I built the dew heater linked on the first post on this thread, but I have a question regarding the 12v battery. I'm using this connected directly to a Li-Ion battery, could that damage the battery?
Thank you
Yes, it can damage the battery if you over discharge it.
Lithium batteries are very fussy about voltage. Like many things, too high or too low causes problems. But with Lithiums this is even more important.
As far as the upper limit goes, I'm going to assume(hope) you are using the correct charger for the chemistry.
It may be worthwhile checking with the manufacturer for the lower voltage limit, if you can't find that information then a minimum discharged voltage, under load, of around 2.7 to 2.9v per cell.
For you though, with such a low discharge current the cells will probably fall over at an even higher voltage, so shoot for a conservative minimum somewhere between 3 to 3.5v, this will also extend the cycle life of the battery.
If it is an 11.1v battery(3 cells) then don't go any lower than 8.1v.
For a 14.4v battery(4 cells) don't go any lower than 10.8v.
Also, when not using the battery for extended periods (months) do not store it fully charged. 40-50% charge is ideal for storage.
I have read the article and crunched the numbers for putting together dew heater straps made from resistors. Even though I do not have a background in electronics, most of it does make sense. That said, there will always be areas that I will not understand unless I go away and study the field in depth.
I have decided to stick with the original design for the Pulse Width Modulated Dew Heater Controller except that I would like to run four (4x)output RCA sockets instead of three. This will cover 12" Dob's primary, secondary, finderscope and eyepiece.
Due to the large number of resistors required for the primary of my 12" Dob I would like to use Nichrome Resistance Wire instead of resistors. Will it create any problems if I run a Nichrome dew heater to the primary and 390R resistor dew heaters to the secondary, eyepiece and finderscope from the same PWM controller?
I used Nichrome in place of resistors using Al's sheet.
1333mm of 0.33mm Nichrome from Jaycar came in at 18.4R for my Canon 200mm prime lens. I double checked with an online Nichrome calculator. Commercial heaters typically use 0.75 watts per inch.
At 12v the heater can be handled. It's as warm as a freshly brewed hot beverage in bare hands.
Operating over 2 nights ambient down to 3C it worked flawlessly using an Arduino and logic level n channel MOSFET (low rds on) for switching. At the suggestion of an IIS member, I applied a dew point calc, as the heating control reference, polling a humidity/temperature sensor - a freetronics breakout board, again from Jaycar.
I used Nichrome in place of resistors using Al's sheet.
1333mm of 0.33mm Nichrome from Jaycar came in at 18.4R for my Canon 200mm prime lens. I double checked with an online Nichrome calculator. Commercial heaters typically use 0.75 watts per inch.
At 12v the heater can be handled. It's as warm as a freshly brewed hot beverage in bare hands.
Operating over 2 nights ambient down to 3C it worked flawlessly using an Arduino and logic level n channel MOSFET (low rds on) for switching. At the suggestion of an IIS member, I applied a dew point calc, as the heating control reference, polling a humidity/temperature sensor - a freetronics breakout board, again from Jaycar.
Thanks for the feedback Rowland. I have just started dabbling in constructing electronics. So far, I have built two kits for my Son and have just got to the point of understanding most of the aspects of constructing dew straps and using the PWM controller described in Al's original post.
I have briefly looked at the Arduino system but I do not feel ready to tackle it yet. I have to say that the option of being able automatically adjust for Humidity and Temp is appealing.
Being a beginner I think I might have to stay close to the original design but I will talk to the people at Jaycar when I visit today.
Last edited by Shark Bait; 24-11-2012 at 10:16 AM.
Programmable microprocessors are another learning curve on top of electronics and a layer of complexity that is perhaps not necessary for a dew controller. It fits with my existing system which is built around the Arduino. Essentially, Nichrome works very well.
The Jaycar wire, at 0.33mm, has a resistance of 0.0138R/mm. Using Al's sheet I needed 18.4R for my lens diameter.
18.4 / 0.0138 = 1333mm - when complete R was actually 17.8R; that is, 18.3 - 0.5, the native resistance of the ohm meter.
Accurate measurement up to the solder points at each end of the wire and allowing for equipment tolerance seems to make quite a difference.
With heavy cloud cover removing any chance of see the eclipse tonight, it was time to make up my first dew heater strap. I treated the work like it was going to be a final product worth keeping, but it was still a test run. Twelve 1/2W 390R resistors spaced at 15.7mm fit nicely over the end of the 60mm dia finder scope. There was just enough overlap to allow a good solder joints.
It will be interesting to see how it performs when the scope is out in the dew again but the heater is very warm at full tilt. I will have to wrap the outside face of the heater to direct the heat into the finder scope. Only three straps left to make.
I am very happy with how well the system works. There is plenty of adjustment to get the heat just right. Very little cost and very rewarding to know that I put the whole lot together. Of course it would not have happened without the author and other contributors to these threads.
I used Nichrome in place of resistors using Al's sheet.
1333mm of 0.33mm Nichrome from Jaycar came in at 18.4R for my Canon 200mm prime lens. I double checked with an online Nichrome calculator. Commercial heaters typically use 0.75 watts per inch.
At 12v the heater can be handled. It's as warm as a freshly brewed hot beverage in bare hands.
Operating over 2 nights ambient down to 3C it worked flawlessly using an Arduino and logic level n channel MOSFET (low rds on) for switching. At the suggestion of an IIS member, I applied a dew point calc, as the heating control reference, polling a humidity/temperature sensor - a freetronics breakout board, again from Jaycar.
Well done Rowland,
Do you have the model number or a link to the humidity/temp sensor?
If its as warm as a beverage, isn't it a bit too hot?
I've read that too much of heat can cause adverse issues
Heater straps, when you place them on your lips to feel the heat, are barely warm
I guess the heating over prolonged periods negates the need for too much heat
I had a look at the Arduino system when I was a Jaycar over the weekend. It wasn't too busy so the Manager spent some time going through the basics. When I have my current setup finished it might be time to take it to the next level and get into Arduino.
Thanks Rowland
Have you taken readings of the temp on the strap and compared to ambient and dew point?
It'll be interesting to see the 3 values.
I'd assume targeting regulation to one or two degrees over dew point should be sufficient.
Cheers
Im not sure if this has been answered before, but has anyone tried to build a Dew Heater from a computer fan controller?
Im asking this question as last year i built a gaming rig, which has 6 x 125mm fans and 4 temperature probes controlled by a very neat fan controller and temp sensors.
The unit runs on a 12 volt rail from the PSU (power supply unit) through a standard molex plug.
Im guessing a Jaycar speed controller works in the same way?
if it would work, you could have a damn fancy dew heater controller for under $50, and for a few more dollars a touch screen model
if one of the electronic DIY gents would like to check out this link, and perhaps have a look to see if this indeed would work as a Dew heater controller, for people like me who dont have a clue on how to build a circuit
Cheers
Im not sure if this has been answered before, but has anyone tried to build a Dew Heater from a computer fan controller?
Im asking this question as last year i built a gaming rig, which has 6 x 125mm fans and 4 temperature probes controlled by a very neat fan controller and temp sensors.
The unit runs on a 12 volt rail from the PSU (power supply unit) through a standard molex plug.
Im guessing a Jaycar speed controller works in the same way?
if it would work, you could have a damn fancy dew heater controller for under $50, and for a few more dollars a touch screen model
I don't have a lot of expierence with electronics but it would have to work in the same way if used on dew heater straps. The added bonus being that you can have a probe for each giving the current temperature.
It is a pity they do not provide a photo of the board.
Re: Measured temp of the resistor dew strap....
I took multiple readings of the resistor dew strap that I recently made and at maximum it is averaging just under 60 deg C. I don't think I would ever need to run it at maximum. I will need add some extra thermal insulation to the outer surface to direct the heat more efficiently into the areas it is meant for.
