Brain exercise project, watts, temperature hotplate

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A friend of mine bought one of those maker bot type CNC 3d plastic oozing machines. I guess they need a hot plate to make the parts on so that the plastic doesn't cool too fast when the machine starts making the part.

He showed me a circuit board "resistor" plate and asked if i would make him a 7 x 7.5 inch plate. I attached a picture of the plate I made. It has 3.9 ohms resistance.

I powered it up to the maximum 3 AMPS my power supply could deliver and got it to heat up to about 110 degrees F
I guess they need it to go up to 240 degrees F

Assuming the PCB board can take that heat (I don't yet know if it can) instead of just increasing amperage to it I thought I would try and figure out how to calculate how much power the board would need.

There are 43 traces .085 in wide 6 5/8 inch long
About 285 inches. One ounce copper board.

Where do I start to calculate watts per square inch needed to get the surface temperature to 240 degrees?

From my 3 amp test raising the temp from 70 to 110 (40 degree rise) I would think a rough estimate to get up another 130 degrees would take 130/40 or another 3.25 x 3 amps another 9.75 amps so roughly 13 amps

So 13 x 3.9 is 50 volts. This would require an expensive DC supply compared to what I think he should use noted below.

That is the traditional wing it method I typically use but I know someone who had a good math/physics background would put a pencil and calculator to it with some fancy graphs and all. is there anyone here that could give that a go? It would be a very nice introduction to thermal dynamics.

By the way I suggested a $19.00 plug in hotplate with a good adjustable heat setting and an aluminum plate or I could use some nice 1/4 inch SS tube heaters from OMEGA and embed them in an aluminum plate and run it on 110VAC using a zero cross SSR.

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the guys at the RepRap community have designed their own heatbed, eagle board can be downloaded too.
check here :
http://reprap.org/wiki/PCB_Heatbed

As a note :
240 degrees Fahrenheit = ~115.5 degrees Celsius

Also see Joule's first law :
http://en.wikipedia.org/wiki/Jou...
http://en.wikipedia.org/wiki/Jou...

Q = I^2 Rt

I is the current.

R is the resistance.

t is for the amount of time the current is flowing.

Q is for the energy exchange from electrical potential energy into thermal energy in Joule.

1 Watt is 1 Joule/sec

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it"

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I would recommend a strip heater, and add temperature control. They come in various wattage, voltage and sizes. They are inexpensive, industrial strength, enclosed in metal, and electrically insulated. They are used for your application or in outdoor electronic cabinets, for moisture control. I purchased mine from McMaster-Carr. http://www.mcmaster.com/#strip-h...

Actual temperature depends on ambient conditions, thermal load and losses. The temperature control will hold the temperature if you have enough watts to meet or exceed the worst case demands.

It all starts with a mental vision.

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Thanks for the replies. The Ultra-Thin Polyimide Heat Film looks like a very nice option to mount under a piece of aluminum.

I also see some very nice silicone square heat pads at OMEGA engineering.

I gave the guy the one I made, he likes it and bought me lunch. I made a connection in the center to make two 2 ohm sections so parallel it's 1 ohm. My 14VDC power supply (300 watt) heats it up to over 240F so he plans on using a 250 watt old computer power supply. I will make a little PWM controller so he can dial in the temp. Heck I have a bunch of the IRFP4410ZPBF mosfets just sitting here, the ones i was going to make a 20 amp stepper driver a month or two back. I decided to take advice and used gecko 7 amp drivers and new nema 42 stepper motors.

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FR4 will NOT withstand that temperature for an extended time. The resin will evaporate and you will be left with glass fiber mat and copper traces hanging in the air.

I write from personal experience on this. The board was maybe 120F, max.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Metron,

That's a sharp looking board.

I'd just be worried about the low load impedance on the power supply, but if you are within its output current rating then so be it.

If I were considering options I might consider a string of power resistors mounted to the bottom of a metal sheet, (aluminum, copper, etc.).

I'd put a little smear of the thermal transfer goop used when mounting TO-220's to heat sinks between the resistors and the metal sheet, then bolt them in place with a small U-shaped clamp. The clamp would additionally pick up much of the heat from the side of the resistor not attached directly to the sheet, and would transfer the heat to the sheet itself.

Being a metal sheet, it will distribute the heat throughout itself well.

JC

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Are the elements interleaved, vcc and gnd? Isn't that inviting an accidental short? Maybe spray paint it black with muffler krinkle paint?

Imagecraft compiler user

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I will email him Jim's concern, like I said FR4 and this board would not be a choice i would make. Initially i thought 120F is all he needed but your testing shows that is even outside the FR4 longer term range.

Elements are from one end to the other and I did buy the 500F black high temp paint it also protects the copper from oxidizing. His plan was of Friday to cut a pocket in a 1/4 inch piece of MDF leaving small islands every inch and mount the board copper side down so air would insulate the bottom side and a piece of glass placed on top. I guess that is the way people are doing it.

I like the resistor idea, that is actually what the www.seemecnc.com folks did to make the nozzle that melts the plastic. I have a bunch of square white ceramic 5 Watt 1.1 ohm resistors that would fit this project well. I may just make a table out of 1/4 inch 6061 and mill it to fit the resistors. I have High temp silicone molding material the same as used in the silicon heaters from Omega Eng. That would make a very nice heater plate. The operating range on the SQP500JB-1R1 is up to +155°C

http://search.digikey.com/script...

Since I also did a project using the max6675 Direct Digital Conversion of Type -K Thermocouple Output chip, I think this would make an excellent little project to refresh my programming skills and perhaps a better option for the folks working with these cnc machines. Perhaps i will make a kit and get on the money making bandwagon.

I have my EXLO-Knee mill rebuild to full CNC finished and making pockets and cutting 1/2 inch aluminum is a simple task now. Here is an example of a marching band bass drum 18 watt red led I make for the Last Regiment of Syncopated Drummers. It has 3 of the 3watt leds on both sides and uses the cat4101 constant current driver and a tiny13 controller.

O heck, I just did some calculations on how many resistors it would take and just buying the Omega silicone 6 x 6 heater would be a much better and cheaper solution. I guess I am just looking for things I can mill...

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More about strip heaters, per McMasterCarr catalog:

Use for high temperature applications in plastic injection molding. These heaters have mica insulation and are extra thin for tight spaces. Maximum heat output is 1200° F for 1 1/2" wide heaters; 900° F for 3" wide heaters. All are single phase, UL recognized, and CSA certified.

1 1/2" wide heaters are 3/16" thick and have a Type 430 stainless steel sheath and 10-24 screw terminals with 3/4" high threaded studs. Mounting holes are 1/4" dia. 3" wide heaters are 3/16" thick and have a rust-resistant steel sheath and two 3/4" high stainless steel stud and nut terminals with 10-32 threads. Mounting holes are 9/32" Lg. × 3/8" Wd.

A couple 3619K821, 3" x 6" are less than $30. They would provide 360W.

I am not affiliated with Mcmaster, just think these would suit your application.

It all starts with a mental vision.

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I think you have me sold there Kit, Would you have a look at this controller and tell me if it could run 2 of the specified 180 watt heaters at 120VAC?
Part No. 3215K76 PD control http://www.mcmaster.com/#7981k/=...

It is rated at 250VAC 3Amps
Two 180 watt heaters would be 3.13Amps at 115VAC
Not really sure the nominal value to use for line voltage in the standard U.S. home.

Question is if it is rated 3 amps at 250VAC would it be 6 amps at 125VAC?

My initial worry is a failure of an SSR controller I might build getting stuck on and getting too hot, I would think a cutoff switch even with an industrial controller would be in order but I am having trouble finding something like this (from China) (and properly fused of course)

http://www.ebay.com/itm/2-Pcs-25...

Mouser has nothing

Newark has these 125C
http://www.newark.com/microtherm...

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Sorry, I have been out. I would think the current rating is 3A regardless of voltage. Might work in your application with less lifetime if mechanical relay. With variance in heater resistance and line voltage 0.13mA is not much.

For cutoff I have used a simple dryer bi-metal cut-out NC switch. Also used in furnace bonnet control.They are available for various temperatures and are inexpensive. It might be possible to use as the temperature control in your application. http://www.devale.com/disc-therm... Also many others.

Most likely will need to add snubber to lower emi.

It all starts with a mental vision.