Dew heaters from Kitchen appliances

Posted by Malcolm Bird   01/08/2016 08:00AM

Lurking within every toaster is about 20 feet of potential dew strip in the form of the elements it uses for burning your bagels.

These elements have a resistance of about 0.3-0.4 ohms per foot, which when connected to a 12V (or similar) power supply can provide gentle heating for your
scope optics and accessories. If your toaster had a fancy bagel or one-sided
setting, there will be a couple of different heating elements used with lower
resistances. Two toasters gave me quite a range of resistances to choose from.

Just take apart the toaster (a fascinating study in production engineering) and keep the mica insulating boards with the elements wrapped around them until you’re ready to take the elements off and make use of them. You’ll need to get hold of a multi-meter and measure the resistance and length of your element(s). This will tell you what the resistance is per inch.

As a general rule of thumb, you need the following wattages for dew control.

10in SCT 16-19W
8in SCT 12-15 watts
6in SCT 8-12 watts
Eyepieces 2-3 watts
Diagonal 2-3 watts
Finder 3-4 watts

It is better to err on the higher side as there is more heat loss with this home brew design than with some of the commercial dew straps. Also, as your battery voltage declines with use, so does the wattage produced.

This is how you figure out what you need.

After measuring the length and resistance of one of my elements, it measured
36.2ohms and was 93in.long. This gave 0.39ohms per inch. (36.3/93)

To go around my 9-1/2in diameter SCT tube would require approx. 30in (3.14 x D) which then gives us 30in x 0.39 = 11.7ohms total resistance.

My power supply is 12V, so..

Wattage = V squared divided by Ohms - so 12V x 12V/11.7ohms = 12.3W , just about right for an 8in
SCT. (It could have been a bit higher but thats how the available resistances
worked out..)

Current = V/Ohms or Watts/V – either one gives you 1.025A (you can use this to
find out how long a battery would last powering it)

If you have a choice of resistances from your toaster sacrifice, using a higher
resistance would drop the wattage in the same length. Using a lower resistance
element would up the wattage for the same length.

You can also double up the length by going twice around your tube for 1/2 the
wattage (but avoid uneven heating caused by too much overlap or gaps around the

Using two elements in parallel will half the resistance – but make sure that they are
insulated from each other.

You can also use a higher (more watts) or lower (less watts) voltage supply.

So there’s some wiggle room available by juggling the variables.

Some general construction notes:

1. Cut the nichrome wire to length with side cutters or tin-snips. It’s tough stuff, but avoid over-flexing the folds where it went around the mica forms. Too much flexing will break the elements.

2. The nichrome ribbon used for the heating elements is impossible to solder (the factory used spot welded terminals), so mechanical crimp connections have to be used.

Standard quick connect crimp connectors like you use for trailer wiring work fine. Just fold the heater element in the crimp tube to make sure you get good electrical contact. Use the insulated connectors.

3. If you need more length than one element can provide, join them with a crimp butt splice. You don’t need to leave the plastic sleeve on because it’s going inside the following…

4. To insulate the element, thread it through a length of heat shrink tubing. (Do this before attaching end crimps.) Heat shrink is rated for 300-600V levels and is tough stuff, so it’s a good thing to have between the heating element and your telescope. You don’t have to shrink it afterwards. A certain amount of shrinkage will happen over time, but you want it to remain flexible and a skin-tight outer sheath would not be.

Depending on the length, you may have to poke some stiff wire thru first and then attach the element to this and pull it back thru.

5. The ends of the completed strap can be connected by a stout elastic band or spring to provide tension and keep the strap in place.

Note: there is nothing fancy about these heaters. They are always ON or always OFF (unplugged), but if dew is a problem, you want them always ON anyway. A little bit of extra warmth will have far less of an affect on your observing session than a fogged up corrector/lens will…

As an aside, if you do have dew controller, it will work with these elements – your controller doesn’t care if it is switching a fancy self-regulating carbon matrix or nichrome toaster wire, resistance is resistance.

To find out how long your battery will last, you need to know what the Amp/hr rating of your battery is (sometimes expressed as watt/hours). Once you know this, divide your battery rating by either the current (for amp/hr) or wattage (for watt/hr) to find out how long it will last.

So with a 5A/hr battery, our 12.3W heater would last 4.87 hours. Using the 1.025A current instead gives the same result (5A/hr / 1.025A). Slight differences are due to rounding errors.

Strangely, toaster designers did not have the amateur telescope community in mind when they chose the element resistances for incinerating things for breakfast… you may not be able to find a combination of length/resistance for your particular requirement at 12V– but it’s worth a look because fancy dew heaters and controllers can set you back some serious $$$$, and at the end of the day, a watt is a watt.

So have fun experimenting and keep your optics ‘toasty’.