Wilmslow Astro
Astronomy from a Cheshire suburb
Make Your Dew Heater Controller
Please note that I cannot accept any responsibility for damages however caused by your attempts to recreate the dew heater shown below.
I present here the construction details for a dew heater that I made for my 10" LX200. It provides variable heat output and is fairly cheap to construct.
I've put a shopping list of components together at the end of the page.
[Click on the images below for a larger 640x480 version]
 | Here is the heater controller circuit diagram. This is a pulse width modulator based on 3525 PMW chip. The potentiometer VR3 controls the duty cycle and hence the power output. You can either make your own cct, or buy a kit from Velleman (K8004). I bought the kit from Maplin (£15), but you can get it cheaper at CPC (about £12) :) This kit will handle up to 6 amps, works on anything between 8.5V and 35V, and has reverse polarity protection. I suggest you add in an in-circuit fuse rated just above your projected maximum load. |
 | The made up circuit board mounted in a box. The kit calls for a 1K linear pot for control, I used a 5K that I had hanging around and that works fine. |
 | And mounted in a box - I've put four phono sockets on the box for plugging in 4 heater elements. The 12v feed is via a standard 2.5mm socket, this allows the power cord to pull free in the event of it being caught. |
 | The heater element is made up of 15 x 1R8 3 watt resistors spaced at 6cm centres. Just overlap the resistor legs the appropriate amount and solder them together. I copied the basis of this design from that of Chris Heapy. |
 | These are then placed in heat shrink tubing for extra strength and electrical insulation. |
 | The whole lot is then fed into an 8mm external diameter micro-bore copper water pipe. |
 | Finally the pipe is bent and mounted in the LX200 inside the corrector plate mounting rim - but in front of the correct plate itself! Don't worry, I will did paint it black :) |
Parts list (and Farnell order codes)
15 x 1R8 3 watt wire wound resistors (343-705)
Heat shrink 1.2m (143-721)
Phono plug x 2 (150-866)
Phono panel socket x 4 (149-269)
1k ohm panel potentiometer ()
2.5mm DC panel socket
Project box
Velleman DC to Pulse Width Modulator K8004