Would a Basic Stamp be better? OR maybe a Atmel AVR based system.
Not really, as they also have limited outputs, start to get costly, and require specialised hardware.
The beauty of the PICAXE is its cheap and simple to use.
Ie the 08M and 18X both have PWM but dont require oscillators etc
Just a 5V supply and they are away.
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Why not just use a LM35 to feed back a voltage to a LM317 to maintain a steady temperature.
LM317s only work down to 1.2V, are only rated to 1A and also need a bit of headroom between input voltage and output voltage. As such, they will give a DC output, but need a lot of heatsinking if you want to draw any large currents. You may need bypass transistors or paralleled 317s to get the reqd output. You also need some way to turn it off as it will never drop below 10% due to the lower 1.2V regulation reqmt.
If you always have 240V nearby its OK, but PWM is much more efficient at doing wide range DC-DC conversion, hence its growing use.
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no2 circuit is quite complex. My head hurts looking at all those op amps.
Nah. Its all good fun, but a uP that can read digital temp sensors makes it soooo much easier. The end result is the same, ie a ( temperature controlled ) PWM output stream driving a Mosfet
If you want to drive a "Starter Motor" in a car, then use a transistor on the output stage, but you are only using it to heat a wire.. Jeepers, all you need is 1 amp for starters, so if your windings are 12 ohms, lets say, thats 12 Watts of power your radiating. Also the correct LM317 will do 2A if you heat sink it correctly (2x1 inch). I use it to drive my steppers on the 22" fork mounted Newt i have.
Like i said, you can use the OPAMP to reduce or increase the desired voltage that will be fed into a resistor bridge to get the right offest needed.
Also from memory, you can also use a 1N914 as a temp sensor too, which is good as the diode is used inside a working circuit, and changes its Reverse current according to junction temp. But thats a little more tricky.
Also the correct LM317 will do 2A if you heat sink it correctly
Fully agree, but heatsinking the controller means wasted energy.
Also, with regulators like the 317s etc used in this way, the lower the required output, the more energy that is wasted in the controller vs what goes into the strap ( as a percentage ).
For a battery supplied unit, this is a design factor to take into account.
Ie you trade low noise for low efficiency
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Like i said, you can use the OPAMP to reduce or increase the desired voltage that will be fed into a resistor bridge to get the right offest needed.
Not totally true. Look at the Spec sheets for a 317 ( part attached )
It will only regulate down to 1.2V ( due to its internal circuitry )
and will only output about 10Vmax for a 12V input ( based on load )
You will note that as Vin-Vout, gets close to or below 2V it loses regulation, and also its ability to output any current
ie exactly when we want the most current.
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Also from memory, you can also use a 1N914 as a temp sensor too, which is good as the diode is used inside a working circuit, and changes its Reverse current according to junction temp. But thats a little more tricky.
Yep, same with transistors ( all you need is an unbiased NP junction ).
Meade do it in that way in the GPS scopes.
Power is lost if you voltage drop is high and current is high across the regulator. If your only drawing 200mA from the wire, then, then your not going to have a huge loss in power. What you are talking about is 20 hours over 17 hours lets just say as a ball figure of usage time. Even then, it should be less than that. But if your that fussy, put a 7812 switchmode regulator in line. They look a little larger than a 7812, but is a switchmode item. Same pinout and soldering footprint size as the normal 7812. Costs were around 15 buks many years back, so check out the price now. Then use it in a CC mode.
As for OPAMPS, you need to go back to what can be done with them. I can make the output swing to what ever voltage i wanted. Feed this into a resistor bridge, and you can get what ever ratio you want to offset the regulator. Gain ratio can be what ever you want up to the supply limit. Im not using it to drive the heater wire, just a feedback to alter the current output.
So if i need a 1 or 2V change for a 10 to 20mV change in the sensor, then obviously i need a 100 to 1 gain. This can either be a neg or pos gain. But look, we are both chasing tales here. Theres more than one way to rob a bank, so its up to the individual to decide how complex or big he wishes to make it. Same goes for the original idea of using resistors. It works, its cheap, its easy !, end...
Power is lost if you voltage drop is high and current is high across the regulator.
Which we dont get when talking dewstraps.
As i mentioned earlier, the design for a dewstrap, where you have a very low resistance load driven over a large voltage range, is very different to driving set loads at set voltages.
To drive a regulator in CC mode, you need a series resistor in series with the load. When the load is only about 7ohms, this series resistor dissipates a lot of energy also. Again, its all a tradeoff.
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As for OPAMPS, you need to go back to what can be done with them.
I understand what can be done with OpAmps, ( i use them in my no 2 design ) but thats not the problem, its the regulator efficiency and voltage limits.
My designs are all based on max efficiency for use with a battery,
not minimal components.
As you say, many ways to skin a cat.
I remember now the most important thing we learn as engineers and why it is the most important.
1. Problem Definition (Specs, requirements etc established).
2. Problem Analysis and Solution Design.
3. Prototype Implementation
4. Testing
5. Repeat 3-4 until it meets Specs, and requirements.
6. Final Solution and documentation.
Without the first step there is no point. Gamma and Andrew you both have different methods, perhaps because you are working to different criteria.
It depends how strict you wish to be with the specs, and that depends on how deep your pockets are or how much time you have etc etc. Building the perfect system is impractical because you will remain in step 3-4 for ever.
That process is widely applied, with minute differences. It is also the reason why one should not jump on the Web and simply surf. The computer is a tool, the web is called the web for a reason you can get lost, if you dont define what you want from it. Even if you just want to pass time, you need to define how much time. An important lesson for the kids.
You dont need to vary your voltage over a large range, use a variable voltage switching regulator, like this one http://www.robotparts.com.au/dimension.htm#swadj , i cant see the need for more current, but you know how to increase it....
Anyway, thats enough yip yap for me.. Good luck with everyones design, lets keep them dry !.
You dont need to vary your voltage over a large range,
True, but the discussion thread started on thermostatically controlled dewheaters ( Not just on/off or manually set )
For this type of controller ( and max efficiency ), you need to be able to vary the output voltage proportional to temp difference.
Straight PWM gives this
If you just want an on/off type, then thermistors feeding a comparator feeding a mosfet is all you need.
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use a variable voltage switching regulator, like this one
Nice neat unit. Looks like a shrinkwrapped "homemade" special.
I note it still only goes down to 1.2V, so an on/off switch is still required.
What do ppl use for their power supply for a dew heater, I've built one after a few goes at it but the problem I have is that it only works(heats up) if its connected to a 12 v car battery, is there a smaller battery I can use, rechargeable if possible, the problem with a car battery is that its gonna get in the way when I slew my scope, at least the battery for the fan goes under the ota, and sits inside the base.
Any help will be appreciated.
You can get small sealed lead acid batteries that are about the size of half milk carton.
I was looking at a 12v 7.2ah SLA in Jaycar today for $19, the next size up was 18ah for $40 I think, though it was somewhat bigger. The 7.2 would easily fit somewhere on the ground board of a dob.
I use a jump starter pack (they seem pretty popular these days for astro work). I have a 7AH power tank that I power my scope with, a 24AH jump starter pack I use for the dew heater, and a 38 AH jump starter for the lap top when required. Buy them from auto accessory shops.
I'm looking at your dew heater design to possibly make a set for my 20x80's.
Firstly how long to you power them for or can you just leave them running
for the duration your observing for. If I give you the plan of what I would
like to achieve could you tell me what I need to build it? sorry I know that
sounds a bit slack but when it comes to electronics... I'm good with a
soldering iron and I can tell you a few diferent components but thats about
it.
Secondly, how do you think of, instead of soldering the legs together using
two strips of braided wire and poke the resistor legs through the braid at the
required intervals and then clip the ends off. I was thinking this could give
the required fexibility to wrap and unwrap around different objects. Since the
braid would always be in contact with the resistor ends/legs would you need
to solder? you could design a cover to hold it all in place.
Also could you make something like this that you could run off a couple of
"D" size batteries?
This may seem a very crude design, but several years ago I built a dew heater out of the following:
I bought an electric heater element from the local electrical shop. This was a coiled 'spring-looking' element which would be fed down a glass tube of an electric bar heater.
I cut a small length and set it aside.
Them from an old electric blanket, I cut strips of material, and wrapped one layer around the eyepiece, then coiled 2 revolutions of wire over the top, then covered it with a top layer of electric blanket material. Making sure the 2 ends of the wire was protruding, I connected them to a 12VDC source.
This was enough to apply just enough heat to stay off dew. So really, I actually made a little electric blanket!
Look, it was very rough, but when you have no money and no resourses, it was a great improvisation.
Sorry for the late reply, Rob, I must've missed this post with all my mooving and dial up business...
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Originally Posted by CoombellKid
Hi Al,
I'm looking at your dew heater design to possibly make a set for my 20x80's.
Firstly how long to you power them for or can you just leave them running
for the duration your observing for.
I use a controller and generally leave them on. It is far easier to prevent the dew forming in the first place than to try to evaporate the dew after it has formed.
Quote:
Originally Posted by CoombellKid
If I give you the plan of what I would
like to achieve could you tell me what I need to build it? sorry I know that
sounds a bit slack but when it comes to electronics... I'm good with a
soldering iron and I can tell you a few diferent components but thats about
it.
You don't sound too different to myself when it comes to electrickery! I think you'll be fine with the dew heaters.
Quote:
Originally Posted by CoombellKid
Secondly, how do you think of, instead of soldering the legs together using
two strips of braided wire and poke the resistor legs through the braid at the
required intervals and then clip the ends off. I was thinking this could give
the required fexibility to wrap and unwrap around different objects. Since the
braid would always be in contact with the resistor ends/legs would you need
to solder? you could design a cover to hold it all in place.
I think you'll definitely still need to solder: a) to get a good electrical connection; and b) to stop the resistor tails from pulling out of the braid during handling.
Quote:
Originally Posted by CoombellKid
Also could you make something like this that you could run off a couple of
"D" size batteries?
regards,CS
You could, but I'm not sure of the capacity of D cells and how long they would last. The dew heater designer spreadsheet will help you design the resistors for it though. The default power supply in the spreadsheet is 12V. Simply change this to 1.5V for 1 D cell, 3V for 2 D cells in series, etc. Adding D cells in parallel will not change the voltage but will increase the time the dew heaters can run for.
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?
I don't see how it could damage your battery unless the battery is way too small for the job (i.e. you will fully discharge it every time you use the dew heater).
What Ah rating is the battery? As long as the current draw for the dew heater(s) times the number of hours you expect to use them for between charges is less than the Ah rating of the battery (the bigger the margin the better!) it should be fine.
I will stand corrected by anyone with more electrical nouse than me.
I don't see how it could damage your battery unless the battery is way too small for the job (i.e. you will fully discharge it every time you use the dew heater).
What Ah rating is the battery? As long as the current draw for the dew heater(s) times the number of hours you expect to use them for between charges is less than the Ah rating of the battery (the bigger the margin the better!) it should be fine.
I will stand corrected by anyone with more electrical nouse than me.
Al.
The battery is rated at 9800 mAh, and I also have a second one with 7 Ah.
I'm asking this because someone told me it could damage it.
An important lesson for the kids. 
I think you'll be fine with the dew heaters.
