Wanna make a scope? Or better still, grind a mirror yourself. Or, you have some good tips in making a really useful accessory? This is the place to show what your hands can do...
The current draw will be about 1A for such hot weather; tested with my C5 it it should be slightly less or about 1A.
1A!!! No wonder you need a 17AH battery!!
You can start with a current draw of 1.2A which works out to be 33 resistors. Place them at equal distances by measuring the circumference of your scope and dividing by 33.
33 330ohm resistors in parallel!! I think may be a bit too much... IMHO, may be 8 or 12 resistor should be enough... than don't need to get such a big battery.
Then test the heater by connecting it the battery (using crocodile clips or other connectors)... and using a thermometer and electrical insulator (cos 1A current is high..don't touch it) measure the temperature.
Don't worry la... just use your hand to test the temoperature lor.... just put your hand near the resistor to feel the heat... don't worry that you'll accidentally touch the resistor... 12V won't kill anyone... hee hee hee
Have a nice day.
Yang Weixing "The universe is composed mainly of hydrogen and ignorance."
Ya..12V won't kill but it is not nice to touch it... it will burn hehe; and if you accidentally short circuit it, it can burn up. I used a 17AH battery earlier cos I used the ratings used by Kendrick.. they claim that the current draw for the heaters are that high; 2A for 8 inch. Anyway, it is always better to be on the high side in engineering, so that the heater can work in all conditions. For my case, I used the controller.
Canopus Lim wrote:Ya..12V won't kill but it is not nice to touch it... it will burn hehe; and if you accidentally short circuit it, it can burn up. I used a 17AH battery earlier cos I used the ratings used by Kendrick.. they claim that the current draw for the heaters are that high; 2A for 8 inch. Anyway, it is always better to be on the high side in engineering, so that the heater can work in all conditions. For my case, I used the controller.
Hi Yang Beng,
can you upload your heater controller design? interested to see how practical my hunt for a dew solution is...otherwise will just stick to heat packs...
cheers
The Boldly Go Where No Meade Has Gone Before Captain, RSS Enterprise NCC1701R United Federation of the Planets
Canopus Lim wrote:Ya..12V won't kill but it is not nice to touch it... it will burn hehe; and if you accidentally short circuit it, it can burn up. I used a 17AH battery earlier cos I used the ratings used by Kendrick.. they claim that the current draw for the heaters are that high; 2A for 8 inch. Anyway, it is always better to be on the high side in engineering, so that the heater can work in all conditions. For my case, I used the controller.
Hi Yang Beng,
can you upload your heater controller design? interested to see how practical my hunt for a dew solution is...otherwise will just stick to heat packs...
cheers
richard, why don't you ask Mathew? I saw he made one for his LX-90 over at his website.
Based on hindsight, a 3w (consuming 0.27Ah @12vdc) heater might better suit your needs. A dedicated 7.2 Ah battery with full charge will support it for more than 24hr (usually less) or 2 full nights of observing without charge.
Assuming if you are contented with 3w surrounding the corrector plate, you’ll need a series of resistors (2.2 Ohm 1/2w X 20) to attain 44 Ohm thereabout to achieve the desired 3w heat output. After that is done, solder on your choice of connectors.
Variables factors will include whether a dewshield is employed, RH of the night how well the heat transfer is taking place, location of the heater wrt corrector and the mileage of the battery.
Have fun
Matthew
The following caveat applies, so do this at your own risk. 8)
(If home-made dew heater is used, in the northern hemisphere, its necessary to align the resistor tolerance band must run clockwise or the heater will end up with picking up Earth’s magnetic wave and induce distorted electrons as well as a high amount of capacitance limited impedance increasing abnormal directional heat output. This can results in too much directional thermal radiation on the corrector and potentially capable of inducing hairline crack on the corrector plate due to thermal imbalance.
If the heater is used in the southern hemisphere, the direction needs to be reversed. Any turns on the wires must be counterclockwise in the southern hemisphere.
Fortunately, for us near the equator, the resistors can be aligned at random but the wire must not twist on at the equator. Polarity is not issue but the connector must be pushed in straight-type.)
thanks yang beng and matthew,
I bought components for BOTH your designs.
As I was making the dc jack for my 12V battery, i took the opportunity to proceed with the simpler design (Matthew's). After soldering the resistors together, I think I made a mess with the insulation tape. visually it looks ugly and sticky...but it does generate heat. Personally, as what matthew have indicated, I don't think its enough unless I use a dew shield as well....
just have to test it out in mersing one day...
rich
The Boldly Go Where No Meade Has Gone Before Captain, RSS Enterprise NCC1701R United Federation of the Planets
Hi Airconvent,
You can take a look at the heater system during the Mars watch. I will bring it along although I haven't made the heaters for my Quickfinder, 2 inch EP and for my dob's secondary mirror.
Airconvent wrote:After soldering the resistors together, I think I made a mess with the insulation tape. visually it looks ugly and sticky...but it does generate heat.
welcome to DIY, is fun to build your own stuff and see it works! I've built a few heater and would like to share....
If you find the heat is insufficient, increase the wattage (use ohm law calculation). For my case, I've inserted 2 rolls of resistors in series (one with 3w, the other 6w) into a 5mm diameter copper tube and place it in the inner corrector plate ring. I built a controller as a selector to switch either 3w or 6w, if in need for fast heat-up, ON both switches to give you 9w.
My next heater version using resistors to joint in parallel, combined with series (29pc of 300ohm x2) to give max 7w and a PWM controller works on 555 IC. this is better than the above as it gives variance heat range by adjusting the knob and use less current.
May change the resistor's heater to nichrome wire, (a 26 ohm/m is suit my need and is only $6)
HEATER
---------
My current series-configuration draws 0.27A and dissipates about 3W.
They are made up of 20 x 2.2 ohm 0.5W resistors as recommended by Matthew.
In order to get 6.4 W, I will have to reduce the resistence to 1.1 ohms.
This will draw about 0.54 A. With 20 resistors, one resistor would be expending about 0.3W. Would that be too hot on a 0.5W resistor?
I am not familiar with use of copper pipes but I know I do not like to attach stuff permanently to the scope like what some of the US guys (and even Matthew) did. I prefer to attach it along the outer edge of the OTA temporarily.
My design uses simple heat shrink as an insulation followed by (probably unncessary) an ugly layer of insulation tape.
But constant bending of the resistor assembly will break it which is why your fixed design is good in this aspectl....
At 0.54A and reserving 3AH for the scope, the new 6.4W heater would last about 7 hrs, slightly short of the 10 hrs for a typical Mersing night. Unless I withhold switching it on till around 10pm and close shop at 5am.
I hope to see your design at Mars Watch 2005 too.
CONTROLLER
----------------
BTW, any efficient controller typically uses switching mode method but these also generate heat which is wasted at the controller heat sink.
Also, some Americans claim that connecting the controller to the same battery as your scope introduces noise in the power lines that will affect the operation of the goto scope. One claimed his Nexstar goes haywire at intervals but stop when he disconnect his controller! (then I should have bought two 4AH batteries instead, one for scope and one for heater!)
In anycase, I went througha number of design but shortlisted to this one
as its simple and easy to solder onto a copper strip board.
It is adjustable from 5-100% duty cycle but I think the darlington pair would be quite hot.
Component List :
R1: 6k8 (1/4 W) IC1: 555
R2: 10 k (1/4 W) Tr1: TIP 120
R3: 47 k (1/4 W) D1, D2: 1N4148
P1: 1M linear (not on PCB) D3: LED, 3mm, red (not on PCB)
C1: 10 nF further: TO -220 heat sink
C2: 1.5 µF (tantalum)
Maybe we should just use a Pot to adjust current! :-)
rich
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The Boldly Go Where No Meade Has Gone Before Captain, RSS Enterprise NCC1701R United Federation of the Planets
"The universe is composed mainly of hydrogen and ignorance." 
