I've previously used Al Sheehan's fantastic dew heater shield resistor usage guide to make several heaters for my 12" SCT, 66mm guidescope as well as for several eyepieces. The technique I used to construct these has led to dew heaters that are a little stiff - especially when cold, because I soldered adjacent resistors together in a ladder pattern by using the actual resistor legs themslves.
I thought about changing this so that I could have a more flexible arrangement and came up with the thought of using solder wicking braid as the two parallel power bus bars. Much neater, and works a treat. Nice and flexible, even encased in heatshrink tubing and reinforced by a strip if 1.5mm plastic strip 2cms wide.
Great idea Chris, thank you for sharing this. Any chance of a "cutting list" what specifically the resistors are, the heat shrink size, and the braid? PM if you want, or post here as I am sure others will want one of these, I do.
Regards,
Gary
very schmicko--this solder braid stuff-would Jaycar know what I was talking about if I went and asked them by that name?
and would they also sell the heat shrink tubing?
thanks
Gary, Al's article has a very handy little spreadsheet that works out how many resistors and all that. He has another article that describes how to build a controller for the heater (if you havent seen these that is)
Narayan
Great idea Chris, thank you for sharing this. Any chance of a "cutting list" what specifically the resistors are, the heat shrink size, and the braid? PM if you want, or post here as I am sure others will want one of these, I do.
Regards,
Gary
Gary - Narayan's spot-on. This post is a follow-on from another I did previously (dew heater for 12" SCT), but if you go to Al's article it'll sort out the parts list for you: http://www.iceinspace.com.au/index.p...63,292,0,0,1,0
I use 390 ohm 1/2w resistors.
Yes, if you go to Jaycar, ask for 1.5mm (or 3mm if they haven't the smaller) solder wick - "Gootwick" by name. It's about $4.00 for a 1.5m length. They also have the heatshrink tubing, and the stuff I used is 25mm across (folded flat).
Thank you Chris. You've just fixed my prob re; the 'flexibilty' of the main cables.
I never even thought of using desoldering braid. Mucho gracias... L.
I just whipped another one for my multiple-reticle red-dot finder (ala LightBridge owners) using the same technique except that I left out the plastic strip.
Interestingly, if the desolder wick is any good for its intended purpose it wouldnt be suitable for this use . When hot the solder is meant to wick its way up the braid, and you would be left with a stiff section of braid.
My advice is to pre tin each resister leg and then be very sparing with the solder. A bit of pre practice will help.
Interestingly, if the desolder wick is any good for its intended purpose it wouldnt be suitable for this use . When hot the solder is meant to wick its way up the braid, and you would be left with a stiff section of braid.
My advice is to pre tin each resister leg and then be very sparing with the solder. A bit of pre practice will help.
Yep, you've said it, and good point - be sparing with the solder.
I've managed to solder each resistor leg with a good wet joint without it propagating along the wick - but I've been building these sort of little electronics projects since the early 70's, so my soldering technique is pretty good. With 1.6cm spacing, I get maybe 2mm either side of each leg stiff with solder - leaving over a centimeter of virgin wick - as the pictures show. The secret is to do it QUICKLY and don't overdo it.
So yes, practice will help, but it really isn't difficult for those who want a go.
Just FYI - because these things get VERY warm if connected straight to a 12v DC supply, I've built the variable pulse-width heater controller described on this site: http://www.backyard-astro.com/equipm...l#Anchor-49575 and the heaters are fully adjustable from luke warm to hot. These pulse-width controllers can put out a bit of RF noise, so be careful what you run in close proximity if you have equipment susceptible to this. I've never had problems with anything - including webcams and laptops. As an alternative, you might also try a simple DC motor speed controller from your favourite electronics store, and, as long as it was beefy enough (i.e. be capable of supplying at least 10 amps) then it'd do the same thing for you.
... and here's mine (still waiting for the 4th output plug to go in:
Likewise I have experimented with resistor strips using the braid outer from small coaxial cable similar to the solder wick you describe and still found it a little inflexible or awkward due to the stiff spots where the resistors are located.
I am now using the some very flexible wire from an old electric blanket, already insulated and fire resistant. Electric blanket cable contains resistive nichrome wire wound in a tiny spiral over fibreglass threads which can be flexed as much as you like. In the US i think they call it 'heat rope'. I originally constructed a loop 300mm long of the cable so my mates who were having problems with green lasers which don't like the cold simply coiled it around the end of the laser fixed with some tape and it works like a charm!
The particular cable I pulled from the blanket was approx 70 Ohms for a 300mm length. With 12 volts applied this is approx a 2 Watt heater (as reccommended for eyepiece heaters). (Use a controller though). Using various lengths with appropriate series/parallel arrangements you are only limited by your imagination as to the many ways you can make heaters for different purposes.
One point though you can't effectively solder nichrome wire so I used the back end of a small spade crimp terminal ie cut off the tubular bit and crimp the power lead and heater element together and place some small heatshrink over it. The big advantage I find is that the heat is continuous over the element not in fixed spots like the use of resistors, it can be made smaller and more compact too.
Kind Regards IanG
I've gone down the nichrome wire route before, and I've not found that there's any real advantage to the way it radiates heat along the entire length as opposed to the resistor method. If resistors are well spaced I've found that it only takes a few to heat an area. which is all you need when it comes to good fog performance. I agree with you in that the big disadvantage with nichrome wire is that you can't solder it - which means crimp connectors which I think are a weak spot in themselves. The other weakness is that one break and you're done. Each to their own I guess - but there's one thing for sure whichever way you go - heating makes for longer viewing sessions for sure
Hi Chris,
Yes, time will tell but so far so good. I still use my resistor strips too. It's good to have a couple of options for different applications. Very true that "Heating definately makes for longer viewing" ... in particular if you apply the heating in anticipation rather than wait for dew to form and then try to get rid of it.
Regards IanG
L.
