Friday, March 21, 2014

Stacked Peltier Devices

I stacked the Tellurex C2-30-1503 and Tellurex C2-30-1505 devices as noted.  Just needed to pull out the old pad of paper that was supporting the copper plate and add some extra foam insulation to properly close the area around the parts. Ideally, I should be using some chemical foam, but that's pretty messy stuff and I'm not finished yet.  No pictures of the assembly this time, I was working quickly. 

My math was off with figuring out what the amp draw of the two devices would be stacked.  With 12V applied, the draw is 5.5 to 5.7 A.  This is only about 1 A more than the 1505 by itself.  I believe that the difference is because the pull and temperature is derating the resistance of the devices. Handling 6A is not a small amount of power for use in the field.  I'm considering what it will take to provide this much juice to the scope. I'll play with putting a splitter closer to the battery/car vs putting it at the Kendrick power distribution box.  Running two cords should give the effect of less cable resistance.

The only part that really complains about low voltage are the dew heaters.  If necessary, I'll put a voltage regulator on it to help support them  They draw 1.1 A under full load so it shouldn't be much of an issue.

So far, the air based heat sink seems to be keeping up with the upgraded power requirements.  I'm testing it on the desktop and I can feel that the tabletop is warm.  That shows me that there's quite a bit of heat being moved.  I looked into some closed-circuit water cooling devices to see if they would be good for this setup.  While the concept is good, the hoses are too short for my use.  Most are one foot to 18 inches - great for use inside a computer case, horrible for trying to route hoses from the back of a camera on a telescope. If I need to go this way, I'll roll my own system as I'll either need 2 feet of hoses to mount the system on top of the scope (piggyback style), 6 feet to mount it at the tripod head, and 10 feet to go all the way to the ground. 

After all the testing on the desktop and again in the refrigerator, I'm disappointed that the EXIF temperatures did not drop below 17C.  I believe that either I've misconfigured the TEC devices OR there's not enough watts pulled out of the camera OR that the EXIF sensor is no where near the cold finger.  Best I can do at this time is continue to take darks and compare with earlier configurations.

As another followup, I see the reason why the PWM controller stopped working.  Apparently the thermistor has gone open.  I'll get another to replace it and when I can, I'll put it into the system.  May be able to wire up the assembly with the new stacked devices to better control the power needs.