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Basic Electronics » Thermocouples, relays, and fun!!!

February 15, 2011
by jasongillikin
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Almost finished with my project. Now, I just need to find the right thermocouple and relay for the job.

I don't think the LM34 is the right sensor for my project (solar differential controller) for the following reasons

1) length of wire needed (sometimes up to 100 ft)----

2) number of wires needed (LM34 is 3 conductor, where thermocouples are 2) less waste is better

so this leads me to my question... where do i find a good thermocouple to use. I want one that already has the compensation for not having a constant reference such as 0degC or 100degC... can't remember what its called, all I can think of is cold junction (don't know if thats right or not been a long time)

okay... on to problem number two. RELAYS

I need to run two 12V .5A (under load) motors... so I need a 12V 1A relay. anyone have any suggestions? and how far does it need to be from the ATMEGA so there is no EM interference.

also, I imagine I would need a cap across the +/- of the motors to cut down on current spikes when they turn on and off. how do you calculate the capacitance needed?

and Finally, Since I am hooking this up to a solar panel to charge a 12V battery, do I need to put some bypass diodes?

thanks to everyone for being most helpful

Jason

February 15, 2011
by 6ofhalfdozen
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Sounds like you are getting close with your project, congrats!

I work with thermocouples all the time and plan on putting a few into my upcoming NK projects. That being said, here are a couple thoughts. To the best of my knowledge, there are currently no TC's on the market that have build in cold-compensation on the TC itself. Most converters, interfaces, or hookups use cold compensation on the ADC or yC software side. you can use some of the commonly availible chips like the Maxim6675 or Analog devices AD595 chips which convert K-type TC signals to an 10mV/CorF output "similar" to the LM34 which can be sent to the NK 10bit ADC. I have a couple of the AD595/596's that I use and like, but I am a bit more obsessive-compulsive than that so I am working on doing a 24bit ADC and LM34 for cold-compensation seperately and sending digital to the NK. (but that is still a little ways off.. ) There a couple places that sell TC to mV converters as plug in modules but most are >$100 per channel, so not an option for me.

For most everything I use or work with, I end up using either bare ended J/K-type TC's or MGOinsulated-ungrounded K-type TC's. so keep that in mind with the suppliers lists. thermocouple suppliers:

  1. Omega Engineering - good place, the "standard" most people trust and use, but usually a decent bit more than most places.

  2. McMasterCarr - good place if you want semi-standard off-the-shelf models, decent if a tad higher prices.

  3. Cleveland Electric Labs.. my personal preference. their part configurator is a little wierd, but their prices are usually lower than many places, their TC's are pretty rugged and they can make almost any thermocouple, though they don't do much with Type E.

  4. misc robotics places do sell small short corded TC's for robots and quick test setups, but their prices are usually higher than all of the above by a decent bit.

I am not much help for the relay info, but hopefully that points you in the right direction.

February 15, 2011
by mongo
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Check this little gem out from Analog Devices. The AD-590.

It is a very accurate temperature sensor and because it is a current source, it can use pretty long wires. We use them in our instrumentation systems at work and I have been using them for a good 25 years or so now.

Very simple operation. Simply connect the + lead to a power source around 5 volts or so. Connect the - lead to a 250 ohm resistor and the other side of the resistor to ground. Measure the voltage at the resistor and you get 1uA per degree C, staring at -373.15. So, if you get a voltage across the 250 ohm resistor of .75V, you have a 300uA draw. (3mA) That would equate to 300 - 273, or 27 degrees C. If you want to go with Fahrenheit, just do the math. Or go through an instrumentation amp with the gain set to .555 to get the output. There is a little to do with setting the zero and stuff because of that pesky 32 degree thing but it is really a simple circuit.

February 17, 2011
by jasongillikin
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Thanks for everyones help, I have one question about these two suggestions I got. while reading the data sheet, I notice they have a max temp of 50degC (AD595) and 125degC (AD590).

I was wondering if thats the ambient case temperature of the actual component or the temperature of what I am mesuring? I hope its the case temp, b/c thats awful low if TC's can measure upto 1200 degC.

Thanks again for everyones help.

I Thought I found a relay at radioshack (but it requires 9v across the coil) ATMEGA only has 5V, so back to the hunt

Jason

February 17, 2011
by devinsbusiness
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Hi jasongillikin, You might check out these relays. http://www.mouser.com/ProductDetail/Coto-Technology/9007-05-01/?qs=93uzuGORGqeEb2jSXpLSHQ%3d%3d. I used them with good success to drive 12V @ almost 1A to a solenoid and they only require <5v to drive the coil. I will say that the thing did finally stop working, but only after some pretty heavy use. I can't remember, you could look at the datasheet, but I think they also have a built in quenching diode also. Good Luck Devin

February 17, 2011
by mongo
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1200 deg C? Yikes!

I maintain a thermal oxidizer that runs 1600 deg F. But 1200C is darn near 2200F.

Thermocouples are about the only thing that can go that high.

There are modules that read T/C junctions and translate them to mV outputs that can be used. Check Omega.com for these and compatible T/C's.

February 18, 2011
by jasongillikin
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@Devins - Thanks for the link to the relays. they are super cheap, hope they hold up for a couple years at least.

@ Mongo - yeah, put the wrong temp down, eitherway, I just wanted to make sure the AD590 could go over 125 degC (yes Im sure this time :)

thanks again everyone for their help, hopefully everything will be up and running next week... first NON-KIT included project, glad to see its finally comming together

March 15, 2011
by jasongillikin
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OK, so I hooked all my components up on the breadboard and everything worked great. using the AD590 temp sensors. Now, I transfered the project to a PCB and nothing works...

for my purposes, I don't need the LCD screen, the serial connection, programming switch, so I didn't transfer them. are they required for opperation of the microcontroller???

any ideas? I checked my solder joints and they all look good, and all diodes are in correct bias.

any advice would be great

March 16, 2011
by Rick_S
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Photo's ... It's very hard to diagnose what you can't see.

Rick

March 16, 2011
by mongo
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Make sure the AD590 is wired correctly. They don't tolerate much in reverse. Are you driving a resistor with it. (I use a 250 ohm resistor and read the voltage from the junction between the two components).

The LCD and programming switch shouldn't have any effect.

The one thing that many people get mixed up on are the pins of a chip. Make sure they are all the right ones and connected to the right places. Some times, they can be offset by one in either direction.

March 16, 2011
by jasongillikin
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I just decided to rebuild the unit... and found out the problem was in the relay... I had the coil hooked up backwards... i didn't know there was a bias on the relay coil... diagram doesn't say anything about it.

anyway, problem solved... thanks for the help

March 16, 2011
by mongo
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Some relays have a built-in clamping diode. If it is backwards, it will look like a short circuit.

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