I’ve been struggling to keep my corrector plate of my 11 inch Celestron EdgeHD free of dew lately.  It remains free of dew for most of the night but seems to always dew up just before dawn when, critically, I need to be taking flats.  Since my system is now automated it has been frustrating me to wake up only to find a corrector covered in dew.

I’ve been setting my dew heater’s target power as high as100% recently but it doesn’t seem to be enough to keep ahead of dew for the entire night. The premiere controller can measure both the ambient air temperature and  the corrector temperature.  It also senses relative humidity so it can calculate the dew point.  Knowing dew point, it can adjust the power to the heater dynamically, allowing it to keep the corrector a preset temperature above dew point.  This has always worked really well for me… until recently.

Although I purchased a second dew heater strap for those times where the dew is absolutely atrocious, it occurred to me that there could be something wrong with the Kendrick controller.  As such, I measured the input and output power while running the dew heater. 

As I suspected, an enormous amount of power was being dissipated in the controller itself.  While the input and output currents were similar at just over 2 amps, the voltages were wildly different.  The input voltage was up at 13.3 volts while the output voltage to the heater was sinking below 10 volts on the primary heater output.

I opened up the controller and probed the various components.  Everything seemed okay until I probed Q1 – the FET.  It was clearly out of spec so I went searching on Digikey for a replacement.  Of course the FET was obsolete so I had to do a little digging to find a newer replacement.  I found one with similar specs but with a lower Rds – the ON state resistance, which would allow the controller to run more efficiently!  Since the part was $2 and shipping was $8, I decided to order 20 which brought the cost per part down and will allow me to use these FETs going forward in a dew controller of my own which I am currently developing.

The FETs arrived the next morning all the way from Texas!  I quickly de-soldered the old FET and had the new one soldered in place.  While I had the controller opened up I soldered in a new power cable as well.  The insulation on the old one was too stiff in the cold.  I Re-attached the two circuit boards together and screwed the whole thing back together.  Again measuring the input and outputs, the voltages and currents were back to spec!

I suppose the second dew strap will rarely need to be used now that the controller is functioning correctly, although I’m sure it will come in handy somehow or other!  In the meantime I’m going to enjoy not fighting the dew anymore.