YADH (Yet Another Dew Heater)

[wimvb]

A few nights ago I tested my guiding setup and lost about half my subs to frost collecting on the guide scope. The dew heater I was using was too short and had too low output power.

A short discussion on this forum and a DHL package later, I may have solved the problem.

The box contains 2 12 V lead batteries. The left battery is connected to the dimmer for my guide scope dew heater (attached). The other battery is not dimmed and can power the dew heater for the secondary mirror (is lower power and doesn't need regulating). The dew heater itself is made from NiCr wire, 4 strands wide, and fits my ST80 guide scope.

Now all I need is clear skies (the frost is here already).

[cjdawson]

4 strands wide.  That's some serious heating power.    I've got a single strand that I've placed directly in front of the optics and that's more than enough to keep my scope ST-80 clear.

[wimvb]

Sorry, need to clarify this: one long strand folded, to get 4 next to each other = same as winding it 4 times around the tube. Wire length is abt 1.3 m. So, not 4 short parallell strands.

Sorry for any missunderstanding

[cjdawson]

Gotcha.  It won't be putting out as much heat as mine, but that's fine as I have a PWM on both of my heaters and tend to use them at about 1/3 or lower normally.    Sounds like your setup will give out lower heat and spread out over a larger area.

Nichrome heaters are great

[wimvb]

11 hours ago, cjdawson said:

Gotcha.  It won't be putting out as much heat as mine, but that's fine as I have a PWM on both of my heaters and tend to use them at about 1/3 or lower normally.    Sounds like your setup will give out lower heat and spread out over a larger area.

Nichrome heaters are great

And I found out during "field trials", that mine uses only half of its power for the telescope. I forgot to have some insulation on the outside, so half of the heat contributes directly to global warming. For the time being, I am saving the environment by wrapping a towel around the guiding tube. Looks ridiculous, but it seems to work.

My other two dew heaters are made from resistors. They work fine for my secondary mirror and 135 mm lens, but not for the larger ST80. My guess is that the power that is required must scale at least with the square of the tube diameter. For larger tubes, NiCr heaters really are great.

 

Cheers,

[cjdawson]

I made a Nichrome heater for my ST-80.  The way I did it was a single loop of nichrome, threaded through some heat shrink. That in turn threaded into some copper brake pipe.   The pipe has been bend into a circle, and I intend on spray painting it matt black.    This loop is was then pressed into the end of the tube, just in front of the main lens.  There's no need for insulation, as the heat is being radiated directly in front of the part that needs to be kept dew free. The nichrome wire comes out of the tube, then is connected to normal wire.

The next result of that is that there's a very powerful heater that delivers the heat where it is needed.

[wimvb]

Oops!

Shorted the dimmer after loading the battery. These things can double as an orange, one time flash. Ordered replacements already. (Found another supplier xenonhuset.se)

I opened the burnt dimmer and what I saw inside makes me doubt the rated output power (96 W, 8 A). There is no heat sinking to speak of. The circuit consists of a simple timer and a power transistor, which is soldered to the pcb. The whole thing must get red hot when put to the test.

I think I will fit a 5 amp fuse in the output line, just to be on the safe side.

[cjdawson]

I use a couple of these for my PWM circuits.

http://www.velleman.eu/products/view/?country=be&lang=en&id=9391

They're easy to make and fun too.

Over the 10 years that I've had them, I've had no reason to doubt their power output.  Also, as they pulse at about 4K, I cannot say that I have noticed any issues with electrical noise.  I power everything - laptop, heater (2 pwm, and 2 elements), telescope, light box, and various other accessories all from the same leisure battery.