Quite some time ago I built two Peltier lamps as shown in Make issue 06 in 2015 (https://www.heise.de/select/make/archiv/2015/6/seite-12). This was more or less my first real make project.
Unfortunately one of the two lamps stopped working after a short time. It’s time to fix this mistake. Question is, what’s the problem. I speculate on the Peltier element itself. Maybe it just got too warm and got destroyed. The Make manual says they tested several elements and had no problem with any of them. Therefore, at the end they went for the cheap TEC1-12706 which I also used for my lamps.
Although I found some high temperature Peltier elements here. At this point I rather check if the problem can be found somewhere else. Why? The TEG1-12610-4.3 which look good for my application cost 28USD each plus 50USD shipping costs. I would spend the money if I was sure that’s the problem. But the other lamp with the “normal” Peltier element works just fine, so in principle the high tempereature elements seem to be not neccessary.
The installation instructions on the TEC site point out, there is a hot and a cold side of the Peltier element. When both wires point in your direction, the hot side is up when the black wire is on the right hand side. The Make instructions say, it doesn’t mater which side is up and which is down. No matter if there is an asymmetry or not, the Peltier element in the working lamp has the same orientation than the elemet in the defect lamp. That’s not the problem.
I am not sure if the connection to the step up converter is the error source, as it doesn’t feature a reverse polarity protection. I definitely have to check that. On the other hand, tha lamp worked for a short time. In case I mixed up the connection, I would have expected the lamp to not work from the start.
The distance between the flame and the Peltier element also plays an important role. The Make instructions point out for distances greater 23mm everything was fine. As every candle, or better, flame, might reach a different hight, it could well be that I boiled this particular Peltier element.
Last thing I can think of is the heat conversion tape I used to conect the Peltier element to the heat spreaders. Honestly, I think it’s unlikely this is the error source. Anyway, I will see if there are buubles or something like that after dissasembly. A possible upgrade would be carbon heat conversion tape.
Enough for today. Next time I will dissasemble the lamp and replace the Peltier element.
Update – It’s “fixed”
So I removed the upper wooden part including the step up modul from the wooden base plate. Unfortunately, I fixed both using hot glue instead of screws back when I put the lamp together for the first time. Because, what could possibly go wrong. The current separation lead to some damaging of the wood on both parts.
At first I tested the Peltier element itself. I lit up a candle below the power block and measured the output voltage via multimeter. Voltage increases slowly to some ten millivolts above 600mV and then finds its equilibrium at around 600mV.
Next up is the step up converter. This I connected to a sourcemeter and, voilà, it’s power LED and the USB lamp connected to the step up converter both turn on at approximately 600mV.
In conlusion, every single component seems to be just working fine. So I put everything back together and gave it a shot. In wise foresight, I did NOT glue the parts together, but tested everything lying around loosely.
Why so suspicious? It works. After soldering the wires from the Peltier elemet to the step up converter, the test failed. Disconnect. Works. Soldering again. Works. Hot gluing in the step up modul. Fails. Back out. Works. Back in. Works. Gluing everything together. It fails!
Long story short, the whole thing seems to be right on the edge of functioning. Sometimes it overcomes the initial switch on and sometimes it doesn’t. When switching the lamp on using the sourcemeter it was visible that for a moment when switching the lamp (or the step up module) on it requries a higher amount of current in the range of 100mA befor going back to lower values of around 10mA. How to boost Peltier element output for a short period of time? Just blow on the cold side for some seconds. Guess what? It really works.
Once the LED is on, it runs more or less stable. You should avoid under any circumstances air movement that might cool down the hot side of the power block. I also think the higher the room temperature the smaller gets the temperature difference between hot and cold side. Which again decreases output voltage of the Peltier module.
In summary, the lamp works but it barely works. Often only with a little help at the start. At the end I reconsider buying a better Peltier element with more power at the same temperature difference. In principal stacking two elements on top and connecting them in a row should also deliver considerable more output voltage and make the whole operation of the lamp much more stable.
But before those optimisations, some better woodworking tools and a better fixture of the power block, maybe even height adjustable, would be nice.
Update – not really
As it turns out, the lamp was working for one complete tealight. The next time it became even harder to get it to run and the intensity of the LED lamp was super low. Third time I tried I couldn’t even start the lamp. The same thing seems to happen with my second build lamp. I guess it’s the cheap TEC1-12706 that degrade over time when exposed to high temperatures. Therefore, I ordered SP1848-27145 type Peltier elements. Let’s see how this turns out.