Nubie needs consultation
KenG
Posts: 4
I don't know very much about electronics but I want to make a control device for my wife's kiln. I have attached a PDF diagram of my plan and would appreciate a more knowledgeable person (or people) to look it over and see if it is workable. I borrowed from plans on the Parallax site, the Maxim site, Terry Allen (another forum member) and various other places. I do have a few specific questions.
1) Do I need another 0.1uF cap across pins 14 and 16 of the Stamp?
2) Do I need one across pins 3 and 4 of the Solid State Relay? BTW, I have seen plans for building a relay from components but I already have the SSR.
3) Do I need a diode at pin 3 of the SSR?
4) I have the K thermocouple. It is attached to a dial pyrometer. The bi-metal part is about 7" long and is connected to a ceramic block. There is a 3 ft. wire connected from the block to the Pyrometer. Is the wire part of the thermocouple? If so, does the length affect the accuracy of the reading?
Thanks, Ken
Post Edited (KenG) : 1/20/2009 4:22:13 PM GMT
1) Do I need another 0.1uF cap across pins 14 and 16 of the Stamp?
2) Do I need one across pins 3 and 4 of the Solid State Relay? BTW, I have seen plans for building a relay from components but I already have the SSR.
3) Do I need a diode at pin 3 of the SSR?
4) I have the K thermocouple. It is attached to a dial pyrometer. The bi-metal part is about 7" long and is connected to a ceramic block. There is a 3 ft. wire connected from the block to the Pyrometer. Is the wire part of the thermocouple? If so, does the length affect the accuracy of the reading?
Thanks, Ken
Post Edited (KenG) : 1/20/2009 4:22:13 PM GMT
Comments
1) No, since the BASIC Stamp is a hybrid module it already has an on-board capacitor.
2) No.
3) No, solid-state relays do not generate back EMF. Driving them from an I/O pin is the same as driving an LED.
4) I don’t have enough information (can’t see what you have) to give an answer on this.
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Chris Savage
Parallax Engineering
Here is a photo of the thermocouple as promised.· The yellow coil of #20 chromel-alumel wire is 3 feet long.· It connects to the thermocouple with a ceramic screw block.· The part that goes into the kiln is 7" long and except for the tip, is protected by sections of ceramic tubing.
The wire connects to the back if the pyrometer.·Between the wire connections and the actual pyrometer connections are 2 resistors.· The one on the positive side is 8.2 ohms ant the other is 6.8 ohms.
Will I need resistors between the wire and the MAX6675?
Are these resistor values affected by the length of the wire?· If so, how is that calculated?
Thanks for the pictures…it definitely paints a clearer picture. Thermocouples use two different wire types depending on the type of thermocouple. Length can affect the values to some degree, but the junction is where the two wires come together at the opposite end. The voltage·generated across the junction varies with temperature. In your case it looks as though yours forms a sort of ‘probe’ that has a ceramic retainer. It almost looks like the ‘probe’ has five ceramic insulators over it as well.
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Chris Savage
Parallax Engineering
Post Edited (Chris Savage (Parallax)) : 1/21/2009 7:17:03 PM GMT
…remind me not to follow up on the thread measuring temperature using an LM34 now…I’ll have that acting like a thermocouple in no time.
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Chris Savage
Parallax Engineering
I have gone thru a similar thought process but have not gotten to the 'real' phase. It will be fun to see how yours turns out.
After you get your wife's kiln control device operational, you can test it to see how accurate it is with the old fashioned cones. It will be interesting to see what degree (pun) of accuracy you can get.
Mike2545
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Chris Savage
Parallax Engineering
I already have the MAX6675 which takes input from a type K thermocouple so I don't need your module for this project.· The answer I was hoping for was that the resistors with the analog dial pyrometer are unique to that device and are not needed with the MAX6675.
I think I will take Mike2545's suggestion and get kiln cones to test the results of my device when I get it built.· I will try it without resistors.
Mike 2545, I will send you a message when I get this thing working. Please be patient.· I am new at this electronics stuff and I may screw up things a few times before I get it right.
KenG
Thermocouples use the same wire as they are made of as the extension wire back to whatever device they finally attach to.
When a thermocouple gets connected to something else, either the terminals of a pyrometer, or a length of ordinary copper wire, for example, the junction of the tc wire and whatever it connects to forms a secondary juction that either adds to, or subtracts from, the millivoltage that the tc produces at the juction reading the temperature that you want measure. This causes error in the reading of what it is the tc is trying to measure.
Those resistors are there to bridge out that error introduced where the tc wire connects to non tc wire.· This is referred to as "cold junction compensation".
The MAX6675 does the cold juction compensation, so you won't need them.
Be sure to read the datasheet: ·http://datasheets.maxim-ic.com/en/ds/MAX6675.pdf· for a lot of additional information. Page 4 gives you the details on what you're wondering about.
Darlene
·this is an amplifier with cold junction compensation (corrects for the error induced by the secondary voltage produced by the connection of the thermocouple to the measuring device).the 595 produces10mv /degree c.If the 595 is provided with a 15 volt differential it can read the output of a type K thermocouple to 2400F
which is where they start to fail. Its output then must be measured by analog to digial converter. im now using a ltc1298 which is a 12 bit job that gives you about a .5 degee resolution if you can dampen the noise in the ad.I have this working well and can output the data to stamp plot to graph it. im still stuck in the control software if you just want to set a temp limit its a piece of cake but im try ing to make a ramp controler so time comes into play. the idea is to set a max number of degrees/hr·from one·setpoint to another·and have several different ramps and holding periods(for crystal glazes)going up and going down in the firing. first i tryed counting and pause statments,·no good ,now i have a ds1302 real time clock on board.
·because of timing issues now im thinking propeller· Ill report back later· Jim
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Thank you for the explanation about resistors, that will make my project easier (and me a little smarter). I also downloaded the MAX6675 datasheet.
Jim Fox,
I knew about the AD595 before I learned about the MAX6675. It looked more complicated than I thought I was capable of doing and I had also not yet heard of the Basic Stamp, so I set the project aside for several months. A friend who was in the market for a kiln re-sparked my interest and as a result, I learned about the BS and 6675. I know the 6675 is only good to 1024 C (1875 F). That is not a problem for me because our kiln is small and will be used for enameling on copper, PMC clay and maybe some small glass fusing projects; all of which work at or below 1500F (816C).
KenG
all this tech is great but for the last 20 years all ive been using for pyrometer is a $5 digital volt meter(harbor frieght) connected directly to a type k thermocouple
that works great out puting about 50 mv at 2250 Understanding that error is the difference between 0degrees C and ambient temp.there are charts to convert to temp if its really necessary .I have a fancy digital ramp hold controller that i use for crystal glazes that works great but i allways use cones as a backup because
as the thermocouples age or if it gets damaged the readings decline and temp run away Jim