What are these long pads for? - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: October 08, 2015, 02:43:45 pm
content: Hi,http://datasheets.maximintegrated.com/en/ds/MAX6675EVKIT.pdfSee page 5. There are long round ended pads in the right picture of the pcb. You can see some of them labeled VIN, GND in the left one. I keep seeing these in layout examples. Are they just a wire solder point? Test point? I was looking at a max997 layout in its datasheet that had the same thing...ALSO - what type of connector do you need for a type K style thermocouple? Do you need one that has the right kind of metal for each side, or will a simple 2x1 100mil female type jack you can press the wires into close to the IC work well?Thanks!

author: tszaboo
author_rank: Super Contributor
timestamp: October 08, 2015, 03:13:10 pm
content: QuoteYes and yes.

author: katzohki
author_rank: Frequent Contributor
timestamp: October 08, 2015, 03:13:51 pm
content: Yeah, looks like those are wire points. They correspond with the schematic on page 4. You can use them to connect various signals into your circuit, or as test points. They're there for you, it's up to you how you wish to use them.K-style and thermocouples in general: It depends on what kind of accuracy you need. I've seen thermocouples wired directly into a board and that works OK for a sort of Hot/Not Hot measurement. If you're not familiar with thermocouples, they use two different metals that meet to form a thermal junction, where the measurement is made. Soldering directly into a board, or using a terminal jack will create two more thermal junctions at the points where the wires meet two more different metals (e.g. Constantine -> tin/lead solder). In theory the two additional thermal points will cancel each other out if they are close to each other (if I recall correctly), but with a reduced accuracy overall. There's also the "cold junction" temperature that you have to consider for accuracy, but I'm assuming Maxim's IC takes care of that internally.Looking at the datasheet for your evaluation board; it looks like they provide a place to plug in your thermocouple (J1). Thermocouples tend to use a standard kind of plug thing.Looking at the datasheet for the MAX6675, they've taken care of cold junction for you:https://www.maximintegrated.com/en/products/analog/sensors-and-sensor-interface/MAX6675.html/tb_tab0Maxim provides a handbook that should, among other things, give you a little more information about thermocouples and their use:http://pdfserv.maximintegrated.com/en/an/AN4679.pdfI think this eval board looks like it should be fairly plug and play!

author: alank2
author_rank: Super Contributor
timestamp: October 08, 2015, 03:22:15 pm
content: What is the point of a specialized connector with two types of metal when they both get connected to pcb tracks that are the same type of metal? Does it really make a difference?

author: katzohki
author_rank: Frequent Contributor
timestamp: October 08, 2015, 03:48:11 pm
content: For one thing the special connector is polarized. Besides that, having a standard connector helps with easy identification and use. (Remember that these are not always used by people that know about electronics, physics or can necessarily spell thermocouple).http://www.asrichards.com/products/temperature/Thermocouple_plugs_and_jacks.htmlHere's a couple more pages, don't know if they'll be helpful:http://www.pcbheaven.com/wikipages/How_Thermocouples_Work/http://www.nphheaters.com/technical/thermo_letter.htm

author: LukeW
author_rank: Frequent Contributor
timestamp: October 09, 2015, 06:45:15 am
content: Some of the fancy connectors from Omega etc allow the cold junction temperature sensor to be mounted right on there at the cold junction.http://au.omega.com/pptst/PCC-OST-SMP.htmlIt probably doesn't matter much if you use an ordinary terminal block or something, the extra junctions cancel out their thermoelectric potential.But you should have a layout where the cold junction sensor (or one of those all in one chips with cold junction compensator on board) is as close as possible to the cold junction so the cold junction temperature is as true as possible.