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Guest Lizardo

Resistors and batteries.

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Does anyone on the forum have a working knowledge of resistors used in hobbies that would be willing to answer a few questions via e-mail?ThanksTim

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OK, here's the scoop. One of my other hobbies is amateur astronomy. I have a Meade Instruments LX200 8 inch Schmidt-Cassegrain telescope, and I'm designing a dew heater for it. During the course of an evening, the telescope cools below ambient temperature which allows the "front lens" or corrector plate to cool the air just above it. When this thin layer of air cools, the water in it condenses and forms dew on the corrector plate which ends the evenings viewing. One way to tackle this problem is to put a dew shield on the end of the telescope (an extension of the telescope optical tube) which will delay this cooling and buy you some extra viewing time. The other answer to this problem is to actively prevent the dew by very slightly heating the corrector plate and keep it at or above the ambient temperature thus preventing dew formation. There are commercial dew heaters available at a cost of about 180 USD shipped for a telescope of my aperture. On the other hand, a "home-brew" dew heater made with resistors that runs off of a deep cycle 12v battery can be made for about 35 USD and works just as well.The common designs involve making several parallel series of resistors that fit on a strip of foam. This strip of foam with the resistors is then wrapped around the end of the telescope, and when you turn the power on, the resistors heat up and just slightly warm the corrector plate to keep the dew formation at bay.So, here are my questions:My favored design involves making 4 separate and independent parallel circuits using resistors. The power usage is 4 watts per circuit using 28 1000ohm resistors in parallel drawing 336mA from a 12v deep cycle battery. These four circuits of resistors would each have a switch on the control panel and can each be turned on or off allowing for using the minimum amount of battery power in the field that is needed to heat the corrector plate. Are my calculations correct? 28 1K ohm resistors in parallel35.7 ohms total resistance at 4 watts power drawing 336mA.Did I calculate this correctly?Secondly, since each parallel series of resistors is independent of each other, and will be switched on, will the resulting heating be additive? What I mean is this: When I flip the first switch, the resistor series will put out 4 watts of power and "x" amount of heat. When I put the second independent parallel series online, I'll be adding an additional 4 watts of power and "x" amount of heat. Will I have just doubled the amount of heat being produced by having both sets of resistors online? That is my goal, because I want to be able to control the amount of heat applied in steps.As you can tell, my knowledge of basic electronics is just enough for me to be dangerous around electricity. If anyone can shed some light on this topic, I would greatly appreciate it.Thanks.Tim

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It looks like your calculations are correct. I would say yes to the additive heat generated by turning on the circuits separately. Each circuit is capable of putting out 4w of power. As long as you aren't overtasking the battery then each should be able to put out the 4w. The total equivalent resistance when you have all for circuits on is 8.93 ohms and since I=E/R, I=12/8.93 = 1.34a or 4 x 336ma = 1.34a.

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Here's a cheap and dirty way: find or buy a normal bed heating pad. {they can also be got in 12v from camping outfits). Option #2: go to drug store and get "Therma Pads"..they chemically make heat when get damp. (about $5 US a box).

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