DIY dew heater

[D Wright]

I understand the principals of making a dew heater strap from Nichrome wire, but I don't understand the controller. Why do I need a controller ?. Do I need a PWM controller and why ? For a single dew heater stap can I connect it straight to a 12V supply. Is it possible to use a potentiometer to control it ? Help would be greatly appreciated thanks.

[wookie1965]

http://www.ebay.co.uk/itm/251451402931?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT.

I was told to buy one of these my friend going to show me how to wire it up I have also bought a dew strip from china probably about the same i would of paid for the parts make one. I did find this it may help

http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html

 

[cjdawson]

9 hours ago, D Wright said:

I understand the principals of making a dew heater strap from Nichrome wire, but I don't understand the controller. Why do I need a controller ?. Do I need a PWM controller and why ? For a single dew heater stap can I connect it straight to a 12V supply. Is it possible to use a potentiometer to control it ? Help would be greatly appreciated thanks.

Let's start out very very basic.

So you make your dew heater from a piece of Nichrome wire.    I've done this myself so I'll use my scope's details as an example.

Here's the details of the heating element that I made.

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

In short, my heating element is nichrome wire, threaded through heatshrink then that is threaded into copper breakpipe.  The whole thing is bent to the profile of my scope and is held in place by pressure alone.  The nichrome wire attaches to normal wire when then connects to the power supply.

 

This is a replacement for my original dew heater which was a resister ladder type. Here's the link to that page.

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

 

Both heaters did the same job, but the nichrome one is placed better.  Overall it works much better than the resister ladder version - because of the placement right in front of my corrector plate.  Would be even better just behind it.  I've never opened the scope and don't intend to, so that's not happening.

 

Anyway back on topic.

 

Dew strap connected straight onto a battery.

It will work just fine.  However, it means that your dew heater will be turned on to 100%.  In the case of my heater, that means 32 watts of power.  Working out at about 2.7 Amps of 12V.   That's alot of power.  It also means that the heating element will get hot.  I mean hot enough to shrink heatshrink.   Great for clearing a misted up scope, but will also create heat currents which are not desirable.

 

This is where the controller comes into play.

Let's start with your idea of using a potentiometer or variable resister.    To make this part of the circuit, you would need to connect this in series with your dew strap.  We already know that the dew strap will be using 2.7Amp, so the potentiometer will need to be able to cope with that amount of current.  It's a lot so you'll need a powerful one.  As you swipe the wiper from one side to the other, the power will be diverted from the dew strap straight down to ground.  The potentiometer will eat up the power that isn't going into the dew strap.  This means that at all times, your dew heater will be using 2.7Amp regardless of whether it's running at 100% power or at 10% power.    This isn't very energy efficient.

 

So rather than using only a potentiometer, you can employ a PWM (Pulse Width Modulator) to mean make things much more efficient.    How does it work?  When you have your heater on at 100%, you switch on the power and current flows.   If you want it at 50%, a way that you could do this is switch the heater on for a second, then off for a second, then on for a second. Queue homer simpon

 

Stating the obvious, when the dew heater is on, 2.7Amp is used, when it's off 0Amp is used.  (Let's ignore the few milliamp needed to run the PWM circuit)   The PWM circuit lets you see the ratio of on verses off - the duty cycle. At 25%, it's on for 25% of the time.  I'm sure you get the idea now.   This means it's much more efficient when the power is turned down.

 

Most of the time you won't need to run your heater at more than about 20% (at least that is what I've found with mine).  So over the course of an hour instead of using 2.7Amps for the hour, it's more like 540mA.

My dew heater's PWM circuit operates at about 4000Hz, so it is flipping the switch on and off 4000 times per second.   This has the effect of smoothing out the power supply.  With my LX-90, it is perfectly happy to share the power with two PWM's and my other kit.  Then again I'm using an 85Ah leisure battery which can handle this amount of load without any problems.

 

You don't need to be an electronics whizz to use a PWM, you can pick them up cheaply from ebay and amazon.

 

Hopefully that will explain the Why for you

[Demonperformer]

That is a really clear explanation of what is going on - so clear that even I can understand it now!

[D Wright]

Thanks for taking the time to explain that. I have seen these PWM controllers on Ebay and rather than fiddling about I will just buy one of these. I am wanting a small heater to go around my camera lenses for long exposures. I am lucky that I dont suffer much from dew on my scope. I have a 12" foam dew shield for the main tube and have used the scope down to -10' with no problems except for the telrad and the finder scope dewing up. I have fitted a resistor running from a 9V battery to the telrad and it keeps it clear very well. I have the battery inside the telrad so it's nice and neat. I would be able to use the same heater for the finder scope but usually I just keep the caps on until I want to use it.

[Pippy]

and ..

I believe that in order to clear and keep clear a misty piece of glass (lens, mirror etc), you only need to create a 2 or 3 degree C temperature difference between the glass face you want clear and the area in front of it.

 

[D Wright]

19 hours ago, wookie1965 said:

I was told to buy one of these my friend going to show me how to wire it up I have also bought a dew strip from china probably about the same i would of paid for the parts make one. I did find this it may help

Did it arrive OK? How long did it take? I am wary of ordering stuff from Hong Kong or China. I don't know why as they are probably very reliable.

[wookie1965]

Actually I was pleasantly surprised turned up Last Wednesday, It had up to this Wednesday to come so it was a week early. I have bought a few things fro China and they have always turned up either before the dew date or on it.

[wookie1965]

This is what I was sent very good I think well made.