tbhausen Chen, do you know what type of heater Dew-Nots are?
No, I don't. You would have to ask the manufacturer. The dew strips that I currently use are the More Blue strips, that have their own PWM controller and able to set both ∆T and absolute T (plus, I also have not powered anything through the ASIAIR DC ports in an abundance of caution -- my mount goes through its own buck/boost converter, for example, to get 15V power and won't cause noise to my cameras, my cooled cameras go through a DeepSkyDad DC power hub that have individual self-healing fuses -- the cameras need as clean a power as you can achieve).
They [dew strips] claim to draw less power.
The "dumb" dew strips are passive devices. Less power simply means less heat. A Watt is a Watt. I*V has to be turned into heat, just from conservation of energy.
A dew strip that takes less power just means that it is capable of less heat generation -- but it might be sufficient to maintain a ∆T the keeps the glass from dewing up; dew point, by definition, is never higher than ambient temperature. An IR thermometer (you may already use one in the kitchen) will show if the temperature of the central part of the objective glass is a degree or two higher than ambient. I actually have a lab IR thermometer that I occasionally use in the kitchen -- reversed priorities, ha ha.
The problem is the exposed central part of a thin piece of glass radiates heat to the night sky and therefore can reach a temperature that is below the dew point. You just have to warm up that central part of the exposed glass to above the dew point.
One difference between good and poor dew strips is if the outer surface is well insulated, so that all the heat will go to warm the tube instead of the heating the surrounding air.
Many cheap dew strips are constructed literally from a string of carbon-composition or wire-wound resistors. The carbon composition resistors will have less inductance than the wire wound ones. About a dollar worth of resistors. The rest that you are paying for is the fabric and perhaps some elastic/Velcro material (and what the manufacturer pays the YouTube shills). If the resistors do not have sufficient wattage, if one happens to burn up and takes on a lower resistance, the rest will rapidly burn up too.
Unless they get too hot and run into danger of catching fire, you do not need to PWM the strips. PWM is only mandatory to save battery life when you are out in the field. If you are AC powered, you can just run constant power into them if there is no danger of fire, or damage to the OTA (I have seen photos where the paint on the OTA even gets discolored; usually from a malfunctioning dew strip, though). You also don't want it to be too hot to cause thermal problems inside the tube.
Just remember that PWM means that the controller is switching the power to the dew strip many thousands of times per second. So, PWM circuitry that are not specifically designed to handle the extra dissipation when the load is reactive will just burn out. The higher the PWM frequency, the worse the problem is. Some controllers actually choose higher PWM frequences so that the components do not "sing" at frequencies that humans (especially young people) can hear, In the computer world, you can read about "coil whine" from switching power suppies and PWM devices. The cheap way out is to raise the PWM frequecny, instead of getting to the root cause. I suspect that ZWO simply raised the PWM frequency sometime ago when there were complains about whining sounds when they turned the DC output to less than 100%; but that step would also have affected how the PWM functions with reactive loads. The (good) engineer's job is to strike a balance between extremes (in this case, the choice of PWM frequency) while keeping the cost down, and not adversely affecting the customer.
I do not know if the ASIAIR DC output is designed to take non-resistive loads into account ; I have never seen the proper engineering specs for the DC ports. If they have done any product testing, they should have that data on file.
Chen
