Tuesday, February 02, 2010

Lantern modification

I have a GE camping lantern that stopped working, so instead of just fixing it, I decided to make it "better".

This lantern uses 4 D cell batteries and a CFL bulb.  4D cells last a long time, and the CFL put out reasonable light for camping.

But for no reason, it wasn't dropped or left in the rain, it stopped working.  I checked around, and there are no replacement parts for this lantern.  A lot of people online are complaining that their GE lanterns have also stopped working.

This is probably a symptom of the "capacitor plague". As long as I needed to fix the lantern, instead of mucking around changing capacitors, I figured I'd make the lantern "better".  I decided to replace the 4 watt CFL with a 3 watt LED and get even more energy efficiency out of the lantern.

I ordered a Cree LED on a star heatsink and a regulator from DealExtreme.

The important thing to know is Power = Volts * Amps.  A 3 watt LED with a rated forward voltage of 3.7 volts needs 3W / 3.7V = .81A, or 800mA of current.  The LED regulators are designed to put out a specific current, not a voltage.  So I bought a regulator rated at 800mA.  Matched with this LED, the regulator will adjust the voltage to be around 3.7 volts to get the delivered current to be 800mA.

A 3 watt LED can get mighty hot, and I didn't want my lantern to fail early due to overheating the LED.  So I  found a little 1-inch square heatsink I had laying around and a package of thermal paste.

Then I  had to figure a way to secure the LED to the heatsink (thermal paste is NOT glue) in a way that would survive car trips and the banging around at camp.  I came up with a bent paperclip design that is sort of based on the bails you get on CPU heatsinks.  I bent it into a "W" shape as you see in the picture, then twisted the arms to the sides so I could get the flat part up into the fingers of the heatsink.

Then I soldered the power supply to the Cree.  Red is positive, Black is negative.   Solder to any of the appropriately labeled pads on the top of the star.  The bottom of the star is electrically isolated.  It just conducts heat, so don't try soldering to it.  Also, it is a heatsink, so you probably won't be successful soldering to the bottom anyway.  In retrospect, I probably shouldn't have soldered the power supply to the star at this point.

I also applied way too much paste to the bottom of the star.  A very tiny bit goes a long way.  You just need a very thin film of thermal paste to ensure good thermal conduction between the star and the heatsink.  Then I put the star on the heatsink and twisted my paperclip bail back into position, making sure I wasn't shorting out power. 


 As you can see, the bail is pressing down pretty hard on the star, squeezing all that excess paste out. You should use just a tiny drop.

The bottom of the lantern unscrews, for replacing the batteries.  To get to the CFL bulb, unscrew two screws in deeply recessed slots revealed by removing the bottom.  To separate the glass from the base, you need to unscrew four nuts that are sealed with Locktite.

I cut the battery lead right at the CFL bulb assembly and removed it.  Then I replaced the CFL assembly with a little bit of plastic to support the heatsink and LED at the bottom of the lantern's glass.


I didn't come up with a clever method of mounting the heatsink.  I just used my hot glue gun.  It turns out, hot glue doesn't stick all that well to flat plastic nor heatsinks, so it took a couple applications to get it mounted.  This is a weak point in my design.  There must be a better way.  The 1 inch heat sink was slightly longer on the diagonal than the hole in the lantern, so I had to file out a couple notches.  Hopefully, this adds some stability to the heatsink.

To assemble this, I needed to solder the power supply to the battery wires with the case off, yet position the heatsink on the platform after putting the battery holder back into the case.  This isn't what I did, but what I would do if I had to do it again:
  1. Solder the battery leads to the power supply.
  2. Use a spot of hot glue to stick the power supply down to the top of the battery holder.
  3. Position the power supply output leads so they are sticking above the platform where the hole in the base will be.
  4. Lower the case over the battery holder, threading the power leads through the hole.
  5. Solder the power leads to the star.  Red to +, black to -.
  6. Attach the star to the heat sink with the paper-clip bail.
  7. Hot glue the power supply to the platform.  Use plenty of glue.
In the end, it should look like this:

A 3 watt LED is piercingly bright.  You do not want to look into the LED when it is on.  Mounted like this, most of that light gets lost in the top of the lantern, yet you can see the LED directly, which is hard on your eyes.  So I added two features to the lantern:  a plastic ball as a diffuser around the LED to protect my eyes, and a reflector at the top of the lantern to reflect more of the light out in a useful direction.

It turns out, a ping-pong-sized ball works nicely for the diffuser.  It diffuses the light coming directly from the LED to your eye and it fits the lamp's reflector on the base just right.  I cut holes on opposite sides of the ball to get more light up to the reflector.  Then I used hot glue to attach it to the reflecting ring from the lantern. 

Forgot to take a picture of the ball with the holes cut.  I made them about an inch in diameter.

For the reflector, to reflect the light 90 degrees, I need a 45 degree surface.  I know the diameter of the top of the lamp is 3.5 inches.  Therefore, the length of the side of the cone then needs to be 3.5 * cos 45 = 2.47 inches.

I drew a circle of radius 2.5 inches on a bit of cereal box cardboard.  Then I made a cut from the edge to the center.  I covered one side with aluminum foil and taped it into place.  Forming it into a tight cone that would fit through the opening in the bottom of the glass housing, I pushed it to the top and let go.  The cardboard tried to flatten out, which resulted in a friction fit with a cone about the dimensions I wanted.  Fortunately, the glass diameter is largest at the top, so it all hangs in place pretty securely.


Then I reassembled the lamp:

Turning it on, I get about the same light intensity that the fluorescent light produced, but thanks to the efficiency of LED technology, even better battery life.

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