Homemade Secondary Dew Heater Questions
Posted 03 August 2011 - 04:18 PM
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.
Posted 03 August 2011 - 10:19 PM
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.
Posted 04 August 2011 - 12:55 AM
Posted 04 August 2011 - 03:56 AM
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.
Posted 04 August 2011 - 08:10 AM
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.
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,
Power = voltage X voltage divided by resistance.
hope this helps,