Jump to content


Photo

Homemade Secondary Dew Heater Questions

  • Please log in to reply
5 replies to this topic

#1 Bluestar456

Bluestar456

    Vostok 1

  • -----
  • Posts: 138
  • Joined: 22 Dec 2010
  • Loc: Florida

Posted 03 August 2011 - 04:18 PM

I just finished building a home made dew heater for my secondary mirror using 330 Ohm resistors in parallel. The design I used is on this page... -->Dew Heaters<-- The one I built is very similar to the one, second from the last diagram at the bottom of the page.

My first question is what would be the best way to attach this to the back of the mirror? That particular design almost fills the entire back of my secondary mirror. Can I just silicone it flat to the back of the mirror, and then silicon the whole thing, mirror and resistors in place on the 45 degree angle holder? Normally I'd duct tape them together, and use an insulator on one side, but this design fills almost the entire back of the mirror, so it's a little tricky, and I want to make sure it'll stay in place with the silicone.

Secondly... Does anyone have a schematic that would function as a controller for the heating element? I'd think that plugged straight into the 12 volt battery will make the resistors get too hot... I don't have a 12v battery here at the moment to test this for certain though.

Anyone with experience with this type of heater can steer me in the right direction, I'd be very happy! Thanks in advance.

#2 Bluestar456

Bluestar456

    Vostok 1

  • -----
  • Posts: 138
  • Joined: 22 Dec 2010
  • Loc: Florida

Posted 03 August 2011 - 10:19 PM

Well I didn't hear from anyone and I got anxious so I ended up using a few dabs of quickset epoxy to hold the resistors in place on the back of the mirror. Then I siliconed the mirror to the secondary holder, which incidentally I made by turning a piece of pine on my lathe, and cutting at a 45 degree angle on a circular saw.

Still not sure if hooking up 12v directly will make the resistors too hot, but I'm guessing it will. I'll have to do some tests later when I get a 12v battery. I might just be able to hook up a variable resistor pot in the line. Again though, if anyone has any experience here, please let me know your thoughts!

Here's a photo of what I did today, and the secondary set up. Still need to make the adjustment screws and the part that holds em in place. I've wrapped electrical tape around the whole thing here.

Attached Files



#3 Achernar

Achernar

    Cosmos

  • *****
  • Posts: 9119
  • Joined: 25 Feb 2006
  • Loc: Mobile, Alabama, USA

Posted 04 August 2011 - 12:55 AM

No, do not directly connect the heater to a 12-volt battery directly, it will get much too hot and if a short circuit develops and there's no fuse to protect the circuit things can get really dicy. Batteries can exlode and the heater could get hot enough to damage your telescope. I'm an electrician and I assure you that not using a fuse is a very bad idea even with a dew heater. I have installed disconnects and power panels that are protected by both fuses and circuit breakers, but all you need is to use a fast acting fuse rated to handle the load of all your heaters running together at full power if you wish to build your own controller. I don't have any schematics that would be of use, I built a simple switch box protected by a fuse. Dividing the voltage by the resistance will give you the amperage so you can choose the right fuse, or simply use the I = E/R formula. I is amperage, E is voltage, R is resistance....which you will get with a multimeter. I would buy a Dewbuster controller, they can be used with homemade heater strips without problems or voiding the warranty. They protect against short circuits and reverse polarity and with a temperature probe will ensure your heater will heat just enough to keep the mirror dew free. I switched to a Dewbuster and got better control over the heat and much longer battery life between charges after I bought one for my 15 and 10-inch Dobs. You will pay more for the materials than simply buying a Dewbuster if you wish to make one of equal quality and workmanship.

Taras

#4 Bluestar456

Bluestar456

    Vostok 1

  • -----
  • Posts: 138
  • Joined: 22 Dec 2010
  • Loc: Florida

Posted 04 August 2011 - 03:56 AM

Thanks Achernar for the information... Sounds like a Dewbuster is the way to go. I suspected that 12v would have been to much directly, but I was confused... One web site I found showing more homeade heatstrips said that the person would simply disconnect the heater from the power line on or off as needed. But on the other hand, that site I posted gave a good warning that running the heater directly from a battery may or may not cause a problem, including over heating.

While I probably could build some kind of controller, I'll probably get a dewbuster like you mentioned Achernar... It's been a few years since I built anything electronic wise, even although I've put together a few guitar effects pedals in my past. I might have to wait for a little while though... I can't afford a dew heated right at this moment.

#5 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1979
  • Joined: 10 Dec 2007

Posted 04 August 2011 - 08:10 AM

Here's a link to a recent thread on this exact topic.

http://www.cloudynig...ll/fpart/1/vc/1

330ohms may be too small for direct connection to a 12v source. It needs a controller or you run the risk of it frying itself, especially buried inside a secondary holder where the heat is trapped.

You really don't want to run the heater any hotter than is absolutely necessary to prevent dew or you will be generating thermal currents in the optical path that will really degrade the views.

dan

#6 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1979
  • Joined: 10 Dec 2007

Posted 04 August 2011 - 08:56 AM

That particular design almost fills the entire back of my secondary mirror........Normally I'd duct tape them together, and use an insulator on one side, but this design fills almost the entire back of the mirror, so it's a little tricky, and I want to make sure it'll stay in place with the silicone.


You can buy high temp silicone. It is red in color. Look in auto supply stores or electrical parts outlets.

You don't need to faithfully copy the design on the web page you referenced. Since the resistors are in parallel, each one is acting independantly. You only need to use as many as you want, or as will fit. Of course, the total power is the sum of the power in each resistor.

A 330 ohm resistor on 12 volts will generate power according to the formula P=IxV, (or using Ohm's law, P=I^2 x R, or P= V^2 R) .

In this case I=12/330 ohms= 0.036amps so P= 12V X .036amps = 0.432 watts in each resistor. One resistor generates 0.432watts, two =0.864watts, three= 1.296, etc, etc. That is a lot for a resistor rated 1/2 watt so you could use 1 watt resistors if you have the space.

To build this, determine the maximum power you wish to utilize and then determine how you will generate it. For example, If you want 3 watts total for your secondary heater, you could use 4 resistors generating 0.75 watts each, or 6 resistors generating 0.5 watts each, or you could use 12 resistors generating 0.25 watts each. Much depends on how much space you have and how you mount the components. Resistors in any value and power rating are not difficult to come by. Ideally, a gentle overall warmth is better than a single very hot spot. You want to spread the heat out as much as possible. Also keep in mind that a secondary heater does not require a great deal of heat to be effective since the heat is captured inside the mirror holder and doesn't disappate well. A little goes a long way to prevent dew. Too much heat will create thermals in the optical path.

Power in a resistor = voltage X current,
or
Power = voltage X voltage divided by resistance.

hope this helps,

dan






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics