Dew Heating basics

[ebeygin]

Hey guys,

I've been reading instructions on how to build a resistor or nichrome dew heater.

What I got from all the reading was, you make some resistance, using resistors or nichrome, attach it to controller to vary current, and attach that to 12V.

My question is, why is there so much info on current draw, ohms, and watts, if in essence, all you're trying to do, is make a strap that is warm to the touch... any commercial dew heater is never hot, but just barely warm. If that's the case, why not then just adjust you resistors or nichrome length to the point until your strap becomes just warm at max current.. shouldn't this be good enough?

The reason I'm asking, is I'm a bit confused with all these numbers, especially when everyone is suggesting different ones :)))

Thanks!

[wimvb]

Beats me. I just soldered 10 1500 ohm resistors in parallell with pieces of thin wire in between to get the right distance. Sandwiched between two layers of velcro with sticky back. Power plug for 12 v battery. No control whatsoever. Works a treat with my 135 mm lens and secondary mirror (sw 150P). It's just slightly warm to the touch, but haven't had any dew problems with it.

I think many "designs" are overengineered.

Cheers,

(And sorry for the numbers )

[ebeygin]

That's what I'm saying :)))

just get the right number for your length, test it, make sure it not hot, but just slightly warm, and off you go :))

Thanks!

[wimvb]

If you use a car cigarette lighter plugs, take one with built in fuse. Just in case you get a short.

[Gina]

I just use fixed dew heating.  I designed a variable dew heater control circuit but never bothered with it.  Just a bit of warmth is all it needs not trouble with the affects if air currents - just kept the dew off   Often you don't need a dew heater - just a long tube with matt black inside (eg. velour sticky back plastic).

[jtlandreneau]

I've been swamped with dew lately on my newt, it's been killing all of my imaging sessions. on my little refractor I just have a constant heat applied, and that works well. For the newt though, I didn't want constant heating hurting the mirror or inducing tube currents.

so, with that in mind, using an arduino, some temperature sensors, and a temperature/humidity sensor, I wrote a little script that kicks a relay whenever the measured temperature is within 3 degrees F of the ambient dewpoint (calculated using the relative humidity and ambient temperature). The relay is fed by a 19V laptop charger that energizes a nichrome heating pad I made.

I'm going to post the build sequence soon after im done testing

[cjdawson]

As with most things you can make it a complicated or as simple as you want too.

 

The reason for getting into all the technical stuff is so that you can do clever things.   For example.  I made my dew heater so that it would have enough power to clear the lens if it started dewing up.  I wanted to have adjustable settings, so that I can turn it down.

It works great.  For the most part I tend to run it at about 1/2 power or lower.  This saves energy and means that I can have longer observing sessions (Not managed to flatten a battery yet).    I did get into all the maths, for mine I know that the control circuits are able to use about 6A of 12V without breaking.  My heater when as full power pulls about 3A so I know it's all good.   I made my heater from a piece of nichrome wire, threaded through heatshrink and then placed into a thin copper tube.  Doing this means that the heat is transferred out and is defussed by the copper tube.  The heatshrink is there to stop a short circuit.

Knowing what the power output is means that I can tell how much load is needed for the battery, which in turn means how long can I run my setup for before needing a recharge?

Here's the link to my dew heater project.

http://astronomy.cjdawson.com/projects/DewHeater.html

I moved on from a resister ladder, to this version

http://astronomy.cjdawson.com/projects/PowerBox.html

Where I wrapped up everything into a nice neat cable.

Last year, I redid the project, and now have a better setup, but based on the same idea.

Here's my power box version 2.  Still runs NiChrome dew heaters.