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MicroBlocks
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Posted: 08 January 2018 at 5:32pm | IP Logged Quote MicroBlocks

You would need to know the resistance of the wire when it is cold.Then you can calculate how many amps will flow through it. With that you can calculate how much it will heat up.
The resistance will change with heat, maybe the mentioned resistance is when it is used at full power or it is when it is cold.
A test will make that clear.



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Warpspeed
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Posted: 08 January 2018 at 5:39pm | IP Logged Quote Warpspeed

They use nichrome wire in electric radiators, and it runs brilliant red hot.
The change in resistance is not that much, less than 10%

Rising from maybe 20C to 40C its going to make bugger all difference in resistance.

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Phil23
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Posted: 08 January 2018 at 8:36pm | IP Logged Quote Phil23

Grogster wrote:
@ Palcal and others asking about eggs: Velociraptor. It'll be fine - they are all female so won't breed.....


See,

Dragons.....

Quote:
Velociraptor was a mid-sized dromaeosaurid, with adults measuring up to 2.07 m (6.8 ft) long......

Fossils of dromaeosaurids more primitive than Velociraptor are known to have had feathers covering their bodies and fully developed feathered wings.[11] The fact that the ancestors of Velociraptor were feathered and possibly capable of flight had long suggested to paleontologists that Velociraptor bore feathers as well, since even flightless birds today retain most of their feathers.






From Wiki....

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redrok
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Posted: 09 January 2018 at 1:19am | IP Logged Quote redrok

Hi Grogster;
Grogster wrote:
I am planning to use a Fairchild/ON RFD14N05 MOSFET, as I have several of these in stock. 14A S-D current, Vgs(TH) of 2v(so should work well driven directly from a MM 3v3 output), and Rds(ON) of 100 milli-ohms. You can get MOSFETS with better specs then this(specifically the Rds(ON)), but this is what I have, is small, yet can handle the current of the pad no problem. TO252AA SMD package.

P=I2R:
P=4-squared-R
P=4-squared x 0.1
P=1.6 Watts @ 4A load worst-case, so I think this can easily be dissipated within the copper pour with some heat vias to the bottom layer.

Hell, I probably have THAT wrong now!

MOSFET would be a LOW-SIDE switch, with the heating pad between 12v and Drain. Source to Ground. Gate to MM I/O pin.

Do I have that right?
I believe so.
Although it would be prudent to have a 100K gate pulldown and a 1K series to the uMITE pin to protect from charge injection due to the MOSFET Miller capacitance.
I like the RFD14N05. It is one of the standard parts in my junk box.
I measured the RDSon at 64mohm @ 3.3V.
The full on power dissipation would be only about 1 Watt.
This part will have to be insulated from the plate. However, at 1.6W or even 1W, the package would not need a heatsink at all.

Another part, TF2618L in a "Fullpack" insulated package and only 16mohm @ 3.3V.
The full on power dissipation would be only about 0.25 Watts.

And another, IRLI2203N in a "Fullpack" insulated package and only 8mohm @ 3.3V.
The full on power dissipation would be only about 0.125 Watts.

I've a listing of many parts on one of my web pages, see:
Parts with which I've Used, Tested, or have an Interest. Lots of data here.

The resistance of the rubber heater will change with temperature, that's inevitable, however, the absolute operating temperature of the wire is not much higher than ambient so the change of resistance is minimal.

The first order approximation of resistance VS. temperature is:
25degC + 273degC = 298degK ambient
35degC + 273degC = 308degK the Bees
( 308degK - 298degK ) / 298degK * 3.2ohm = +0.11ohm resistance change
3.2ohm + 0.11ohm = 3.31ohm
That's not a significant change in resistance.

Have fun!
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Quazee137
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Posted: 09 January 2018 at 2:19am | IP Logged Quote Quazee137

As my Granddaughter hates this. But back in the day "early 1960's"

My Dad and Grandfather had me help with this task.

We use a big ZIPPO heater and metal cigar tubes wrap the egg with
tissue then in the tube and into the ZIPPO bag.

We'd get about 20 each time.

The incubators for the chickens, ducks and pigons was gas powered.



Edited by Quazee137 on 09 January 2018 at 2:20am
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Grogster
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Posted: 09 January 2018 at 9:15am | IP Logged Quote Grogster

@ Phil23: Trust you to actually research that comment. "Clever girl."

@ redrok: I usually just provide a small-ish zone of insulated copper pour on top and bottom layers for heatsinks for these kind of parts.





Thermal vias are filled with solder during assembly, but I doubt I would even need the bottom copper 'heatsink', but if you have room, why not allow for it. I will add a 100k pulldown to the gate, and a 1k series.

@ Quazee137: Owwwww. Gas powered. Fried eggs.

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Quazee137
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Posted: 14 January 2018 at 3:41am | IP Logged Quote Quazee137

Quote:
@ Quazee137: Owwwww. Gas powered. Fried eggs.


It was small boiler much like an old peculator coffee pot.

Hows the work going?


Edited by Quazee137 on 14 January 2018 at 4:00am
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Grogster
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Posted: 14 January 2018 at 10:53am | IP Logged Quote Grogster

PCB's ordered, made, and on DHL to me now.(have arrived at Auckland Airport according to tracking)
Waiting on heating pad to arrive. I got two.
I will assemble the PCB while I wait.
The PCB is designed as a backpack to a 2.8" SPI LCD with touch, so I can add some basic touch controls to the LCD screen. Old design used a red LED display, but the SPI LCD's can be setup however you want - including red 7-segment if I want it.
Pad has NTC, but I have elected to use a couple of 18B20 temperature sensors, as they are much more accurate then a passive NTC would be. I have allowed for connection of the NTC to an ADC pin on the MM2 chip, just in case I elect to use it.
I will post back to this thread as I progress with the project.
Thank you for your interest.

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grunto
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Posted: 27 January 2018 at 6:45pm | IP Logged Quote grunto

A few thoughts (a bit late I know). Have no idea about bees but I have built a few incubator systems for birds (mostly parrots, although some chicken and quail systems). Learned some lessons along the way:
1. Make sure the incubator has good "thermal mass". This smooths out most of the temperature flucations (such as when you need to access inside the incubator) and makes the control system easier to design. I used to use plastic bottles full of water (3 or 4 x 1.25l soft drink bottles).
2. Use a forced air rather than a passive air system. Forced air eliminates hot and cold spots that tend to form in the incubator. I used a computer fan with a PWM speed controller and then used multiple temperature sensors (I think about 8) to ensure consistent temperature throughout the incubator.
3. Make sure it has an over temperature and under temperature alarm. Bird eggs were very sensitive to over temperature but less sensitive to under temperature (within reason). I also used a data logger for the temperature so I could compare temperature variations with hatch rates and this allowed the incubator temperature to be fine tuned.
4. 12V systems are a great idea - I often had to transport eggs in the incubator so being able to plug it into a car 12V outlet was very useful (although plenty of people I know just used a 240V inverter in their car). I just used a 12V power supply when I used the incubator at home.
5. Humidity control was often the hardest part - not sure if its is an issue with bees. High humidity was required leading up to hatching but measuring and adjusting humidity was always a challenge. I to make use of DHT22 sensors and used little 5V fans blowing across a shallow dish of water to adjust the humidity but the control loop was difficult due to the delay in generating a higher humidity. I was thinking that the little ultrasonic "fountain fogging" devices might be an alternative but never implemented such a system.
6. I tried many different heat sources - nichrome wire, light bulbs (and you can still buy incandescent bulbs in NZ I think), power resistors, peltier devices etc. I finally settled on in slab heating wire - I used it at 12V (it is normally 240VAC) and found a metre or so was about right (I used to just get "left overs" from new house builds - took me a while to learn that you can get it in different resistance / metre depending on the area being heated).
7. I use to keep my system in a the garage which was under the house - the temperature was pretty constant (compared to most rooms in the house, particularly during the day). Wife also preferred the system out of the house. Had a little display inside the house to monitor the system via a 433MHz link so i didn't have to go into the garage all the time - nowadays would probably just connect it to WiFi and write an app so I could use my phone.....

After a few iterations the system was pretty robust and would run almost unattended for the full incubation period.

Looking forward to hearing how you go.
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Posted: 28 January 2018 at 11:55am | IP Logged Quote Grogster

Hi there grunto.

Replies:

1) Original prototype(that sort-of worked) used a 3mm aluminium plate. I plan to make the new one 5mm at least, for the thermal mass you are talking about. I may even go thicker then that if the thicker plate is not prohibitively expensive. More mass.

2) Good idea, but not possible in this version as I have now built the new prototype PCB. Something to considder for another revised version if the new version does not work out though.

3) Something to add. Currently does not have any kind of audible alarm, just the LCD alerting. But if you are not looking at it.....

5) No, not an issue. They are surrounded by bees wax, inside a queen-bee carrier, which is then placed in the pillow(thick foam that sits on top of the plate, and is punched with a whole lot of holes so the carriers are a push-fit). They just need to be kept warm during transportation from the hive.

Here are a couple of photos of the assembled unit:









The last image is showing errors for the temperatures, cos I don't have any 18B20 sensors on it just yet, so the code is written to pick up on that and show an error message. The 'SYSTEM HALTED' message in red is actually blinking off and on. I will try to get some sensors on it later and post another image.

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Posted: 28 January 2018 at 5:13pm | IP Logged Quote Grogster

OK, I have hooked up a couple of 18B20 sensors, and this is what it looks like under normal operation:





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Posted: 29 January 2018 at 5:35am | IP Logged Quote hotwater

delete

Edited by hotwater on 29 January 2018 at 5:39am
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