my diy dew heater

[kingkong]

.
the controller is based on Len Benschop's design - details can be found here: Len's Benschop Dew Remover Controller

the heater is made from nichrome wire i bought off e-bay, some heat shrink tubes, and velcro tapes


the nichrome wires are inserted into the heat shrink. both ends crimped on to some 1mm multistrained wires which are connected to the controller box by RCA connectors.


the nichrome wires are sandwiched between the velcro tapes.


the circuit is built on a small piece of prototype board. i elected to use the cheaper FET instead of the Opto22.


everything is then fitted into a standard electronics project enclosure like those you can buy in sim lim tower... as you can see from the Benschop webpage, it can fit into a smaller enclosure...


i bought a 12VDC 3A power adapter to power the whole thing. the typical 12VDC wall adapters are normally rated 1A or less. since the heaters are rated around 10-15W, a bigger power supply is required.


two heater straps are made. one for the 8" SN8 and C8 and one for 4" refractor. and i still have nichrome wires left over....



i've tested it indoor. it heated up the 4" refractor no problem.
will put it on my C8 in the obs and test it the next clear night.

all comments and suggestions are welcome.

thanks for looking.
--kong
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[anat]

Kong:

Well done! Do we need any specific nichrome wire ?

Thanks,
Anat

[cataclysm]

It looks really neat.

[Canopus Lim]

Nice

[kingkong]

Kong:
Well done! Do we need any specific nichrome wire ?

thanks Anat. this is nothing compared to some of your super-duper works/ideas :oops:

this is the first time i work with nichrome wire.
it's thin - 0.2mm
it's flexible - bending radius not a problem

the main things to keep in mind are:
1. it's not insulated - that's why i stick it into heat shrink tube
2. its resistance - the one that i got is 35ohm/m.
3. the wire gets hot (as opposed to only the resistive elements get hot in a resistor-train kind of heater) so soldering may not be a good idea. use crimps. in theory any way. i've not actually tested using solder but i suspect the wire may not get too hot (>300degC) to pose a problem...

i used the attached excel table to calculate/approximate the required length. the thing is the bigger the scope the higher power you'll need, and smaller power for finder and ep's. however, for a fixed-voltage design like this, to get a lower power you'll need a longer wire and for bigger diameter scope you'll need shorter wire. which can be a little anti-intuitive.

examples:
to get 15W for an 8" scope, you need 9.6 ohm which is slightly less than 11", but the circumference of an 8" scope is 25"-30" i.e. about three times the required length. you'll have to use 3 wires in parallel to get near the required resistance.
to get 15W on a 14" scope will need 4 wires in parallel. to get 18-19W on a 14" scope will need 5 wires in parallel and so on.
to heat a 2" ep with 5W, you'll need to wrap the wire 5 times around. :roll:

--kong
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[anat]

Thanks Kong. It is very useful information

Anat

[mrngbss]

Wow!! Amazing!! Good work!

[kingkong]

the heater had first light last night.

found that the heater strap didn't nicely wrap around the c8 corrector because the dovetail plate below and the guidescope rail on top were obstructing the velcro tape. d'oh! will have to rethink a new method to secure the heater on to the c8... may be put it into another sleave which is narrower than the velcro... or trim the velcro tapes. or to tape the heater inside the tube on the bracket holding the corrector glass!

it did keep the edge of the plate warmer than ambient temperature but didn't feel quite hot enough. may be partly because of the gaps between the heater and the tube it was losing heat to the air. will need further test after an improved strap is made. might have to add another wire in parallel to up the power. 4 wires will bring it up to 20W. wow...