OK, fair enough... I guess there is some interest. I don't want to sound like a pouty forum baby, but I also don't want to waste my time and this forum's bandwidth writing longwinded posts if nobody's reading.
Since it appears that a few of you are reading, here's a longwinded post.
I finished making the protoboard version of the controller. I didn't think that this was the right time to learn how to etch traces, so I just used jumper wires. It's not THE most elegant thing in the world, but it worked on the first try. I was pretty chuffed, I must admit.
So, I've been trying different PWM duty cycles to see what the TEC likes versus an acceptable current draw. My power supply didn't like running the protoboard... as soon as the TEC's high duty cycle kicked in, the voltage dropped to about 6 VDC. That seems strange, because it ran the bare TEC just fine. I assume that it has to do with the PWM aspect that I've now added to the mix.
I lugged the real RV battery over to the bench and fired everything up. I managed to get my 40 C delta pretty easily after a couple reflashes of the ATTiny to nail down the duty cycle.
Here are some pictures. Be gentle.
The completed protoboard. I used a little buck converter that I had laying around to feed the ATTiny its 5 VDC, and fed the raw 12 VDC into the MOSFETs.
The blue wire heading north out of the picture is the temp sensor lead that I'll mount inside the fridge. The two small bare wires will be the door switch, but I'm just touching them together at this point to test the functionality (which works). The temp sensor and door switch are connected to the protoboard using recycled computer motherboard fan and LED plugs.
I used mini Dean's connectors from my local hobby shop as the TEC, fan and light connections. I knew that the 20 years of money I've thrown at R/C cars would come in handy eventually.
The test setup with RV battery. I won't lie to you; it wasn't pretty, but it worked.
To the right of the RV battery lies the hairdryer that I've been using to simulate a rising temperature on the temp sensor. If I had a wife, she'd kill me.
As you can see on the multimeter, the total system current draw at high duty cycle, including the liquid cooler, the TEC and both fans is 3.076 A. It continues to drop very slowly as the TEC attains its maximum delta, stopping at about 3.05 A. When I put my palm on the TEC to give it a heat load, current draw rises to about 3.15 A.
The fans have some pretty noticeable PWM whine, which will be as annoying as hell in the little trailer. I hoped that I'd eliminate that by increasing the PWM frequency to 3.8 kHz, but it doesn't seem to have worked. I don't have an oscilloscope to verify the frequency, but I'll keep testing to make sure that I used the right Arduino code for the frequency that I want.
After running this test setup for about 45 minutes, there were no surprises. The MOSFETs didn't warm up at all, the CPU cooler didn't heat soak in the slightest, and the current draw stayed consistent in both high-duty and maintenance modes.
So far, so good.