DIYEFI.org Forum
http://forum.diyefi.org/

Thermo couple input conditioning
http://forum.diyefi.org/viewtopic.php?f=9&t=740
Page 2 of 5

Author:  mk e [ Thu Sep 03, 2009 8:35 pm ]
Post subject:  Re: Thermo couple input conditioning

Fred wrote:
That is great, but there is no point ignoring the fact that he is obviously talking about designing the circuit to read the otherwise useless sensor and continuing to talk about the sensor voltage. Knowingly talking at cross purposes does no one any good, wouldn't you say?

Fred.



I do agree and I was simply attempting to respond to this request :

jharvey wrote:
What is the temperature range we are looking to measure? Do we care about - temps? These thermo couplers are intended to be in the exhaust gasses right?

Can the brain ADC convert negative voltages? I suspect it can not. I'm assuming the ADC will range from near 0V up to near 5V. Is 10mV/C a good accuracy? I suspect it will suffice.

Author:  jharvey [ Fri Sep 04, 2009 12:46 pm ]
Post subject:  Re: Thermo couple input conditioning

Attached is a PDF of the thermo coupler schematic. It should allow for ADC readings, as well as signal a failed coupler. It's based off the datasheets fig 6 schematic.

Do we know the impedance of the ADC? Right now I'm attenuating the output with two 15K resistors. However, in reality the ADC is parallel with one of the 15K's which will cause that resistance to be lower than 15K. This will cause the impedance seen by the op-amp to drop under 30K.

Attachments:
thermo_couple.pdf [15.79 KiB]
Downloaded 191 times

Author:  MotoFab [ Mon Sep 07, 2009 12:44 pm ]
Post subject:  Re: Thermo couple input conditioning

If you can do with 4Hz conversion time, Maxim has a nice SPI enabled thermocouple converter.
MAX6675 http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149

12bit resolution, 0°-1024°C range, 3.3-5 volt Vcc, a little less $ than the Analog Devices part.

No external parts, save for the thermocouple and a decoupling cap.
Looks like a very handy item.

Author:  jharvey [ Mon Sep 07, 2009 1:06 pm ]
Post subject:  Re: Thermo couple input conditioning

I believe the AD chip has a refresh time close to 1KHz. So much faster I believe. Feel free to show me differently, I don't exactly know how fast it will refresh. I figure empirical data will show the real latency.

Author:  MotoFab [ Mon Sep 07, 2009 1:43 pm ]
Post subject:  Re: Thermo couple input conditioning

I'm sure the bandwidth of the analog part is much higher. Of course that may be moot depending how often the processor samples it. But if 4Hz will work, then the Maxim may become an option, especially if other parameters are taken into account.

The AD595 part needs another, higher, voltage supply (5V will only provide an output voltage equivalent to 500°C). Say you use 8 volts to make 800°C, the higher voltage output will need to be compressed to fit the range of the ADC.

The analog output requires more layout, and interconnect, attention due to noise susceptibility than a serial part which has the ADC on board.

I get it though, if you need the higher bandwidth then the AD595 or equivalent it is a required choice.

Author:  jharvey [ Mon Sep 07, 2009 3:03 pm ]
Post subject:  Re: Thermo couple input conditioning

Good points, I suspect the slower refresh rates should be fine. I don't know of a use for these other than checking for problems caused by manifold length, failing parts, ect. So I don't see why the and SPI interface wouldn't work, I know we need it for the knock detector, so lots of those components will already exist. I'll look into the data sheet a bit closer, and check availability ect. Thanks for the recommended chip.

Author:  jharvey [ Mon Sep 07, 2009 3:40 pm ]
Post subject:  Re: Thermo couple input conditioning

The availability might concern me a pinch. I see it from Newark, Farnell, and one other supplier. Farnell and Newark are really the same, so it's basically a 2 vendor item. I bet there has to be others just like this that are more commonly available. On a good note, both Maxim and Newark are fairly good sources. So I'd bet this chip will be obtainable in a couple years.

Also the $13 vs $5, that's an increase of $7 per cyl, which is kind of annoying. I'd bet you're on the right track, but I'd also bet there is a more common chip out there. Lets keep looking for a bit more, and see what happens. National semi seems to have quite a selection of chips, not sure if they have something quite like this, but they did pop up alot with octopart.com. I'll see if I can find some time to browse thier chips.

I need to go but was looking at the digitals listed here.

http://www.national.com/analog/tempsensors/products

Author:  Fred [ Mon Sep 07, 2009 6:24 pm ]
Post subject:  Re: Thermo couple input conditioning

Thanks Jim, nice find :-)

You have to wonder about the thermal mass of the sensor. I would imagine 4Hz would be fine for a totally optional sensor though.

Kudos for the metric units too! :-)

Fred.

Author:  mk e [ Mon Sep 07, 2009 6:51 pm ]
Post subject:  Re: Thermo couple input conditioning

MotoFab wrote:
I'm sure the bandwidth of the analog part is much higher. Of course that may be moot depending how often the processor samples it. But if 4Hz will work, then the Maxim may become an option, especially if other parameters are taken into account.



Temps don't change very fast so4 hz would be ok I think and if FreeEMS plans to use this design, the 5554 team is ok with it.

If this is a 5554 only circuit, I'd put in the faster chip and not give the sample rate another thought. Anything over 100hz and the temp will be sampled every time the cylinder fires which is the ideal situation.

Author:  jharvey [ Mon Sep 07, 2009 11:37 pm ]
Post subject:  Re: Thermo couple input conditioning

Looks like the two above mentioned chips are basically the chips that every one uses. As mentioned above, the AD chip is fast, and the max chip is digital. How about I draw it up with both, but we plan for the MAX chip to be the primary one. That way if someone wants a faster response, or is trying it on the cheap cheap, they can, but we'll focus on digital support first.

Any objections or thoughts? Some times I wonder if the PCB will fit under the hood. Hi hi. I'm sure it will fit just fine, perhaps with a bump in the hood, but it will fit.

Page 2 of 5 All times are UTC [ DST ]
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/