Try this one, its cheap to build, and all parts probally set you back no more than $15. The heating element can be purchased from Jaycar, part no. WW4040. Also required is a length of heat resistant tubing to feed the element into, part no. WS5504.
The heating element is a special type of wire known as Nichrome wire whereby its resistance measured from one end increases as length increases. Its the same kind of stuff used in toasters, although you would definitely not want your element to glow red hot.
I feed the wire through the tubing then using some insulating type of flexible foam/rubber mat (roughly 25mm wide x dia of OTA) taped the tubing (with the element contained within) to the rubber/foam with a long strip of gaffa tape, then just folded the edges round the back to make it neat.
You'll need to solder the ends of the nichrome wire in order to make a good connection with the output of the PWM circuit. I tried a few ways (nichrome wire is difficult to solder) and found that by lightly removing the surface of the wire I was able to solder it to a couple of female spade terminals (remove the plastic insulation before soldering, and apply stacks of heat + solder) once cooled down just slide the insulation back on, or just use tape!
Stuff the circuit (made mine on a piece of viro board, also at Jaycar or dicksmith etc..) into some kind of plastic jiffy box and mount the led through the lid (so you can see the PWM in action), connect the load (element) and you're away.
The resistance wire on those old boiling jug immersion heaters is ideal for the elements. You can pick them up pretty cheap at supermarkets and hardware shops. Cut off a length to give you 20 ohms resistance, about 9-10 watts at 12volt. Mine were made by cutting a 25mm wide piece off a PVC pipe and cutting it lengthwise to allow it to clip over the scope tube. Drill a couple of small holes in each end and and anchor one end of the wire by looping it through the holes. Wrap the wire evenly spaced around the 25mm (takes a couple of tries to get it right) and feed the other end through the holes again. Feed the connecting wires through the holes at each end to anchor them and solder to the element. I wrapped mine with some tubular hemming material from the local sewing shop and put a strip of waterproof tape on the outside. Works a treat and the soft material prevents scratching of the OTA. Does'nt get hot enough to damage the material but will melt plastic tape at full output if put in close contact with the wire. There should be enough length on the jug element to make at least 2 heaters for a 4" OTA.
The resistance wire on those old boiling jug immersion heaters is ideal for the elements. You can pick them up pretty cheap at supermarkets and hardware shops. Cut off a length to give you 20 ohms resistance, about 9-10 watts at 12volt. Mine were made by cutting a 25mm wide piece off a PVC pipe and cutting it lengthwise to allow it to clip over the scope tube. Drill a couple of small holes in each end and and anchor one end of the wire by looping it through the holes. Wrap the wire evenly spaced around the 25mm (takes a couple of tries to get it right) and feed the other end through the holes again. Feed the connecting wires through the holes at each end to anchor them and solder to the element. I wrapped mine with some tubular hemming material from the local sewing shop and put a strip of waterproof tape on the outside. Works a treat and the soft material prevents scratching of the OTA. Does'nt get hot enough to damage the material but will melt plastic tape at full output if put in close contact with the wire. There should be enough length on the jug element to make at least 2 heaters for a 4" OTA.
Cheers
Bill
And if you want to take it a step further you could use a PIC to measure ambient temp and adjust o/p.
And if you want to take it a step further you could use a PIC to measure ambient temp and adjust o/p.
Or measure the tube temp like I do this allows me to regulate the focus by regulating the tube temp.
I like the level of detail in the website. There is plenty of good information there. I have one comment about something that seems to get lost in all the effort to explain how to make the heaters and controllers. The effectiveness and efficiency of wire heaters is directly linked to how well the heat they generate gets into the metal of the scope. As many people use batteries this is super important. I have seen too many people with set-ups that warm the night air far more than the scope. By midnight they have flat batteries and a dewed up scope or worse they create heat shimmers that pass in front of the scope.
Long story short, consider installing the heater permanently by wrapping the wire around the tube (over a layer of tape to not short out on the tube) and then covering it with an insulating layer to not loose to much of the heat to air. When mine is heating, so much of the heat goes directly into the tube that the heating element never feels warm to touch. This is a pretty good test. If your element is warm to touch when it is on the scope the heat is not efficiently entering the scope.
I've made a start on a microcontroller based controller for dew heaters. Based on an Ardunio Duemilanovo and an SHT15 Temp/Humidity sensor. Fiddling with some code off the web I can measure Temp and humidity and calculate the dew point.
I'm also planning on using some Dallas 18B20 1-wire temperature sensors to measure the tube temp at the heaters. I've not yet tried out the 1-wire stuff but there is plenty of material on the web to support that. http://datasheets.maxim-ic.com/en/ds/DS18B20.pdf
I've not decided on how to control the heaters yet, an idea I've considered is for each heater unit to have 3 * 1-wire devices and a power switch device associeted with it. If I could get that to work I'd put one each of 1-wire eeprom, switch and temperature sensor at each heater. The eeprom would hold the addresses of the switch and temp sensor. The Arduino could search and find all the eeproms connected to the 1-wire bus and get the info it needs from them to know which temp sensor and switch were associated with each other.
The weakness in that is that I've not worked out how to take advantage of PWM to control the rate of heating. The strength is that I could have a number of heater unit's to plug into a common bus as suited the occasion.
I've still got to work out some aspects of the heaters, I found a supplier on ebay selling bulk rolls of nichrome wire and have made a start on one. I got some thermal transfer tape from Jaycar and wrapped it around some aluminium tube then wound nichrome wire onto that. I've covered that in some insulation sheet I have as leftovers from building a furnace. Some mechanical issues still bug me about that so I may revisit the whole thing.
I've attached a photo of the Ardunio, the SHT15 and an 18B20. I've also attached a screenshot of the software to control the Ardunio, in real use the Ardunio would run stand alone. I've attached the code I'm using at the moment as well for those who want an idea what's involved.
Wow! All this time, I thought I was working on a uC - dew point based Dew Controller without any one else doing the same. I have chosen and aquired the Relative Humidity sensor (a Humirel HS1101) and temperature sensors (LM135s) and am now at the point of chosing a uC. This is where I hit a wall.
Do I choose a uC that can do the math (dew point calculations using relative humidity and ambient temperature requires logarithmic functions, especially at less than 50% RH), do I choose one that can do the A2D for the temperature sensors or do I choose one that can do the PWM for the O/P? The alternative is to choose one that can do it all - which is not really a problem (except for the math), if you are making a single O/P controller. I plan on making a 4x O/P controller (Dob primary, secondary, eyepiece and finder). I have now discovered http://www.awce.com/index.htm. They supply coprocessors for use with AVRs, PICs or any othe uC and have scaled my uP back to a simple I/O pin choice leaving the math and A2D up to the PAK-IX (combines ALU and 5x 10-bit A/D channels) and the PWM to the PAK-V (8 simultaneous PWM outputs). They do seem a bit expensive, however I am willing to pay more to simplify the design. The PWMs will need power switchs on the O/P - likely FETs.
The "stand alone" operation will raise the optics to a couple of degrees above the dew point temperature (or frost point temperature - which can be below the ambient air temperature below freezing) in a effort to preserve batteries and make the heating efficient. I believe I can fit the circuit onto a board about 2.5" by 4" in size and plan on mounting it in my power center (tool box with 2x 12v12AH batteries, power supply / charger) which travels with my scopes.
Tom if you have not done so already have a look at the Ardunio.
It would seem to do the stuff you want to do. The basics don't need an external programmer and there is a big online community. There are other versions, some simpler and cheaper and I may eventually use one of them for the actual build and use the Duemilanove for learning and experimenting. A lot of work also seems to be happening with PIC's but
I've not worked out just what is needed to get going with them.
The broad specs of the Duemilanove are as follows http://arduino.cc/en/Main/ArduinoBoardDuemilanove
Summary
Microcontroller ATmega168 (mines a 328)
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 14 (of which 6 provide PWM output)
Analog Input Pins 6
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 16 KB (ATmega168) or 32 KB (ATmega328) of which 2 KB used by bootloader
SRAM 1 KB (ATmega168) or 2 KB (ATmega328)
EEPROM 512 bytes (ATmega168) or 1 KB (ATmega328)
Clock Speed 16 MHz
After posting last night I breadboarded 4 or the temp sensors with a resistor and some jumpers, made three connections to the Ardunio and loaded some software from the web and was reading the temp from all four sensors with very little effort. This morning on the train I worked on merging the code for the SHT15 and the 18B20's but I've not had a chance to test the combined code yet.
The next step will be to make a decision about controlling the heaters.
Do you have a feel for the accuracy of the temperature controller in terms of hysteresis? I suspect the difference between the complex (log) and simplified dew point calculations is only a couple of degrees and this is possibly within the control error of the temperature controller. You might be able to get away with the simple non-log dew point correlation opening up more options for your uC.
I have both types of sensors being measured now and have had it running in a loop for some hours without grief.
I'm still exploring options for controlling the heaters. If I skip the PWM idea there is a version of the temp sensor with two general purpose IO lines which could be used to drive a mosfet power switch - DS28EA00 and a few other options which might keep the circuits at the heater simple but still support fine control.
I've attached the current version of the code and one loop of output.
Good points!
I will look at the differential between the simplified Dew Point Calculation and the more complex logarithmic method in the next couple of days. My application is more geared towards frost elimination which typically occurs with relative humidity well below the 50% which is the recommended floor for use of the simplified formula. I will fire up excel and do the comparison.
An no, I have not looked at the Arduino. I had planned on using a PIC or utilize the original Atmel MCU00100 Starter Kit which I have mothballed as well as AT90S8515s. Then I ran into the calculation complexity. Looks like I have some homework to do.
Will keep you posted.
Tom somehow I missed the attachment last night.
I've not checked on how this is implemented under the hood but the formula I'm using makes calls to a log function.
I've not got my head around the impacts of atmospheric pressure on dew point nor have I looked at wet bulb/dry bulb calcs but my impression is that I don't need great accuracy.
A range of I2C barometric pressure sensor's at http://www.futurlec.com/Pressure_Sensors.shtml could work well if barometric pressure is important. Cheap and I'm already running a variant of the I2C for the Temp/Humidity sensor.
Looked into the Duemilanove. Wow! Have not dealt with uC for a while (evident from my AT90S8515 reference). Subsequently have ordered a Starter Kit and am downloading and trying to absorb the wide range of information available. Looks like this will make implementation simple on the hardware side.
All of my experience has been programming devices in Assembly language. I am looking forward to being able to use a higher level language. Am still looking into the LOG function call - not documented in the Atmel data sheet at first glance - which does not mean it is not a higher level translation of lower level commands. Do I remember my math on this - no!
Now for Dew Point calculations - I too was using the Wikipedia formulas as they are standard. On the praries of Canada, and especially during the winter months, it is unusual for the RH to get over 50%. However, with the memory of the Duemilanove (ATmega328), I can implement using the Simple approximation for RH above 50% and Closer approximation below 50%. I have discovered that there are no published approximations for frost point. This will likely have to be a table.
My eventual hardware implementation - final version - will likely be a custom PCB / Controller with bare minimum components supporting:
1) 4 channel control (with manual on/off selection).
2) Automatic dew/frost point temperature determination.
3) Optic temperature measurement.
4) Option for manual control by varying the PWM for each channel.
5) Channel on/off and heating on LED indicators.
6) Undecided on a possible 7 segment LED information readout (LCDs freeze here - not a good thing).
It will use the ATMEL ATmega328 with Duemilanove bootloader at the core. I sould probably review my sensor selections as well.
Thanks to you and Peter for setting me on a much simpler journey to the end goal. I find it so much better to develop and understand the details in something rather than to purchase a item that may not do exactly what I want it to do. As battery conservation during freezing temperature observing is a priority for me - no commercial product actually meets my needs.
Hi Everyone,
Wow - everything from calculating dew and freze points to $15 do it on the cheap heater controllers in this thread...
What is my point? After people discover that I design electronics in my day job, I have been approched to suggest / help / design dew heater controllers for some of my Astronomy friends. (I actually designed a heater system for a telesope in Antarctica once, but that is another story)
OK, happy to help, but I am not going to build 5 different models on Veroboard in my kitchen. If we can get a minimum of 10 people interested in getting a professionally designed and built dew controller together, we could start a project.
Problem is can we find 10 people wanting the same requirements?
Goal is to get a quality controller for significantly less $ than buying one from the shop - final price depends on quantity and features as usual.
Post a quick reply or send me an email if you are interested and include a note on the key elements you need to have in your controller.
If there is enough interest I might open a new thread for this.
Funny enough I am at this moment getting some cct boards made for a FET driven 4 channel PWM dew heater I designed & prototyped up.
I found that many FET (or transistor) PWM dew heaters switched the heater thru Vcc rail. Thus when the output driver device was off there is Vcc (in most cases +12V DC) sitting on the outer shell of the heater connector. Some/many of these commercial dew heater use a metal RCA connector & +12V sitting on this outer shell is recipe for trouble.
So I designed up a FET cct that switches heater thru earth with the outer shell connected to earth, thus the outer shell is always earth.
I am tossing up whether to market these as I could pump them out at pretty much $100 Aust.
New member here but I have made a few heaters before. I haven't read the whole thread so excuse me if this has been mentioned before. I use a PWM kit from Jaycar Electronics and for the element I salvage the core out of an electric blanket. My 10" LX50 has been using this setup for about 6 years without fail.
Problem is with the the jaycar cct & RCA connectors, 12V DC can be present on the outer shield of the RCA connectors (if the shell is metal)...see below. I knew this was a problem but really only did anything when I purchase some kendrick dew heater straps which had metal RCA plugs.
The Jaycar cct was not really designed for RCA type connectors more designed for something that is hardwired.
There are many Dew PWM ccts out there that switch Vcc & allow +12V on the RCA shield. In reality RCA plug connectors for this use are a poor choice adopted by amaturer astro nuts.
Some RCA plugs have plastic shells but usually even these have a metal skirt that can present the same problem, especially if you use an extender RCA cable wrapped around your tube.
An easy fix is to cover the RCA conectors with insulating heatshrink.
You will probably never have an issue but it would be very easy to short one of these RCA plugs to your mount or scope...passing +12V thru it.
Which would do wonders for all your valuable electronic equipment.
I use the multi pin/socket connectors, from Jaycar
The male/ female ends are shielded by the plastic housing that they are mounted into.
I've used this for many years with 100% success rate.
I found that many FET (or transistor) PWM dew heaters switched the heater thru Vcc rail. Thus when the output driver device was off there is Vcc (in most cases +12V DC) sitting on the outer shell of the heater connector. Some/many of these commercial dew heater use a metal RCA connector & +12V sitting on this outer shell is recipe for trouble.
Is there a way to test the PWM dew controller (using a multimeter) to see if this is a problem? Would you see a voltage between the earth on the input to the PWM and the outer shell on the outlet when the load is turned down to minimum?
You will probably never have an issue but it would be very easy to short one of these RCA plugs to your mount or scope...passing +12V thru it.
Which would do wonders for all your valuable electronic equipment.
Considering my dew heater and scope share the same 12V source the only thing that will happen is the fuse in the heater will blow.
But the most interesting fact I'd like to make is that electric blankets are the best source of heater elements there is. It comes in a lattex rubber tube that makes it easy to use. Being nichrome means you have to crimp the ends because it's next to imposible to solder.
Funny enough I am at this moment getting some cct boards made for a FET driven 4 channel PWM dew heater I designed & prototyped up.
I found that many FET (or transistor) PWM dew heaters switched the heater thru Vcc rail. Thus when the output driver device was off there is Vcc (in most cases +12V DC) sitting on the outer shell of the heater connector. Some/many of these commercial dew heater use a metal RCA connector & +12V sitting on this outer shell is recipe for trouble.
So I designed up a FET cct that switches heater thru earth with the outer shell connected to earth, thus the outer shell is always earth.
I am tossing up whether to market these as I could pump them out at pretty much $100 Aust.
A low tech (cheap) solution indeed.
Sounds good - can you give us some more info on the actual specs for your controller? A photo maybe? Max output currents, indicators, temperature controlled, under voltage protection/cutoff?
High-side switching - meaning you interrupt the 12V rather than the ground return - is the way to go and can avoid some EMC issues as well. Are the outputs in your controller short circuit proof? P-fets can be a bit sensitive and I have seen them go before the fuse at times.
