12 volt Dew Heaters

I've owned Scopetronix EP heaters and had Orion's Dew Zapper for my 8" SCT, but I have never tried Kendrick's Heating system.

Are there any other alternatives out there? Has anyone made their own EP and OTA heaters that run off 12 volt power supplies? I'm looking for some type of dew removing system for my Astroview 120ST and I'd like to find something that I can keep attached to the scope without a ton of wires to wrap up.

If anyone has any ideas, suggestions, building plans or links to other outlets and sources for existing or DIY heaters I'd appreciate it.

Thanks

I made my own out of discrete resistors. It's very easy to do even if you know nothing about electronics. Take this formula:

P = I*V where V=12V. P is the power you want, say 2W. Use that to calculate your current, I = .16A

Now you can use the formula V = I*R to calculate your resistance. In this case, R = 12/.16 = 72 Ohms.

So you want a total of 72 Ohms worth of resistance to generate a total of 2W of heat. So, here is where some drafting skills or a knack for putting things together comes in. I used, as I mentioned, discrete resistors. You could to that and solder them end to end as I did or you can try to get a length of nichrome wire that's perfect for your objective at around 72Ohms in total resistance. If you pick discrete resistors, which is probably harder to implement you need to be mindful of each resistor's power rating. For example, if you choose to make your heater 9 resistors in series (as I did) then you need to divide 2W/9 to get 0.22W power dissipation per resistor and 72 Ohms/9 to get 8 Ohms per resistor. Find whatever you can that's close, or change your design slightly to accomodate what you can acquire easily. I chose 9 for the exact reason that most resistors can safely dissipate about 1/4W of power and I had access to something around 8 Ohms.

After I measured and soldered everything together I attached a wire lead and I put it all in a length of heatshink tubing for electrical insulation.

To make a heater block instead of strip, you can use a single sand filled type, high power resistor instead of a strip of smaller ones.

You can make a very simple control box too. In my case I just used a couple switches, a couple LED's, a project box and a tangle of wires If you wanted some easily controlled heat output you could use a rheostat, for example, to dissipate some power before your heating srtip or blocks. Design the strip with your maximum power requirements in mind and use the rheostat in series to reduce the total power output as necessary. Rheostats are fairly expensive though, so keep that in mind. Remember also to calculate the power your rheostat will drop so that you don't overload it!

EP heaters are hard to make becausae you're always moving your EP's around. You could make an aluminum plate with 1.25" or 2" (or both) holes in it and place resistor blocks around the holes, or you could wrap insulated nichrome wire around the holes, or you can do what I do and use your hands to warm your EP's