linear regulator to drop 12VDC to 5VDC @3A, then your regulator will dissipate 7*3=21 watts. That's possible, but not without some significant heat sinking. No, the AMS1117-5 alone is not suitable; it is rated for 1A max. You can likely find low cost buck regulators (assembled on PCB) on eBay or such. Otherwise, start learning about buck regulators; there are lots of published circuits and devices.
 

Reloadron

Joined Jan 15, 2015
4,591
Indeed rough numbers

Thank all of the attention and commenting. I see some errors I was missing already. Im gonna need a bigger AC adapter for starters. Then from what I am finding a Linear Regulator probably AMS1117-5?
The linear regulator you mentioned is a 1.0 Amp LDO (Low Drop Out) regulator.

OK, I just saw the reply above. Even at 1 Amp you will have it dissipating 7 watts and still need an adequate heat sink. That plus a way to mount things.

12 VDC power supplies are really pretty inexpensive from suppliers on Amazon and similar. Then I would just look for an automotive type USB charger rated for 2.0 amps or so.

Ron
 
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