Home made Dew heater bands

[Kerrp30]

After my weekend of dew'ed out viewing, I have decided to use my technical know how and design a dew band and heater.

I notice that people are using a bunch of 330 Ohm resistors in parallel (usually groups of 6) This produces a power output of 0.436W per resistor, so a group of 6 produces approx 2.6W

I have a 8" SCT and was wondering how many groups do I need, not managed to work out what the proper dew band output in terms of heat. So if I say I need a batch every 3 Inches that makes it one batch per inch of aperture roughly.

I would like to use a single wire which gives a uniform heat around the circimference. Anyone know what type of wire I should use.

As for the control, do I really need this? or can you just run it flat out?

Paul

[PhillyDee]

You need nichrome wire. I made some which work excellently.

I got the needed wattage from the Dew-Not website and worked my calculations backwards.

Nichrome wire cannot be solered by conventional means, it must be crimped, or as I did, I cut the plastic from a chocolate box connector and used the screw block part.

I threaded my nichrome through 1" webbing strapping (I have loads) and then covered it in black heatshrink (difficult to get the damn stuff through!).

I would use a control, if only so you can 'tune' it. But again, you can build your own PWM controller (As I also did!).

The only thing I found is that my nichrome has a buzz when it is on. Virtually inaudible, but you can hear it. I have no idea how to get rid of it!

[Kerrp30]

The buzz is probably due to the duty ratio of the pwm, can you alter the buzz by adjusting the pot that controls the pulse width

Paul

[barkis]

I made my own Dew Bands too. I used Nichrome Wire which I threaded through fine heat shrink tubing, prior to using adhesive fibre glass tape, to facillitate attaching it to webbing of the correct length for the particular application it was intended for. My wife did the stitching by hand. She's a grand lass.

I made a PWM Controller, and also bought a HiTec Astro Controller.

The heater for my 12" Meade SCT, was incorporated into micro bore Central Heating tubing, which when formed, fits just inside the Corrector plate on the OTA.

I bought my wire from the following.

Ron.

http://www.wires.co.uk/acatalog/nc_bare.html

[PhillyDee]

The buzz is probably due to the duty ratio of the pwm, can you alter the buzz by adjusting the pot that controls the pulse width

Paul

Yup, adjusting the temperature through the PWM controller POT adjusts the pitch of the buzz. Actually, I just got my hands on a working oscilloscope, might have a looky through that. All I need to do now is work out how it works

[Space Bat]

Try adding small amounts of capacitance to the circuit to illiminate the noise...

Chris

[sgazer]

I used the wire from those 12v heated blankets. On my SCT I wove and glued it into the dew shield, on my refractor I wrapped it around the end near the lens and put some tape around. Both solutions work perfectly and supply just enough heat to keep the dew off.

[Sponcom]

To get rid of the buzz try upping the frequency of the dew controller - depending on circuit used this normally means changing a reducing the size of one of the capacitors. My dew controllers I have made operate at frequencies inaudible to humans although they may annoy some of the bats in my garden! I have no problems with my homemade heaters using nichrome wire.

[Sponcom]

Just to add all my heaters use nichrome wire encased in heatshrink to eliminate the possibility of the wire shorting out and setting melting or setting alight your equipment. Nichrome wire

is available on ebay in various guages and therefore ohms per metre.

Power (Watts) = Volts x Amps

Amps =Volts/resistance

with ohms law you should be able to work out the lengths of wire needed. Looking at kendrick or other dew straps will give you an idea of the power needed for various lengths.

Also bare in mind that a lot of dew controllers only like to put out around 2 amps max (approx 25 watts at 12V)

Cheers

Paul

[Space Bat]

After seeing this thread I thought errr...I should make one...

So I used 5 groups of 4 off 300ohm 1/4 watt resistors

I simply used lengths of tin copper wire to solder the resistor legs on top and bottom and then cut them off into squares of 4 off resistors in each square and made enough to make 5 sets of them...

Then using 1 mm wire soldered the squares in a chain at intervals enough to wrap the scope....with the last in the chain soldered to about 10 meters of 2 core 1mm equipment mains cable..

All this was passed through flame retardent sleeving and sealed with heat shrink at both ends leaving the 10m tail of mains cable which was to connect up to a 12V source....

Last night put the scope out to cool and wrapped this round the scope and popped the dew shield on - I had to use some tape to make it more secure - but will get some velcro on this weekend - I simply connected it up to my 7ah power tank terminals and left it running.

Went out 30 mins later and powered up the mount (from the mains not tank) and aligned the scope and checked how things were doing on the corrector and checked there was not excessive heat...there wasn't, just barely warm to the touch...was thinking perhaps I needed to change the resistance value to increase the temperature.

But carried on with a nights viewing and imaging...

So from 7.30 till 10.30pm with periods of checking, there wasn't even the hint of dew on the corrector although lots inside the dew shield - powertank still on green. Around 11pm I noticed the start of dew - but in no way the amount I would normally get within an hour of setting up the scope...but things were getting very cold and my boots were sticking to the frosty grass if I stayed in one position too long quick blast of the hairdryer and chased after a few things before calling it a night around 11.15pm.

So a major result - normally once dew is present I am blasting with the dryer 30 - 60 mins but last night just the once after 3 1/2 hours.

So would recommend to anyone to give it a go - and it costs peanuts so bang for buck this is a no brainer.

Chris

[Starjumper]

Hi Chris,

Sounds like a good result, can you post some pics.

John