About ten years ago, my wife bought a fancy china cabinet with a pair of halogen display lights recessed into the top of the cabinet. The halogen down lights highlight two compartments with glass shelves containing pieces that have special meaning to our family. The halogen bulbs do a good job of illuminating the cabinet interior but the bulbs run darn hot inside the china cabinet and they’ve started to burn out with regularity—every month or so. It could be the cheap halogens I’ve been buying ten at a time on eBay. However, I just can’t bring myself to purchase $5 halogen bulbs from lighting stores that might be no more reliable even though they cost perhaps ten times more than the eBay cheapies.
When my latest stockpile of halogen bulbs started to dwindle, I considered another bulk purchase of ten bulbs on eBay. Then I realized that it might be time to replace the halogens with LEDs. As it turns out, that’s not as easy as it sounds even though LED replacement bulbs are now the “big thing” in residential lighting.
Our china cabinet’s light fixtures use 12V halogens with G4 pin bases. You can find many LED replacements for G4 halogen bulbs on eBay (they’re hard to find at Lowe’s or Home Depot), but most of the puck-style, 12V G4 LED replacements are designed for dc supply voltage, which you need if you’re replacing bulbs in an RV or a boat. However, my china cabinet and most halogen-using indoor light fixtures run on 12Vac.
Some slightly more restrictive searching on eBay turned up a very few ac-powered LEDs with G4 bases. The trick here is that the LED puck must incorporate a regulated dc power supply in addition to the LEDs and associated current-limiting circuitry. I finally selected some warm white ac/dc-powered pucks from Wired Communications. The price was $9.95 per puck plus shipping—about ten times the cost of the halogen bulb that the puck replaces—but if I could avoid the need to get out a stool and replace one or two bulbs on a monthly basis, I was willing to spend the extra money.
I placed the eBay order and the pucks arrived in a few days inside of a bubble-pack envelope. Here’s a photo of the front and back sides of the pucks from Wired Communications:
The photo shows that the business side of the LED puck has ten warm-white surface-mount LEDs soldered in place. The back side of the puck has four surface-mount power diodes in a full-wave bridge configuration plus a large electrolytic filter cap and what looks to be some dc power-regulation circuitry. There’s an inductor in there, so I presume the power regulator is a switcher. The full-wave bridge allows this LED puck to run on ac or dc so you need not be mindful of socket polarity in dc applications.
When the pucks arrived, I pulled out my old HP 6236B bench dc power supply (purchased on eBay from Loveland, Colorado and complete with a genuine HP asset tag so you know it came from some HP engineer’s bench, maybe even mine) and hooked it up to the puck. The puck started to light at perhaps seven or eight volts. It was a lot of light. A painful amount of light.
Those SMT LEDs can burn spots in your retina. I now know from experience. Clearly, some additional diffusion was in order.
At first, I considered diffusing the light with some theatrical film used to scrim stage lights. I’ve got sample books of Roscolux filters and there are several white diffusion films in the Roscolux line that would have worked. (I first discovered Roscolux filter books through the Strobist blog. They are perfect for gelling camera flash equipment and I bought a bunch when you could get them for two cents apiece from B&H Photo and Video. Thanks to their success through Strobist notoriety, they now cost $2.50 and are still a big bargain.) However, after some consideration I felt that film diffusion would to be too thin (not enough effective diffusion) and flimsy for use on the LED pucks and so I decided to take a different approach.
Tap Plastics had a partial answer. They sell 1-inch cast clear acrylic disks at 25 for $6.25. I’m lucky enough to live near a Tap retail store, so I picked up two for 65 cents apiece. Unfortunately, the acrylic disks are crystal clear and so they provide no diffusion as sold. Some 330 and 150 grit sandpaper and a little elbow grease provided me with the required diffusion. I sanded both sides of the disks.
Next, I had to decide how to attach the disks to the LED pucks. The first try was a disaster. I was out of epoxy but I had some Pliobond rubber adhesive. I dabbed a bit on the outer corners of each outer LED on the puck and set the sanded acrylic disks in place. The next day, I had finished assemblies to try out. That’s when the disaster struck. You need to use a bit of force to plug the G4 lights into their sockets and the shear force proved way too much for the Pliobond adhesive. The acrylic disk tore off of the LED puck as I was inserting the first puck.
Back to the drawing board.
Some additional research suggested I’d have some difficulty with using most glues and epoxies on the plastic SMT LED housings so I finally settled on a plastics-specific epoxy called Permapoxy Plastic Weld, which I bought at the local Orchard Supply Hardware store for about five bucks. Permapoxy is methacrylate-based so it smells to high heaven. Use it in a well-ventilated area. A really well-ventilated area.
Overnight curing produced some rugged assemblies that look like this:
This closeup photo shows that my sanding might have been more consistent but you can’t deny that there’s diffusion going on here. Another test with the HP power supply demonstrated adequate diffusion. My eyes no longer hurt when I looked at the lit puck.
I plugged the LED puck assemblies into the china cabinet fixtures and hit the power switch. Both lights switched on and my wife approved of the illumination. The LED pucks actually seem a bit brighter than the halogen bulbs they replace, which was a surprise. The color of the LEDs’ white light is about the same as the halogens and the pucks run a lot cooler. Here’s a photo of the installation.
So, what do I conclude? There was way too much effort involved in this particular replacement adventure for mere mortals. Most consumers aren’t going to understand that they need ac pucks not dc pucks. Most consumers aren’t going to look on eBay for these products and they will not find them in Lowe’s, Home Depot, or Orchard Supply Hardware. Most consumers will not be happy with the light from the undiffused SMT LEDs mounted on the pucks because looking at the lights (unavoidable when looking at the items illuminated by the pucks) will be painful. Most consumers do not shop at an arcane store like Tap Plastics and are not familiar with theatrical lighting films. Finally, most consumers are not experts in plastics-specific epoxies.
So the barriers to adoption of halogen-replacement LEDs appear to be many and frankly, I do not see how we’re going to hurdle these many barriers in the short term. But my problem’s solved, for now.