Are power adapters current limiting?

[Murphy625]

Hi folks,
I'm still searching for a cheap power supply solution and have another question.

I have an old netgear firewall/router that uses a power adapter that plugs into the wall.. You know, those little square black boxes that plug directly into a wall socket. The adapter says 12 vdc and 1 amp on it..

I bought some 10 watt power led's on ebay that take 9 to 12 volts and use a max of 1050 mA current..

So would this little power adapter work to light up a single led chip? I guess what I am wondering is if that 1 amp is regulated or would the power adapter blow an internal fuse or something if it went over 1 amp..

Anyone ever try one of these?

 

[LEDite]

If it is a switching supply (100-240VAC input rating) . . .

then it will usually deliver well over the rated current.

It is voltage regulated, so you need to reduce voltage to get the current you want.

LEDite

 

[RetroTechie]

Well if you're shopping on fleabay anyway: have you seen the prices on matching drivers?

Why go out of your way to avoid an obvious, simple & cheap solution? :thinking: (especially if electronics isn't your thing)

 

[Murphy625]

Well if you're shopping on fleabay anyway: have you seen the prices on matching drivers?

Why go out of your way to avoid an obvious, simple & cheap solution? :thinking: (especially if electronics isn't your thing)

It's not that.. I have plenty of power supplies already laying around.. ATX computer supplies, and I have a bunch of different power adapters from various old electronics.

Here's the thing... You can buy 10 watt bulbs that screw into a regular light bulb socket for about the same price as what you spend buying the LED Chip and a special driver for it. I'm just trying to see if there's a work around.

 

[RetroTechie]

Then I suggest you get some low value resistors, say 0.47, 1, 2.2, 4.7, 10 Ohm etc. 1 Watt size should be fine for experimentation I think.

Grab that 12V supply, hook up LED with a series resistor (start with the highest value), measure voltage across the resistor -> you have current going through the LED. Also measure voltage over the LED for each current value. Repeat with different resistor values (while keeping an eye on the LED's operating limits!), plot LED current vs. voltage on a piece of paper, and it'll be easy to work out what power supply / resistor you need for a given LED current. LEDs aren't magic you know... ;)

It's not that.. I have plenty of power supplies already laying around.. ATX computer supplies, (..)

For a 10W LED, an ATX supply is overkill (and too noisy). Some leftover laptop psu, or any adapter in 12~15V or so DC range with enough Amp capacity should do if you work out a suitable value (+ wattage) for a series resistor.

A dedicated LED driver just skips the "series resistor" part. But that doesn't mean plain old DC supply + resistor doesn't work... :)

 

[Murphy625]

Then I suggest you get some low value resistors, say 0.47, 1, 2.2, 4.7, 10 Ohm etc. 1 Watt size should be fine for experimentation I think.

Grab that 12V supply, hook up LED with a series resistor (start with the highest value), measure voltage across the resistor -> you have current going through the LED. Also measure voltage over the LED for each current value. Repeat with different resistor values (while keeping an eye on the LED's operating limits!), plot LED current vs. voltage on a piece of paper, and it'll be easy to work out what power supply / resistor you need for a given LED current. LEDs aren't magic you know... ;)


For a 10W LED, an ATX supply is overkill (and too noisy). Some leftover laptop psu, or any adapter in 12~15V or so DC range with enough Amp capacity should do if you work out a suitable value (+ wattage) for a series resistor.

A dedicated LED driver just skips the "series resistor" part. But that doesn't mean plain old DC supply + resistor doesn't work... :)

Thank you for the response and help. The reason I want to use the ATX power supply is because I plan on hooking up about 10 to 15 of the 10w high power 12v leds to it. Since the ATX supply puts out 20 amps at 12vdc, I figured I should be able to get at least 15 leds on it.

This is why I've been so reluctant to go buy an LED driver for each led.. It drives the cost up fast. I paid $14 for a quantity of 10 of the 10w LED's.. I bought 50 pcs of 2 watt 2 ohm resistors, and I already have the power supply and a bunch of low voltage speaker wire. I don't need dimming functions, I don't need to get every lumen squeezed out.. Just want to put a bunch up in my garage rafters for extra light.

I plan on cutting some 3/16 thick aluminum stock 5" wide x 10" long.. mount the led to it, mount some sandblasted plexiglass about an inch above the LED as a diffuser, and then screw the aluminum plate right to a rafter support.. Mount the resistor to the other side of the aluminum plate, and run the wires to it.

Thank you so much for your input.. anything you can add is really appreciated.

 

[Julian Holtz]

Are power adapters current limiting?

From what I have tested, yes. At least the common 12V 5A types. The voltage goes down once the rated current is overshot.