Yes, I got to that part of the process (thinking it was simple) but I could quickly see the the needle on the frustratometer climbing if I tried to persist... a little innovation de-fuses all that!
Is there any particular reason people use resistors rather than Nichrome wire? I thread soem nichrome into some heatshrink tubing, run 4 strands attached to switches such that I have 4 settings. Works great on my 14" run inside the end lip just in front of the corrector plate. With a simple foam dew shield I get complete protection on nights of 90%+ Humidity and freezing temperatures.
Is there any particular reason people use resistors rather than Nichrome wire? I thread soem nichrome into some heatshrink tubing, run 4 strands attached to switches such that I have 4 settings. Works great on my 14" run inside the end lip just in front of the corrector plate. With a simple foam dew shield I get complete protection on nights of 90%+ Humidity and freezing temperatures.
Cheers
David
The best reason to go for resistors that I can think of David is for small items like eyepiece and finder scope heaters. To get the resistance high enough for these small lenses would require a long length of nichrome wire (unless you have ready access to a supply of a range of wire sizes!). A long wire around something small can be a problem.
Once you get up to reasonable scope sizes, I say nichrome is the way to go. Nice and neat and tidy! But if anyone wants to build a resistor heater for their scope, they can do that too.
Whilst i agree that resistors are a good ( and inherently safe ) solution
for small dia objects, ( and amateur construction )
you can use nichrome wire ( and hence get a thinner more flexible strap ),
as long as you provide some form of current limiting.
If you are using a PWM controller, this can be a simple limit on the max available duty cycle.
If a variable voltage regulator ( like say an LM317 ) in constant current mode , then you limit the max current.
Of course, you do need to ensure you NEVER plug the strap directly into a raw 12V supply ( which isnt a problem for resistors ),
but you can end up with a smaller heater strap if space is a problem
( ie straps for EPs in a Binoviewer,
when the user has close set beady eyes :-) )
You are absolutely right, of course, and its a valid comment to make. I confess to making the assumption that up to a full 12V could be used through the heater.
Even with some power tanks these days having multiple voltage outlets (3V, 6V, 9V typically) you could of course make a simple nichrome wire heater suited to one of these voltages as well - nice, neat simple solution!
As you say, though, you just need to make sure you don't put excessive voltage (and hence current and heat) through it.
Re accidentally putting a solid 12V through a short nichrome element, i agree its a problem, that could result in really red nichrome wire,
but a fuse will fix that short term.
However, what i have been playing with recently is self contained microprocessor controlled heater units ( one per strap ).
If you use a surface mount type PIC controller, you can build the whole controller unit into the heater strap itself.
This can give you a thermostatically controlled unit that only requires a 12V supply. Hence no dramas.
Slightly more expensive than a centralised controller, but much more flexible.
This route also eliminates all power supply problems, whilst still giving a minimal current draw solution to the heater.
It does use PWM to control current, and hence has EMI/RFI consequences,
which Daves switched nichrome wires doesnt, but it results in a much more flexible system.
I have designed/played with about a dozen different types of controller at present.
1) Passive manual ( just a 0-100 pot )
2) Passive manual/thermostatic ( using LM335 temp sensors )
and a dual purpose 0-100%/0-10deg offset pot
3) PICAXE controlled manual/thermostatic ( using DS18B20 sensors )
4) Simple PIC controlled
5) Really flash PIC controlled ( digital setting )
All have their own applications and costs
Also, with minor tweaks, all the above can be made for N Channel or P Channel operation.
1) Is very simple, has true linear 0-100% operation and is about the size of a matchbox ( using full sized parts ) see piccy.
5) is the most complex, as it also has humidity sensors and a LCD display, and they double the cost but arent required for basic units.
For a minimal Picaxe controlled thermostatic unit, cost of parts are in the order of $50. The P Channel is normally a few dollars more.
You also get low battery alarms, and a serial feed out if you want to watch what its doing from yr PC/Lappie.
Whilst i havent succeeded in making a truly integrated surface mount unit yet, option 3 is the easiest to work with for people without PIC burning hardware.
The circuit diag for these units is pretty simple ( as all the smarts are in the software ) and the fabrication ( other than the PCB ) is simple.
Basically, i find a box, then make a board to suit, then change the program to suit the board. It sounds complex, but is really quite simple.
You also get to program it with free software ( the programming hardware is built into the unit, so all you need is a serial cable )
Also, as you get to program it, ( and a dewheater controller is just a variable voltage supply ), you can modify it to drive peltiers, fans, motors etc.
Its all up to yr imagination ( and the current capacity of yr selected Mosfet )
Hi Andrew,
Thanks for the info, food for thought. Electronics was a hobby of mine until about 7 years ago. I've never played with PIC's. Do you make your own PCB's they look terrific?
Regards IanG
Once i have a working design, i find a box to fit it in
then make up a PCB to suit. Its fairly simple and only
takes about 20 min to make the pcb ( once the design is settled )
I use presensitised boards so its easy ( if the sun is out )
As to PICs, i use both raw PICs and PICAXE
The latter is a new product that uses a native PIC that has a proprietary basic interpreter built into it
look up http://www.rev-ed.co.uk/picaxe/ for details on the product.
As such, it is very simple to program, but as a tradeoff, has limited functionality relative to a native PIC, and runs slower.
However, the software to program it is free, and the hardware to program it is a serial port and two resistors. It has native functions for reading digital temp sensors, onboard 10bit ADC and can send hardware generated PWM, so is perfect for one off apps to drive peltiers/heaters/focusser motors.
Ie i have a meade microfocusser. By using one of these chips, i can use ADC functions to give me infinite speed and direction control from a single pot, centre is off.
For simple one function apps, its very flexible, and only takes about 10sec to load programs. As such, if you dont like how is running, change the code, not the hardware.
Couldnt be simpler.
Andrew It would be nice to get some schematics of a PICAXE System to with multiple heater control. I imagine all could share a single Ambient sensor. With individual sensors on each target area. If you can provide a parts list perhaps a mob like jaycar or Microzed could be convinced to carry a kit.
Andrew It would be nice to get some schematics of a PICAXE System to with multiple heater control. I imagine all could share a single Ambient sensor.
Have attached a copy of the schematic for the box i referred to above.
However, i did note the PICAXE runs slower than a normal PIC and has other limited functions. One of these limitations is it only runs one hardware PWM channel. This is not a showstopper, but i havent tried to see what the timings would be like to run a bitbanged PWM, esp 4 channels.
Just one thing. I mentioned in an earlier post re trying to get a surface mount unit going. I never got back to that, but the idea of putting the heater right on/near the strap was to try and reduce RFI from the cables.
Rather than have 4 units in one box, another option may be to use distributed processing. ( Sounds techo but is simple really )
The PICAXES have inbuilt serial comms. If one main unit were placed somewhere, with the ambient sensor in it, as well as the fusing and 5V supply, then individual ( much smaller ) units could be made up for each heater. These would only have a small PICAXE, a pot, a local temp sensor and a MOSFET.
The master PIC would just send out the air temp when reqd
, and the local units would carry out instructions. This would make a very flexible system, and much cheaper for larger nos of heaters.
Now that i just thought of that, i'll have to think about it some more.
Quote:
If you can provide a parts list perhaps a mob like jaycar or Microzed could be convinced to carry a kit.
Not sure there would be a large enough market for them to consider it.
Andrew, now you are testing me on theory learned 9 years ago. Are you speaking of NPN vs PNP Transistors? No idea, please explain pros and cons of each.
Can the PIXAXE handle two or three heaters automatically, what are the input output limitations?
On the subject of dew heaters, here's a page with some info on Nichrome wire dew heaters using silicone tubing and a nice simple PWM cct using a 555 timer and a couple of extra components. I built the circuit (for under $15 incl case) on veroboard and it works very well. It would be very simple to put the board inside the unused battery compartment of the LX90 Fahim.
A dew heater for my eyepieces would be great. Thanks again.
that up to a full 12V could be used through the heater.
