Heater for SCTs

Post by **Airconvent** » Thu Oct 06, 2005 12:02 am

The 3 hot (not warm!) packs from Richard II rescued my scope from dew agony that night...but 3 packs, while convenient, seemed pretty wasteful..so I'm thinking of going for a heater solution. A dew shield is too cumbersome for me..

but looking at scopetronix, seems their heaters are priced too high for a heating coil? A cool US$60 each!

I don't have any experience with heat from resistors so wondering if anyone here would share their DIY designs for an OTA heater. Nothing fancy..a simply one perhaps with a resister based variable heater control?
I know CC soldered a series of resistors in series but I'm wondering how many I need and how much heat is enough?

Anyone can share? or just bite the bullet and buy a commercial one?

rich

Canopus Lim · Post by **Canopus Lim** » Thu Oct 06, 2005 12:29 am

Yup..those heaters on the market are really expensive. I'm using resistors as heaters for my DIY heating solution. I have a heater controller that allows variable heat to the optics. To make it variable.. you need to design a controller... and the controller has to be comprised with variable voltage regulators. Basically you need to have some knowledge in electronics. Voltage regulators are ICs that control the voltage output .. basically any electronic component that requires power will have them. By having a variable one, you can change the voltage across your heater..and thus controlling the amount of current drawn by your heater. The heater is basically a resistor of certain value.. but not so simple too.. as resistors have power ratings.. if you apply too much current.. it will BURN UP. For me, I put parallel resistors.. I think some people do it in series.. which is not a good idea as your resistor has to be high wattage. Parallel resistors of 330ohm and 0.5 W works the best as it allows the maximum possible heat dissipation of the resistor when connected to a 12V battery. (P=VI for DC power). Actually a lot to say about it... feel free to PM me and I can give you the details. I have other ideas on how to further improve the heater system just that I'm too tired to do it now. hee.

Post by **Airconvent** » Thu Oct 06, 2005 1:08 am

Hi Yang Beng
Having a controller would regulate the supply to maintain the required current but I really hate to have to do a circuit board and all. I was thinking of using a plain old pot and to adjust the heater current as the voltage dips through the night...I know CC did one for his ETX (which I accidently destroyed due to a super glue accident..:-) )

As to your circuit, so you are saying the trick is the maintain the current to achieve the power rating of the resistor? In this case, adjust current, so that the resistor will dissipate 0.5W constantly which is how we can get it hot ?
I saw you used a velcro, I suppose the resistors are taped to the velcro strip using strong insulation tape? (I did not get to see it clearly that night in the dark..)

rich

Post by **acc** » Thu Oct 06, 2005 7:50 am

Hi Rich
Wah u destroyed the dew heater!! :bangwall:

The dew-heater I made used about 0.1W. But now I prefer to use heat packs... They are only a dollar each and you don't have to worry about draining ur gel-cels prematurely and more importantly, there are no wires dangling all over the place.

cheers
cc

Canopus Lim · Post by **Canopus Lim** » Thu Oct 06, 2005 9:05 am

For parallel resistors, the resultant resistance is R/N where N=number of resistors. At 12V, EACH resistor(330ohm) will dissipate 0.5W of power. So. the total power dissipated = N*0.5 for voltage of 12V; note if your voltage is lower than 12V, each resistor dissipate less than its maximum of 0.5W. This will maximize the potential of each resistor. Also, it can get real hot doing it parallel and the heat is better distributed as you have many resistors wrap around the optics at equal spacing. You won't have the case whereby in series.. just a few resistors. and you even need a very small resistor value which is hard to find. With few resistors, only certain part of the scope gets real hot. With many resistors giving the same heat distributed around the scope, the effect is that there won't be 'corners' of the optics that are dewed up. Also, with the controller, you can control the amount of current that goes to the heater, and hence control the heat. Thus you can get the best temperature.. not too hot or too cold. The best temperature of the heater should be slightly above the ambient temperature. Dew forms because the scope's temperature is lower than the surrounding. Therefore if the scope's temperature is made similar or slightly higher (by heating) than the ambient temperature, dew will not form. Also, with a controller, you can save the battery life... having just a fix resistance will drain the battery too much if your resistance is not optimized; that means you are supplying more heat than required. I calculated the resistance of the heaters by actually using the Kendrick heater table.. it states the different heater current draws for different sizes of their heaters.. let say 1.25, 2, 5, 8 inch. By knowing the current draw, I work backwards and find the resistance of each heater for the each size. Also, heat packs can heat the scope but the heat may be too high (depends on the ambient temperature) and it can't be controlled. How about the eyepieces or finderscopes? Are there heat pads of that size?

There are ways to reduce the battery drain.. this is by improving on the heater. For me, my heater is made of different tapes. I have teflon tape for electrical insulation and fire prevention (in case of short circuit). Duct tape to improve the heat transfer as it is black colour. I have aluminum tape (at the back of the resistor to reflect the heat and increase the efficiency) and also have foam tape to trap the heat. By having a layered heater the efficiency of the heater can increase and thus don't need so much current. The main objective is to supply the heat, retain the heat and ensure that the highest amount of heat gets to the optics by reducing those getting lost to the surroundings.

As CC said, the only problem is the wires but hey.. those expensive heating solutions all have wires. The other factor is.. time to make it.

Post by **Airconvent** » Fri Oct 07, 2005 12:44 am

acc wrote:Hi Rich
Wah u destroyed the dew heater!! :bangwall:

The dew-heater I made used about 0.1W. But now I prefer to use heat packs... They are only a dollar each and you don't have to worry about draining ur gel-cels prematurely and more importantly, there are no wires dangling all over the place.

cheers
cc

sorry cc..that was 1-2 years back already.heh heh
one of the parts broke off and in attempting to glue it back, the super fluid super glue flowed into the switch without me realising it and soon, everything was glued shut. further attempts to fix it made it worse and soon, it had to be decommisioned...sigh..

yang beng : so the trick is to try and maintain the current to sustain the maximum rating of the resistir, not above or below it? that would mean more sophisticated constant current circuits instead of the usual pot...
I do hate to experiment to get the correct combination..will try to hunt around and see if there is any tested design out there...

btw, what do you think of this :

http://www.telescope.com/shopping/produ ... oductID=10

but at 2A, a 7AH battery would be dead in 3 hrs and that's without the scope load! do you think its possible to put a pot between the supply and the coil to conserve power? I don't think it needs to be so hot since our temp here is relatively cooler.

rich

Post by **acc** » Fri Oct 07, 2005 9:13 am

Hi Rich
The 'trick' is to limit the -current- flow through the resistor to a level that does not exceed the -power- rating of the resistor. Really hor, IMHO not worth the effort lor unless you are doing it for fun. More things to breakdown, short-circuit or trip-over in the field...

cheers
cc

Canopus Lim · Post by **Canopus Lim** » Fri Oct 07, 2005 9:21 am

Hi rich,
I think the Orion one should be ok; only thing is that you can't adjust the temperature. But if it is well optimized for the 8inch, then it should be good enough. I have a 17Ah rechargable battery (cost about 50 dollars with charger I think) that I bought from Sim Lim Tower. You could get one of those to have the dew heater last the whole night (5h); Note that the battery is not big (much smaller than those car battery) but the weight is a few kilos. However, you need to know what connector the dew heater is using so that you can buy or solder the correct connector for you 12V battery. I use a connector and two crocodile clip (+ve and -ve) combination to connect to the 12V battery. Also, 2A current draw is around there for an 8 inch. How about your other accessories like eyepieces? I'm not too sure about the pot solution cos I only thought of using the voltage regulator.

Post by **Airconvent** » Fri Oct 07, 2005 9:57 pm

Canopus Lim wrote:Hi rich,
I think the Orion one should be ok; only thing is that you can't adjust the temperature. But if it is well optimized for the 8inch, then it should be good enough. I have a 17Ah rechargable battery (cost about 50 dollars with charger I think) that I bought from Sim Lim Tower. You could get one of those to have the dew heater last the whole night (5h); Note that the battery is not big (much smaller than those car battery) but the weight is a few kilos. However, you need to know what connector the dew heater is using so that you can buy or solder the correct connector for you 12V battery. I use a connector and two crocodile clip (+ve and -ve) combination to connect to the 12V battery. Also, 2A current draw is around there for an 8 inch. How about your other accessories like eyepieces? I'm not too sure about the pot solution cos I only thought of using the voltage regulator.

frankly, the orion looks overkill to me. an not being able to adjust the temp means unnecessary battery drain. this guys were designed for really cold weather but in mersing, coolest is about 24-25 deg only.

I just pop by sls to just now to see the battery (baseshop battery shop was just closing!) and the 17AH is big! I'm thinking more of the singastro-common 7AH one.
I used to have the more expensive 4AH one due to its smaller size.
while that battery has been sold with the etx, I still have the charger which is used to charge some other stuff.

CC : I'm thinking that I don't need full coverage of the corrector plate. maybe a few clusters near the bottom (hot air rise) and one cluster at 10 o'clock and one cluster at 2 o'clock position. that could save some power. also, I think we need to run the resistor near its max power rating for heat to be generated? otherwise it will run cool, isn't it?

rich

Canopus Lim · Post by **Canopus Lim** » Sat Oct 08, 2005 12:46 am

The current draw will be about 1A for such hot weather; tested with my C5 it it should be slightly less or about 1A. It also depends on how efficiently you can transfer the heat from the resistors to the scope. If there is no insulation to prevent the heat from escaping, then the current draw will have to increase. Also, decreasing the resistance will increase the current draw. If you want, you can follow my suggestions of using 330ohm 0.5W resistors. 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. Solder the connectors to the battery.. make sure there is enough length of wire. 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. If too hot, you can just cut away the resistors from the resistor ladder until u get the right amount of resistors.