LED Reading Lamp Power Conversion to USB

Intro: LED Reading Lamp Power Conversion to USB

I had an LED floor lamp that I bought from the local Scandinavian Cheap Furniture Store that started to rapidly blink rather than stay on. Although I thought that I might have gotten a cool (albeit rather dim and unadjustable) strobe lamp, I was much more interested in reading than partying. I am at that age now. So after tracing the problem down to the power supply, and having little luck getting said Scandinavian Cheap Furniture Store to talk to me about getting a replacement, I figured I could fashion one of my own using the 5 volts used in USB power supplies and devices.

STEP 1: Your Old Power Supply

Check out your power supply. I imagine a lot of companies manufacture LED reading lights along similar lines. Check the output voltage on your existing (or malfunctioning) power supply and find a diagram of the wiring polarity if it is there. Remember that with LED's, polarity matters.

It is important to note that the lamp was designed for 4 volts and I am putting 5 volts through it. You should put a resister in line to bring down the voltage to the specified 4 volts or your lamp will over heat and probably burn out your LED prematurely.

Although I did not do this in this Instuctable, be carfull! I am keeping a weary eye on my lamp and I definatly would not leave it on when I am out of the house.

STEP 2: Find a Male USB and a USB Charger/power Supply

In this case I salvaged an old printer cord and the charger that came with my now defunct Kindle.

STEP 3: Get Your Tools and Supplies...

Tools:

Soldering iron

Wire cutters

Volt meter

Sharp knife

Heat gun (or a cheep cigarette lighter)

Supplies:

Solder

Heat shrink tubing (1/4")

Electrical tape

STEP 4: Get Your USB End

Leave about a foot. Also cut the old plug/power supply off your lamp.

STEP 5: Loose Fit the Heat Shrink Tubing

Put it on now because later will be too late!

STEP 6: Separate Your Wires

With USB your white and green wires go away, they are for data that you're not using. We are just using the red (5 volts) and black (common).

STEP 7: Strip

Carefully go around the insulation with a sharp knife. And make one wire shorter than the other so both splices are not at the same place and end up resting against one another.

STEP 8: Check Your Voltage

To make sure your scavenged parts are okay to use.

STEP 9: Twist, Test and Solder

Once twisted but before the soldering is complete, you should gently test the whole thing to see if your lamp comes on. If it does not, you probably have your wires crossed (polarity inverted) and you need to switch them.

Time to solder. I find it easiest to put my iron under what I am soldering and bring the solder down on top of the wire splice.

STEP 10: Tape It Up

Put a small amount of tape on each splice and a little on the outside of the insulated pair.

STEP 11: Shrinky Tube

Gently move the shrink tube over your work, being carful not to disturb your tape. Then shrink it with the heat gun. I have used a cigarette lighter when I didn't have a heat gun around.

STEP 12: Read a Book

Here my lamp plugged into my old USB charger!

REMEMBER to keep an eye on the temperature! Retire the project if it gets too hot!

Comments

[Vi Kay]

Hi
Is it a big deal if I just wrap them in electrical tape and don't solder?

[vikyngo]

Hi

I have a couple of those, and the constant current driver in on the lamp, close to the diode. If you check it, you'll see you don't need a resistor in series or a constant current, supply. The extra volt means some estra heat on the regulator, that's true, but it will probably handle it just right. And of course. Use usb supplies that can handle the current (at least 750ma supply).

[rafununu]

You guys who don't know nothing about electronics often make this error, voltage has no physical existence, voltage is only the expression of an intensity thru an impedance.

A common white led like this one needs between 750mA and 1A to work normally. A standard USB port can only deliver a max of 500mA. Now, you see the problem. You'll say "it works", yes indeed, until your laptop burns.

[timadamb]

Boy, I'm sure glad you pointed that out! Until I test this out for a while using my computers USB port, I'll remove the picture of my laptop as a power supply. I'd be happy to hear from anyone else who might want to weigh in on this!

[rafununu]

Removing the picture is a good idea.

Your idea of using another power supply is not so bad but, if the supply isn't a constant current one, you'll need a resitor in serial with the led. R=U/I where R is the resistor in ohms, U the voltage measured on the resistor in volts and I the intensity needed by the led (350mA for 1W, 500mA or 1A for 3W). V is the voltage found on the supply minus the led zener voltage (usually 3V), so if the supply is 5V (USB) and the led 1W, R=(5-3)/0.35 so R=5.7ohms and will dissipate 700mW, it's going to be hot. With a 3W led the resistor will be 2ohms and will dissipate 2W, the supply needs to drive 1A.

Sorry for this laius, but you can see clearly that with a conventional supply, 2/3 of the power is lost in the resistor heat (this explains the overheat you found). This is why leds are most of the time driven by constant current drivers.

[timadamb]

:)

[akrims]

Thanks for posting this, i have the same problem with my "Scandinavian" lamp, but mine is blinking because of a bad contact in the switch, so i think that your power source should still be intact.

[timadamb]

I thought it might be the switch as well, but on testing found the power supply at fault. In the end I am glad as it put the idea of USBing it in my head.

[rafununu]

I've got the same lamp and the blink comes from the switch. You cannot test a constant current supply unless you've got special electronics instruments. Don't try this USB supply, believe me it's too dangerous for your USB driver.

[fred27]

Isn't the standard power supply a constant current driver? I know it is for the clamp on version.

I suspect you're only getting away with this because your USB supply has hit its limit. Check the current your LED is drawing and how hot your new PSU is getting.

[rafununu]

Yes, you're right, it's a constant current driver which isn't the case of an USB port !

If you don't want your laptop to burn, don't try this.

[fred27]

Never mind. I can see from the photo that you had a different supply to the one I'm thinking of.

[timadamb]

I notice a heat increase in the light/light housing only. The supply seems cool to me.

[dbuckley6]

I have the same lamp, began blinking, thought it was switch and cut it out of circuit, blinking persisted. Design fault from company that prides itself on design please take note...

[harari]

I have the same experience

[avocadostains]

4 volts vs 5 volts? I think the led will still last basically forever.

[timadamb]

I think your right, but I put the warning in as a good practice. I don't really know the limits on the particular LED in the lamp I am using.

[avocadostains]

Yea that's a good idea. It's hard to know the long term effect of over or under voltage. Then you have bulbs that burn out quickly even though they're driven at the right voltage.