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robert.rozee
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Posted: 07 January 2018 at 10:52pm | IP Logged Quote robert.rozee

Phil23 wrote:
palcal wrote:
Out of interest what eggs are you hatching.

Shhhh,

They are probably Dragons....

Destined for the next GOT Series.


oh no, not dragons! everyone knows that the ministry of magic (called the MBIE here in new zealand) prohibits the breeding of dragons.

(for our australian friends, the MBIE is a government department colloquially known as either 'the ministry of everything' or 'the ministry of magic'. it is widely regarded as being evil)


cheers,
rob :-)


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redrok
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Posted: 07 January 2018 at 11:45pm | IP Logged Quote redrok

Hi Grogster;
Please clarify a couple of things:
1. What is the power source? 120VAC, 240VAC, or a lower DC voltage?
2. What is the desired maximum heater power in Watts?
3. Once up to temperature what is, roughly, the normal running heater power?
4. What is the normal ambient temperature?
5. What is the desired plate operating temperature?

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Azure
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Posted: 08 January 2018 at 1:28am | IP Logged Quote Azure

what about one of these Egg Incubator Heater Element in 110v or 220v

Edited by Azure on 08 January 2018 at 1:28am
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Grogster
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Posted: 08 January 2018 at 11:38am | IP Logged Quote Grogster

@ redrok:

1) 12v ideally, but will settle for 24v. Has to be portable, with battery backup. Mains for charging the batteries or indoor use is fine. 24v pad will draw 6A or so while on, which is manageable. 12A for the 12v one might be pushing the life expectancy of the gell-cell!

2) Don't care. All I need, is to be able to EVENLY heat up the 3mm plate to about 45-ish Celsius and keep it there. There is a foam layer on top of the plate, that the eggs sit in to keep them warm, then another layer on top of that, then you close the lid. Eggs never actually TOUCH the plate, as it's surface temperature is too hot(for the eggs), but cos of thermal lag, the plate needs to be about 10'C hotter then you want it to be on the top of the foam.

3) Off. Heater just warms up the plate, then switches off and on to maintain that temperature. Nothing fancy.

4) Can be anything from zero to 30'C. Eggs can be collected at any time of day or night, any time of year - just whenever the hives are ready.

5) See #2. But needs to be adjustable. Having said that though, generally you set-and-forget.

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

But seriously, queen-bee eggs. They can be collected from the hives at any time, and as the hives can be far away from any power source, the unit has to be battery powered so the eggs can be collected from the hive, and transported to the hatchery. If the queen-bee eggs drop below about 33-35'C, the cold kills them.

@ Azure: Nice element. Just bigger then the wee ones I was looking at yesterday. But cannot be mains powered, must be battery powered for portability.

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redrok
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Posted: 08 January 2018 at 1:35pm | IP Logged Quote redrok

Hi Grogster;
Very interesting project!
You know more about what is needed for building the carrier and caring for the little beasties.

1. The rubber heaters are the way to go silicone glued to the plate. They heat the plate evenly without hot spots. Their relatively cheap and come in a wide array of sizes. Also thin and light weight. They are available in any wattage density and voltage. I might suggest the heater power rating be about 3 times the running power when up to temperature.

2. The uMITE is perfect using the built in PWM for controlling a MOSFET. Say at 100Hz or so. At this low frequency you could directly drive a sub-logic level MOSFET from a 3.3V uMITE pin. I like the IRF3708.

3. A simple thermister sensor glued to the plate should have a fast reaction to changes in plate temperature. (I normally don't like thermisters but in this case you are controlling to a setpoint they would be easy to use.)

4. I suspect you don't need a full "PID" control routine because there will be little thermal lag between the plate and the thermister, pluss you have an insulator between the plate and the little critter's. The "P" part of PID directly controls the PWM from 0% to 100% over about 1 deg C. If you want to limit the heater power reduce from 100% to some lesser value.

5. 12V so you can plug it into the car's power.

redrok

Edited by redrok on 08 January 2018 at 1:37pm
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Grogster
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Posted: 08 January 2018 at 2:04pm | IP Logged Quote Grogster

Hey there.

I am thinking of getting a 24v pad, but running it on 12v. I don't need anything like the heat they are designed for, so the likes of a 150W @ 24v pad, but run it on 12v, so the current should be about 3A and roughly 75W. The pads I am looking at have a 10k NTC built-in, so that takes care of that! Here is a link to the A4 one I am looking at. 24v, 150W, 3.2-Ohm so Ohm's law tells me that running on 12v, that should be 3.75A, which is about 45W or so, which should be plenty, and it will be an easy life for the pad too I would think.

Re #2: Are you saying control the heating pad via a MOSFET and PWM? I think that is what you are saying, but just want to make sure I do understand you right.

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redrok
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Posted: 08 January 2018 at 3:09pm | IP Logged Quote redrok

Hi Grogster;
Grogster wrote:
Hey there.

I am thinking of getting a 24v pad, but running it on 12v. I don't need anything like the heat they are designed for, so the likes of a 150W @ 24v pad, but run it on 12v, so the current should be about 3A and roughly 75W.
No no, 37.5W. You have 1/2 the voltage and 1/2 the current = 1/4th the power.
Quote:
The pads I am looking at have a 10k NTC built-in, so that takes care of that! Here is a link to the A4 one I am looking at. 24v, 150W, 3.2-Ohm so Ohm's law tells me that running on 12v, that should be 3.75A, which is about 45W or so, which should be plenty, and it will be an easy life for the pad too I would think.
I agree.
Quote:
Re #2: Are you saying control the heating pad via a MOSFET and PWM? I think that is what you are saying, but just want to make sure I do understand you right.
Exactly. The uMITE reads the thirmister, calculates the error from the desired setpoint, and sets the PWM output.
The PWM controls the heater power as a percentage of max power.
The control loop is plenty fast enough in basic.

I might add another thermister to measure the the bee's temperature on the other side of the insulating barrier.
This could be a second slow control loop that adjusts the main setpoint a bit, slowly.
This, effectively, acts as the "I" or Integrate in "PID".

To increase efficiency another insulating barrier could be placed on the "Outside" surface of the plate. Less heat loss = more efficiency and longer battery life.
What kind of battery are you considering?
How long does this battery need to run before you plug into the car's power?

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Posted: 08 January 2018 at 3:47pm | IP Logged Quote Grogster

Arrggghhh. Being on holiday I have let my Ohm's law slip!

37W is still just fine I expect, for what I plan to do. Either 3Ah or 7Ah gell-cell. I would prefer the 3Ah as it is smaller/lighter. The unit is brought up to temp using the mains supply or the car cigarette lighter socket power, then to collect the eggs and return to the car would be less then one hour.

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Posted: 08 January 2018 at 4:19pm | IP Logged Quote Grogster

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?

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Posted: 08 January 2018 at 4:50pm | IP Logged Quote MicroBlocks

I am not sure a 24v heating pad will work with 12v. The reason nichrome wire gets hot is because it has high resistance compared to for instance copper wire.
A 24v version would have longer wires and thus needs that voltage to get the wire to heat up.
Also resistance changes with heat.
12v Might not be enough to get it to the required heat.
Have fun with this info: :)
https://en.wikipedia.org/wiki/Nichrome


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Grogster
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Posted: 08 January 2018 at 5:05pm | IP Logged Quote Grogster

Oh, excrement!
Really?

Oh well, I suppose I can get one in(a pad) to play with, and go from there.

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Posted: 08 January 2018 at 5:24pm | IP Logged Quote Warpspeed

He is only going to heat it up to 40C, the change in resistance will be negligible.
It will behave just like an normal normal resistor, half the voltage will give half the current, and a quarter of the power.

A 24v device should work perfectly well on 12v. It will probably be a lot more reliable too if quarter rated power will do the job.

A chickens bum cannot put out that much heat, surely............

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