My astronomy project:
Making of a heating band or dew heater

Content:

1. Introduction, attach a heater to the dew shield
2. Heater band for 5" refractor, field test outdoors
3. Making a USB powered dew heater
4. Lens USB dew heater
5. Dew USB heater narrow band model with regulator

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

3: Making a USB powered dew heater

As you maybe have read on other places here I have purchased a new light weight mount earlier this year, a Star Adventurer. The idea is to have this mount as light and easy to use as possible. Still I need a dew heater on the lens and that heating band needs power. My power source today is a car battery that I use to my EQ6 mount, and that weights more than 25 kg and that's far too much.

The last years it has come a lot of USB power units, often called power banks and the use of them is mainly to charge mobile phones. Here I will try to use a power bank and build a heater band to it.

There are already over the desk equipment for this to buy, but the camera lenses, at least the ones I have has very limited space to mount a heater band on it. I need someone that is more narrower to fit.

Note:
This is just a test, if it overheat or something similar happen the equipment can be destroyed, maybe even burn your house down! Take the information with caution! I do not take any responsibility! Don't try to copy my ideas if you don't have the knowledge and understand this very well of what you are doing!

If you don't have the knowledge or don't want to build you should buy a ready made one, see next page.

The heart of my new dew heating system, the power bank. It has the capacity of 10000 mAmph, they are cheap today, you find them for less then 15 Euro.

The cable you see is for charging and it takes about 10 hours. There are two USB power outputs and you should not load them with too much Amps! 0.5 Amp is standard for one USB output. That is equal to 2.5 Watt at 5 Volts.

To my earlier heating band I used resistors as heating element and solder them together to get the right size and power. Now I want something that's easier to build. I search for different kinds of solutions.

Kanthal wires which is what you normally use when building heating elements. But they need to be encapsulated in something.

Then I started to look after stainless steel wires and found them in electric band for electric fences. Is it possible to use them? It has the resistance of 15 ohm per meter. 200 meters of this electric band cost only 12 Euro, I have a supply for my lifetime if it works! There are three wires of 45 ohm each per meter, maybe useful if I want to reduce the power later.

I bought 6.3 mm flat male connectors to connect the heating band and the USB cable together when I do this test. I will use other connectors later.

I also took one of my old USB cables and cut it and mounted two 6.3 mm female connectors. Use a volt meter to see which wires that has the power, don't shortcut them! You should read 5 volts when you find the right ones. The cable area is very small on a USB cable, keep it short to reduce the looses.

I guess I need about 2.5 Watt at most to keep the dew away from my lens, maybe less. With 5 volt from the power bank the resistance has to be about 10 ohm to give 2.5 Watt).

Resistance=Voltage² / Power.

It's of course very important to not overheat the lens, it can melt down! The wire has 15 ohm per meter and I cut off 0.65 meter and then got the resistance of 10 ohm.

Note:
This are steel wires, use tools that can handle that! I mounted the flat connectors. Each of the three wires gives about 0.8 Watt with this length of the heating band.

Note:
This is just a test cable and is dangerous to use, it must have a 0.5 Amp fuse!

My first test to see what heat I get from the heating band. It feel a bit too hot, if I put thermal insulation of the heating band on the outside it certainly will be too hot! Maybe use of a power regulator or only use two of the three wires will get the right amount of power. To much heating power can destroy the equipment and it also will let the battery last for shorter time. You should of course have fuses installed to protect the equipment, a fuse of 0.5 Ampere, place it in series of the heater band, the battery is of very high capacity and can get on fire if it's overloaded, be careful ! Remember it's just a test I'm doing here, the final construction will be much better.

The metal wires of the heating band is open and has to be electric insulated to not give a shortcut. Here I have put a electric insulation tape on front and backside of the heating band. Maybe better to find a plastic tube to have around the heating band. But it should not isolate it for heat, the heat must come out from the heating band to heat the lens to get the dew away.

Here I have wraped the heating band around the lens, a Sigma 150mm f/2.8 APO. Two turns. The lens has a very limited space to mount this heating band on it. You must also have the mechanical focus to rotate free, otherwise the lens focus motor can be destroyed! Better to chose a lens which is easier to attach a dew heater to.

Here is the heating band connected. What I felt earlier 2.5 Watt is too much. I reduced the power by just use two of the three wires, it will then give 1.6 Watt. Maybe later I can reduce the power to only 0.8 Watt if I put a thermal isolation outside the heating band. I must do some outdoor test to figure out how much power I need. Maybe it will be smart to have switches so I can adjust the power by selecting how many heating wires that will be connected.

In some way I also must have the dew shield mounted, here I have just pressed it on, maybe it will work without falling off.

Ready for a first test, in a few days we have the Perseid shower coming!

Testing the USB heater band:

I decided to built a control box that can let me choose one, two or three heating wires in the heating band to be connected. That correspond to 0.8, 1.6 or 2.5 Watts of heating power from my heating band.

I also have too built an enclosure for it, ordered a box and some components that arrived the day after.

After some work it look like this:

Three LEDs and switches and one fuse of 0.5 Ampere. I also ordered a special USB A connector that let me solder 0.75 mm² cables to it.

No circuit board, just air mounting. What's missing here are the clamp holder of the cables, have to do a complement later. The black component up to right is the important fuse holder with a 0.5 Ampere fuse, it must be there and correctly connected!

I mounted LEDs as indicators of which heating wires that are activated.

The light output from the LEDs are a bit high, I will increase the serial resistors to them to about 1 kilo ohm later, now they are 470 ohm. Or maybe it's handy to have the red light in the dark?

Don't try to solder the stainless steel wires, it will not work, better to use crimp connectors.

Here is all that is needed to have an anti dew heating band on my camera lens. Heating band, cables, control box, battery and charger. The weight are about 0.5 kg together. Note that it are four cables connected to the heating band now, one black common to one end, and three red cables to the other end, one for each wire in it. The black cable has a flat connector so it's possibly to take it apart and wrap it around the lens / tube.

I will be back with more information of how well it works. Later in the winter it will be much colder, will 2.5 Watt be enough then? And how long will the battery last?

Update:
I have now tested this dew heater outdoors under different conditions. I found that it is enough with only 0.8 Watts, just one wire connected. But a problem arise, the power bank feel that the amp is very low and shout down the battery after a couple of minutes. To don't let this happen I have a bit higher power, 1.6 Watts (two wires). But maybe there is another problem, see next page.

Today there are so many cheap USB powered dew heaters, no good idea to built it, better to buy a new one.