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Make Your Own Dew Heater strips - New Construction Technique

By Chris Malikoff

Username - Omaroo

Difficulty Level - Easy

Cost - $10

I've previously made 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 an "S" ladder pattern by using the actual resistor legs themselves to form the chain. These break in no time at all and render your heater useless.

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 of 1.5mm plastic 2cms wide if required to sit inside an OTA tube. By putting the resistors on the outside of the band they are then closest to the OTA which needs heating. For applications such as eyepieces and camera lenses I recommend not using the plastic strip and just encasing the resistor strip itself in heatshrink tubing so that it can be wrapped around the object and secured by velcro.

1. You'll need to buy enough resistors to make a strip long enough for your application - with each resistor placed approximately 15mm apart along the length of that strip. For example, a strip 20 cm long should contain around 14 resistors. I've found that 390 ohm, 0.25 watt resistors are perfect for the job. These are pretty cheap - around a dollar for a pack of 8 at your local electronics store.

2. You'll also need to buy a spool of solder wick to mount the resistors on. I use "Goot Wick", 1.5mm width - for about $3.50 for a 1.5m length. Prepare the solder wick by taping two parallel strips down to a steel ruler about 10mm apart. The reason I use a steel ruler is that it allows me to measure off the placement of the resistors and solder them without burning the ruler. Start off by getting a small soldering iron hot and feeding it a little solder so that a small blob hangs molten from the end. Dab these blobs QUICKLY on to both strips of the wick every 15mm. Do NOT keep the iron held to the wick otherwise it will start of absorb the solder off the iron and it will spread too far. You just want to literally drop a blob and then move on.

3. When you've prepared the wicks, take your first resistor and place the body of the resistor inside the two strips at the points you dropped the first two solder blobs. Hold it with a pair of needle nosed pliers as this next bit will ensure burned fingers otherwise. QUICKLY as you can, melt the blob beneath the leg of the resistor by holding the iron on the leg of the resistor and press down quickly into the blob. The resistor is tacked in place. They are not polarized, so it doesn't matter if some point the other way. Repeat until all have been done. Now untape the strip and cut off the excess legs outside the strips.

4. Make sure that you leave a few mm of strip after the last resistor so that you can solder a two-core pair of wires to the end - one wire to each wick. These will go off to a plug of your choice and connect to 12vDC. Use a controller for heat control. these guys get VERY warm without.

5. Cover the strip & resistor assembly with a length of heatshrink tubing to insulate it electrically. A length of 10.0mm tubing will do nicely. Heatshrink tubing is simply a plastic tube that shrinks when you heat it. Use a heat gun to heat the tubing when it's in place - and make sure that you extend the tubing to cover the first couple of cm of the wires off the end. If you don't have access to a heat gun then a really hot hair dryer will work - or a ciggy lighter held underneath if all else fails. Keep your distance or else you'll burn the tubing. Practice on a spare piece first so that you can get the hang of it.

6. Connect it to 12vDC power - a car or motorcycle battery. Again, I'd recommend building or buying a controller so that you'll be able to vary the heat output. At the end of the day you don't want to fry your OTA - you just want to heat the area up so that it sits just a couple of degrees above the ambient air temperature. A cold OTA will soak up the heat very quickly - so even if you find that your heater strip gets very warm indeed - just remember that when it's pressing up against a near frozen OTA it will be luke warm. Perfect!

See images for more detail -

By Chris Malikoff aka Omaroo