Introduction: 10W LED Fiber Light Source

Over the past few years I have been slowly putting together a Franken-ebay stereo microscope. The stand, microscope, and ring light all came from different vendors and were mostly selected because the price was right. Adapting the stand to the microscope turned out to be a easy job on the lathe. The ring light fit off the bat (I did measure my scope before buying so not a surprise.). But I was left without a light source. Looking on ebay for a matching source they ran from $75 to $300 depending on condition.

Eventually I decided to just build my own. The official light sources typically use a halogen bulb. These are hot and have a useful life measured in the tens or hundreds of hours. Since I was building one from scratch I decided to go with a LED light source. This should result in a light that runs cooler and longer than a halogen bulb would.

Here are some progress videos I took during development of this light source:

Early testing

Testing with the fiber ring light

Final results

Step 1: Bill of Materials and Tools Needed.

Bill of material:
Since the design of the light greatly varies based on the fiber your trying to illuminate this bill of materials will be vague.
  • 1x LED (see step 2)
  • 1x Heat sink (see step 2)
  • 1x Fan (optional see step 2)
  • 1x IRF640 N-fet (optional if using a fan)
  • 1x power jack to match power supply (optional) 
  • Power supply (see driver / dimmer instructions) 
  • 1x 10W led driver / dimmer card (see instructable here
  • hookup wire
  • 6-32 screws and lock washers (various lengths see CAD data) 
  • double stick tape (I like Scotch exterior mounting tape) 
  • Kapton or electrical tape
  • thermal grease
  • Aluminum bar stock (see CAD data for sizes) 
  • heat shrink tubing
  • HVAC galvanized sheet metal

  • metal band saw or hack saw
  • Mill and lathe + supporting tooling (optional can be done on a drill press but much easier with a mill / lathe) 
  • soldering iron + solder
  • 6-32 tap and handle
  • center drill
  • #36 drill (for tapped holes) 
  • #26 drill (for clearance holes) 
  • ? drill to match outside diameter of fiber end (if not using a lathe) or boring bar (if using a lathe) 
  • end mill (exact size not critical just used to clean up the stock and cut the LED channel)
  • cutting oil 
  • calipers 
  • center punch
  • 4 jaw independent latch chuck
  • center (live or dead) 
  • dial caliper and stand 
  • test indicator and stand 
  • 2x 1-2-3 block
  • table clamping kit
  • sheet metal hole punch or metal hole saw
  • edge finder 
  • file or deburring tool 

Step 2: LED and Heat Sink Selection

LED selection:
First order of business is to select the LED.
Most fiber light sources use a 150W Halogen lamp. In general these lamps put out around 1800 lumens @ 3200K color temp. 

Using this as the standard for the lamp a direct replacement would be a 20W led module with 1800 to 2200Lm @ 3000K. But the other thing to take in consideration is the cross section of the end of the fiber your going to drive. In my case the fiber has a active cross section of  9mm circular. The 20W leds have a active area of ~18mm square. Without some sort of optics to focus the light on the end of the fiber a lot of the light would be lost.

To start with I decided to go with a 10W led module. These have a active area of ~10mm so I should be able to butt up the end of the fiber to the output of the LED without optics with minimal loss. Also since I am building from scratch I chose a hotter color temperature of 6300-7000K which produces a more blue than yellow white. Personally I like the blue end of the spectrum when working on small projects.  Looking around on ebay I found a DC-LE14274 from sure electronics. This led is 900Lm @ 6300-700K when driven at 1050mA. My driver card will only go up to 1000mA but even at the reduced current and lumen output it was plenty bright for my ring light and scope. 

Heat sink selection:
Now that the LED is selected we know the max power dissipation the heat sink will need to handle. In my case 10W max. Since some of the power is converted to light the actually heat load is much less but for explanation simplicity I am going to call it 10W. 

There are many choices for heat sinks out there. The 2 most important issues are the amount of thermal mass and the surface area of the fins. The more thermal mass a heat sink has the longer it will take to heat up or cool down. The more surface area the fins have the faster air can remove the heat from the heat sink. Designing a heat sink is a complex engineering task with lots of trade offs. It can be as simple as a large bar of aluminum to as fancy as a multi metal finned heat sink with heat pipes. Digging around in my pile of junk I found a small ~91mmX79mmx75mm heat sink and fan off a old PC. Most modern CPUs run in the range of 50W to 300W so this should be overkill for my application. 

Note if your using a CPU heat sink and it has "heat pipes" (tubes running from the CPU mounting plate though the fins) you MUST make sure you will not drill any holes though them else the thermal fluid will leak out and greatly reduce the thermal efficiency of the heat sink. Also typically these heat sinks must be mounted with the CPU plate down. Other mounting orientations will result in reduced cooling efficiency.

Fan or not to fan:
The requirement of a fan is best decided experimentally. In my case the heat sink would get very warm (but not burning hot) after a half hour of use at full brightness with no fan. With the original 12V fan hooked up and running at 44% PWM (ie slowed down) the heat sink would stay cool to the touch after a half hour. For my application I don't think the fan was really needed since I only use it for short periods of time and it did not get significantly warm. But in the interest of the longest LED life I did end up adding the fan. 

Once your LED, Heat sink, and fan are selected I highly suggest modeling the devices in sketchup or similar CAD program. From this model you can design the mounting hardware and dimension all the parts for machining. Attached is the sketchup model for my LED light source minus the stand mounting brackets.

Step 3: Drilling and Tapping the Heatsink

Once you have laid out all the parts on you heat sink in CAD it's time to drill some holes. 

First mount the heat sink CPU face up on your mill table. My heat sink had 2 edges that protruded out from the body of the heat sink. I used a pair of 1-2-3 blocks and table clamps to hold the heat sink in place. Tighten the clamps to just snug for now. 

Next set up a dial indicator on one edge of the heat sink as shown. The exact reading is not important what your looking for is the reading not to move as you run the axis of the mill perpendicular to the dial gauge in and out. Gently tap on the corners of the heat sink to adjust it. Once the needle does not move more than a thousandths or so tighten down both clamps. Verify the heat sink did not move during clamp tightening by running the axis in and out while watching the dial indicator. If all is good (no or very little gauge movement) then the heat sink is parallel to your table. 

Next load up your edge finder and zero out to one edge of the heat sink in both directions.

Load up a center drill and using the DRO and CAD drawing locate and pilot drill all the hole locations in the heat sink.

Load up the #36 drill bit and drill out all the hold locations in the heat sink. 

Remove the heat sink from the mill and tap all holes to 6-32. Note if this is your first time tapping here are some tips:
  1. Use plenty of cutting fluid on the tap and hole. 
  2. Make sure when starting the tap you keep it parallel to the bore of the hole. 
  3. When tapping only turn in 1/4 turn at a time then back out 1/2 turn and back in 1/2 turn. This breaks the chips off as they form. 
  4. Remove the tap frequently and clear any chips out of the hole and tap
  5. DO NOT FORCE The tap. Taps are hardened and very brittle. Nothing better to ruin your day than a broken tap in a hole. 
Once this is done clean up the heat sink with paper towels and rubbing alcohol and it should look like the last photo.  

Step 4: Mounting the LED

Using your calipers and dimensions from the CAD drawing draw a scribe line as shown for the mounting location of the LED.

Coat the back side of the LED module in thermal grease. Align the LED module with the scribe lines and set down on the heat sink. Start the 4 6-32 mounting screws as shown (note new design files the washers should not be needed). Before tightening verify the LED module is aligned to the scribe marks (and therefore centered under the fiber mount). Then tighten the screws slowly alternating between opposite corners. By doing this you should slowly and evenly squeeze out any excess thermal paste. Wipe up any excess paste with paper towel and clean with rubbing alcohol.

Place a small piece of either Kapton or electrical tape under the LED terminals as shown to prevent accidental short to the heat sink. Solder wires to the LED as shown. Make sure to note which wire is + and -. On my LED the red wire is +. 

Step 5: Wiring and Testing the LED and Driver.

See wiring diagram and ignore the IRF640 and fan connections for now.
NOTE the LED is VERY bright. Best to cover the led with a piece of paper or point away from you before powering it up. 

Wire up the driver card as follows: 
  • LED- to J1 pin 1
  • LED- to J1 pin 2
  • Power supply + to J1 Pin 3 
  • Power supply - to J1 Pin 4
NOTE just solder the end of the wires to the top of the pads. Do not insert them in the holes and solder on the backside. This would interfere with the mounting used in this application. 

Clamp the CAT4101 on the driver card to your heat sink temporally and apply power. Press SW3 once and the led should light up at 10%. If not see the debug guide in the LED driver / dimmer instructable. If it tests out ok disconnect the power and proceed. 

Step 6: Machine the Fiber Adapter and Driver Clamp.

Next using a hack saw or metal band saw cut up a chunk of aluminum bar stock into blocks slightly larger than your fiber retainer and driver clamp. 

Mount a machining vice and dial indicate off the fixed jaw. Adjust the vice until it is parallel to the axis of the mill then tighten down the vice mounting bolts. 

Mount the block for the fiber adapter and using a large end mill take a light pass on 3 adjacent sides to clean them up. On the other 3 sides machine the block down to final dimensions. 

Mount the fiber block led side up in the vice on parallels. First using a end mill cut the LED pocket per the CAD data in several passes. Edge find 2 edges and using a center drill locate and pilot drill all the holes on the back side of the block per the CAD data. Next load a #26 drill and drill all the holes on the backside including the center hold for the fiber. 

Next remove the block and flip it onto its side for the fiber retainer screw. Note make sure you get the block in the proper orientation the U should be horizontal in this case.  Edge find 2 edges and pilot drill the hole using a center drill per the CAD data. Load a #36 drill and drill out the fiber retainer screw hole all the way to the center of the block. 

Flip the block so the driver clamp screw hole side is up. Make sure the LED channel U is vertical and the hole will end up on the right end as shown in the photos. Edge find 2 edges and pilot drill with a center drill per the CAD data. Next load a #36 drill and drill the hole for the driver clamp. Note this hole does not need to go all the way to the center but will not hurt anything if it does. 

Remove the block and mount it in a independent 4 jaw latch chuck using a center (either live or dead) in the tail stock as shown to hold it in place. lightly tighten up the chuck jaws on the part and remove the tail stock and center. Set up a test indicator as shown so it rides on the inside of the center hole in the block. Watch the test indicator while spinning the lathe chuck by hand. Slowly adjust the 4 jaws until the indicator does not move at all when the 4 jaw chuck is rotated 360 degrees. Once dialed in fully tighten the jaw chucks and verify the part did not move by rotating and watching the test indicator. 

Drill out the center bore using incrementally larger drill bits held in the tail stock of the lathe. Continue this process until the hole size is just smaller the outside diameter of the end of the fiber optic bundle. Switch to a appropriate sized boring bar on the cross slide and bore out the hole to a few thousandths over the outside diameter of the end of the fiber bundle. Test fit the bundle and if you have a slip fit you are done and the part can be removed. If it is tight or binds take a few more thousandths off the inside diameter of the block. Repeat the test fit and cut steps until you have a slip fit on the fiber bundle. 

Remove the block and tap the 2 side holes (fiber retaining and driver clamp) with a 6-32 tap. See my previous notes on tapping same rules apply here. Deburr all edges and holes then test fit the block on the heat sink. If done correctly the 4 mounting holes should line up and the center hole should line up with the center of the LED. Also verify clearance between the LED and the inside surfaces of the block. The block should not touch any part of the LED, LED mounting screws, or pinch any of the wires. 

Next mount the rough block for the L shaped driver clamp in the mill vice and using a large end mill take a light pass on 3 adjacent sides to clean them up. On the other 3 sides machine the block down to final dimensions. Use that same cutter to cut out the center of the L shape on the block per the CAD data.

Rotate the block in the vice and load a pilot drill, edge find 2 edges and pilot drill the hole per the CAD data. Drill out the hole with a #26 drill. Repeat these actions for the other mounting hole in the block. Note the holes should NOT intersect. If they do double check your drill locations per the CAD data. Deburr all edges and holes and check fit on heat sink with fiber mounting block. 

Step 7: Fiber Mount and LED Driver Assembly

Take the fiber mounting block and 4x 6-32 screws and lock washers and mount the block as shown in the photo. Verify the LED wires are routed out the driver mounting clamp end as shown and are not pinched under the fiber mounting block. Tighten the screws alternating between opposite corners. 

Clean the top of the heat sink where the driver card and CAT4101 will mount with rubbing alcohol. Apply a piece of 1" x 1" square of double stick tape to the back side of the driver card. Apply a thin coat of heat sink compound to the back of the CAT4101 driver IC. Peel off the backing paper of the double stick tape and place the driver card as shown and press down to stick the tape to the heat sink. 

Mount the driver IC mounting block over the CAT4101 driver IC as shown. Using 6-32 screws and lock washers attach the block starting with the screw that goes into the heat sink. Once that screw is tight install and tighten the one that goes into the fiber mounting block. Check that the drive IC is secure and some of the heat sink compound has been squished out from under the IC. If not loosen the block mounting screws and place a small shim on top of the driver IC and repeat (should not be needed if the parts were machined properly). 

At this point you can test the assembly and determine if a cooling fan is needed or not. As discussed in step 1 you may or may not need a fan based on the heat sink and application. Install the fiber bundle and locking screw. Just tighten the locking screws enough to retain the fiber bundle. Run the light a full brightness and check the temperature of the heat sink over the course of normal use.

Step 8: Optional Fan and Power Jack Wiring.

If you decide you need a fan on your heat sink you have 2 options for control. 
  1. Off when the LED is off and full on when ever the led is on (pin 5 of J2) 
  2. Off when the LED is off, 25% PWM on when the LED is set to settings 1-5, and 44% on when the LED is set to settings 6-10 (pin 4 of J2)
Either method uses the same wiring diagram with the only change of which pin on J2 it connects to. Option 1 gives you maximum cooling all the time. Where options 2 gives less cooling and noise. In my testing a minimal amount of air flow was required to keep the heat sink cool to the touch. The fan PWM rates can be adjusted in firmware see the LED driver / dimmer instructable for more information. 

Some fans have a PWM input and can be driven directly by the PWM / on-off output of the driver card. The fan I had did not. It only had a RPM output wire. In the case of a fan RPM output wire it is not used and should be capped of. Since I did not have a PWM input on my fan I had to drive the PWM signal into the power feed. The following instructions assume your using this method to control the fan speed and your adding a power jack.
  1. Connect the negative side of the power jack to the source pin of the IRF640 and pin 4 of J1 on the driver card. Cover any exposed connections (IRF640 and power jack) with heat shrink tubing. 
  2. Connect the positive side of the power jack to the positive wire of the fan and pin 3 of J1 on the driver card. Cover any exposed connections (power jack) with heat shrink tubing.
  3. Connect the negative wire of the fan to the drain of the IRF640.Cover any exposed connections (IRF640) with heat shrink tubing.
  4. Connect the gate of the IRF640 to pin 4 of J1 on the driver card. Cover any exposed connections (IRF640) with heat shrink tubing.
Since the fan draws so little power there is no need to heat sink the IRF640. But you MUST isolate the tab on the IRF640 from the heat sink. If these short together the fan will receive full power all the time. Easiest way to do this is just put a piece of heat shrink tubing around the entire IRF640 (see photos)

The power jack can be mounted in any  convenient way. The jack I removed from the old laptop had a small mounting screw on one side. Using that and some super glue I attacked it to one of the heat sink fins. 

Step 9: Lamp Mounting

The lamp can be mounted in any wan that is convenient for your setup. A few guide lines when deciding mounting:
  1. Make sure you have plenty of clearance around the intake and exhaust of the heat sink fins. 
  2. Fiber optic cables do not like sharp bends they will break. 
  3. If using a heat sink with "heat pipes" they work best when the CPU mounting flange is pointing down. 
  4. If not using a fan orient the fins of the heat sink so the exhaust points up. This will use heat convection to draw air through the heat sink.  
Other than that find a method that works best for your application. Could be a box that sits on your bench or in my case a bracket that hold the lamp on the post of my scope stand. 

To make the mounting bracket I started with a piece of HVAC galvanized sheet metal. The piece I had already had a small lip bent at a 90 degree angle. Note you will need to adjust the dimensions to match your heat sink and scope post (mine was slightly under 1.5" in diameter)  

First I cut two 2.5" square pieces from the HVAC sheet metal so each piece had a lip on one edge. Next I drilled 3 pilot holes using my drill press (MAKE SURE you clamp the metal down when drilling. DO NOT try and hold it with your hand unless you want to lose a finger.) Two of the holes are located on either end of the small lip for mounting the bracket to the heat sink. The third hole is located half way back from the lip and offset to one side such as it lines up with the center line of the heat sink. Note you will need to locate this hole on opposite end of the brackets to make a left and right side when mounted to the heat sink. If drilled in the same location they will not line up when installed on the heat sink.  Drill out all 3 holes with a #26 drill bit. 

Originally I was going to use a hole saw to cut the 1.5" hole until I realized my hole saw was for wood only. I lucked out and happened to have a 1.5" panel punch. So I drilled out the post mounting holes large enough for the panel punch bolt to slide though then punched out the hole to 1.5". Either method would have worked equally well. Deburr the edges and holes. 

Next line up the brackets with the small lips pointing inward on the heat sink. Using a pen or punch transfer the 4 mounting hole locations to the heat sink. My heat sink happened to have a think backing plate on the top of the heat sink. Other than watching out for the heat pipes I was able to drill and tap this for mounting the bracket. Center punch the 4 mounting locations (if not done already) and using a center drill in a drill press pilot drill all 4 locations. Next drill out the holes using a #36 drill bit. Tap to 6-32 (all the same tapping rules apply as before.) and secure the mounting brackets as shown with short 6-32 screws and lock washers. 

At this point the light source is ready for use. assemble the source to the stand and attach the fiber bundle. Note do not over tighten the fiber bundle retaining screw and use zip ties to secure the fiber whip so that sharp bends are prevented. 
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