DIY dew heaters and controller

[Clive]

A couple of weeks ago I decided it was time I started using dew heaters on my lens and guide scope so I set about designing them together with a controller.  For simplicity and cheapness, I decided to go with a resistor ladder.  Looking on the net it appeared that dew heaters have a power rating between 0.2 and 0.3 W/cm, so with a 12V supply and a resistor placed every 1cm, a resistor value of 560 Ω would dissipate 0.26 W.  A 0.6W rated resistor was chosen that was 7 mm long x 2.5 mm diameter.  The thermal impedance of a resistor that size in free air is about 140 °C/W, so at 0.26 W the temperature rise would be about 36 °C so shouldn’t create a burn hazard.

As the resistors were supplied on tape spaced 5mm apart, alternate resistors were cut out:

 

Two lengths of 22 SWG tinned copper wire were then threaded through the resistor legs at a 17mm spacing:

 

All the joints were soldered and a length of speaker wire attached to one end of the ladder:

 

A length of self-adhesive sealing strip was then attached to one side:

[Clive]

And then inserted into a length of heat-shrink tubing, the ends sealed using double sided tape:

 

 

The heat-shrink is not particularly flexible so small diameter heater will suffer from creasing.  To smooth them out the heaters were wrapped around a suitably sized tin can which was then filled several times with boiling water (remembering to keep the sealing strip on the outside, oops!).

 

 

The heat-shrink starts to shrink at about 90 °C so will not fully shrink at 100 °C but the larger creases will be smoothed out.  A short length of self-adhesive hook and loop was attached to each end to complete the heaters:

 

 

The controller is based upon a 556 timer driving MOSFETS to produce two pulse width modulated outputs and allows control of the power over the range 3% to 100%:

 

 

A couple of spring clips allows the completed controller to be mounted on one of the tripod legs:

 

Having attached one of the heaters to a tin can to simulate the guide-scope (not wishing to cook the guide scope optics!) I have now been able to measure the actual temperature rise.  The temperature rise is linear with knob position and at full power, the internal temperature of the heater band rises by 36 °C, which is exactly what was expected, whilst the temperature of the tin can outside the heater band rose by 11 °C.

[+RonC]

Nice, neat job Clive.. I think you'll use them a lot this Winter!! 

Ron

[BAZ]

Nice job Clive.

[Clive]

Well at least they appear to work. Set everything up prior to sunset under cloudless skies but by the time it got dark, cloud started rolling in. Managed a few shots of the moon before giving up but just left everything outside. After 2 hours at least the camera lens and guidescope were clear of dew whilst everthing else was dripping (but I did need to crank up the volume more than I expected). And now, having packed everything away, it's cleared up again - but work in the morning.

[Clive]

Finally pulled the pictures of the moon taken last night from the camera whilst trying out the new dew heaters.  This is the best of the 8 images I snapped through the increasing cloud using a Canon 200D + Canon 400mm F5.6, 1/250 sec at F8 ISO1600 (yes I should have dropped the ISO but I was gearing up for M33).  The first time I've imaged the moon for over 30 years so quite happy with it, but it doesn't match up to the quality of Derbyshire Dave's image which must have been taken at about the same time.