DIY dew heater HELP Please

[spaceboy]

Hi All

 

I am gathering all the materials together to make myself a dew controller and tapes. I have most the info I need to make them from the searches already made on the world wide web but I seem to be struggling to find what caballing I should use for the heater tapes ?? I have the Nichrome all sorted but what gauge and type of cable is recommended for the power lead ? My guess was to use rubber cable as it's suitable for outdoor use but I am coming across some threads suggesting that a dew heater can draw up to 4.4 amps ? which in my mind seems an awful lot ? I was wondering if they meant 4.4 watt ?? If it is 4 amp it means that the cross section of a 2 core 0.75 (up to 6A) rubber cable will be around 6mm which is no doubt going to be noticed at the eye piece if used to heat the secondary.

 

My thoughts were that I could get away with an ordinary phono lead but this is obviously only intended for indoor audio systems and not powering a "heater" outdoors on a freezing cold damp night.

 

Any advice welcomed.

[MarkyD]

It's usually about 14Watts for the tube and approx 3 -4watts for eyepieces, secondary mirror etc.  Dew Not has a guide on their web site.

http://www.dew-not.com/specifications.htm

Use Ohms law to calculate the current required.  A 14W heater at 12V will draw 1.167Amps.  0.75mm2 cable is good for 6Amps at 230Volts.  For my secondary I used heavy duty servo cable and sticky copper tape along the spiders - It's a 4watt heater taking max 350mA at 12V.  For the main cable to the heaters I'm using defense standard multi core as I need 3 wires for the temp sensor and 2 for the power

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Mark

[MarkyD]

Just found some pictures of my setup for the secondary

 

 

 

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Marki

[spaceboy]

Which is the more important volts, watts or amps ? IE which one is likely to go up in flames if I get it wrong ??

[MarkyD]

They are all dependent on each other.  With a resistive load the only constant to a degree is the resistance of the heater.  ie A heater rated at 14Watts with 12V is 10 Ohms which will draw 1.167 Amps.  Up the voltage to 13 volts and the current will then go up to 1,2 Amps which is in effect 16Watts - if the heater can take the extra current.

You know the resistance from your calculations on using the Nichrome wire to make the heaters.  Then calculate the current at any given voltage which will also give you the watt output of the heater.

With cable it is the current and the volt drop that decides what mm2 you need.  0.75mm2 is plenty capacity for what we need it for with negligible volt drop with 12V at the length you are using.  I also use HF05 (pond cable) as it is pretty flexible and resilient to being stood on etc

 

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Mark

[spaceboy]

Thank you for your help Mark and a very impressive solution to your secondary dew problems. I may give the idea some though to my set up but as I have multiple OTA I was going to make just the one heater tape and use it across the each of the scopes. I guess I could still go with the copper tape method but just add a socket on the secondary end for the tape.

 

Thanks again

[MarkyD]

Since those pictures were taken I have installed a 5 pin locking chassis connector into the tube

http://www.maplin.co.uk/p/maplin-5-way-audio-locking-connector-chassis-plug-fm53h

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Mark

[spaceboy]

Well all done

 

Have plenty of materials left so may give it another go at some point as my sewing skills leave a little to be desired so there is room for some improvements.

 

Anyone wanting to have a go themselves this is a good place for advice http://www.blackwaterskies.co.uk/2013/05/a-cheap-multi-channel-dew-heater.html

[MarkyD]

Looks good. You would never know at first glance that it was a DIY job

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Mark