omaroo
May 14th, 2007, 09:16 PM
Hi everyone. :)
I wanted to construct my own dew heater and save a few bucks.
I took 2 hours off this evening and built a strip dew heater to Al Sheehan's spec (on IIS) and it works absolutely brilliantly. The spreadsheet called for 52 x 390ohm resistors 18mm apart and it calculated 1.6 amps at 12v to run it. The article is here, along with the spreadsheet: http://www.iceinspace.com.au/index.php?id=63,292,0,0,1,0
Well - I soldered all 52 in parallel in a 45 degree staggered ladder pattern and lightly taped this ladder to the center of a strip of plastic 20mm wide. After wiring it and anchoring the end, I sheathed the whole shebang in 28mm heatshrink tubing and butted the ends together. I then used an additional section of heatshrink to cover the join and finish it off nicely. The resistors are on the outside of the internal plastic strip within the sheath - next to the scope's corrector plate holder assembly. The flex in the plastic skeleton means that it is a very snug fit - no loosness but not too tight. It actually looks like a bought one I think - it turned out very nicely because the resistor bumps aren't visible - all you see is smooth plastic.
I plugged it in and it became warm immediately. In fact I'm going to make a pulse controller using a TIP120 Darlington transistor and a TTL 555 timer tomorrow which will let me vary the heat output and run multiple heaters (scope, guidescope, 8x50 finder and Telrad). Circuit here: circuit here: http://www.backyard-astro.com/equipm...dewheater.html
Thanks Al - you were spot on. It draws 1.55 amps at full bore, which in theory is 19 watts or so. With the controller I'll be able to vary that to around 0, 5, 9, 14 & 19 watts in five steps.
Cheers - and thanks again Al for your spreadsheet
Chris
http://www.omaroo.net/albums/album34/DSC_0195.sized.jpg
http://www.omaroo.net/albums/album34/DSC_0194.sized.jpg
I wanted to construct my own dew heater and save a few bucks.
I took 2 hours off this evening and built a strip dew heater to Al Sheehan's spec (on IIS) and it works absolutely brilliantly. The spreadsheet called for 52 x 390ohm resistors 18mm apart and it calculated 1.6 amps at 12v to run it. The article is here, along with the spreadsheet: http://www.iceinspace.com.au/index.php?id=63,292,0,0,1,0
Well - I soldered all 52 in parallel in a 45 degree staggered ladder pattern and lightly taped this ladder to the center of a strip of plastic 20mm wide. After wiring it and anchoring the end, I sheathed the whole shebang in 28mm heatshrink tubing and butted the ends together. I then used an additional section of heatshrink to cover the join and finish it off nicely. The resistors are on the outside of the internal plastic strip within the sheath - next to the scope's corrector plate holder assembly. The flex in the plastic skeleton means that it is a very snug fit - no loosness but not too tight. It actually looks like a bought one I think - it turned out very nicely because the resistor bumps aren't visible - all you see is smooth plastic.
I plugged it in and it became warm immediately. In fact I'm going to make a pulse controller using a TIP120 Darlington transistor and a TTL 555 timer tomorrow which will let me vary the heat output and run multiple heaters (scope, guidescope, 8x50 finder and Telrad). Circuit here: circuit here: http://www.backyard-astro.com/equipm...dewheater.html
Thanks Al - you were spot on. It draws 1.55 amps at full bore, which in theory is 19 watts or so. With the controller I'll be able to vary that to around 0, 5, 9, 14 & 19 watts in five steps.
Cheers - and thanks again Al for your spreadsheet
Chris
http://www.omaroo.net/albums/album34/DSC_0195.sized.jpg
http://www.omaroo.net/albums/album34/DSC_0194.sized.jpg