Resistance measurement switching/multiplexing? For thermal testing - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: March 13, 2013, 03:08:10 pm
content: So if you read my previous posts you will renember that I performed experiments involving a styrofoam box, a heat lamp and a resistor in order to determine the TCR (temperature coefficent) of resistors.Is it feasible and cheap to somehow to multiplex 4 wire resistance measurement?The design I had in mind is a styrofoam box with heat lamps and circultion fan + temperature measurement. The idea beind that the heat lamps would heat air which would be circulated around the box using the fan. The temperature would be measured and a number of resistors would be in the box connected to 4 wire clips. This way TCR could be potentially measured for a large batch, rather then taking 30 minutes to catalog every resistor.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 13, 2013, 05:03:28 pm
content: Yes, it is quite common to multiplex 4-wire resistance measurements. There are a number of commercial solutions both in the form of generic banks of signal relays and also specifically designed for 4-wire resistance measurements. You can also build your own. The cheaper USB controlled relay boxes are actually not much more expensive than just buying the relays.Analog switches are cheaper, but harder to design with.You could also use a mechanical rotary switch.

author: robrenz
author_rank: Super Contributor
timestamp: March 13, 2013, 05:40:39 pm
content: With all the thermal emf stuff going on you better be using offset compensated ohms or current reversal techniques to get valid readings when measuring the lower resistances.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 13, 2013, 08:52:59 pm
content: Quote from: robrenz on March 13, 2013, 05:40:39 pmAt what resistance level do you think that alternative methods should be used? How would you implement the current reversal, using technology like ad7730?

author: robrenz
author_rank: Super Contributor
timestamp: March 13, 2013, 10:26:15 pm
content: Just glancing at the ad7730 data sheet, You could shut off the source current and use the tare function of the chip to zero out the thermal emf voltages then turn on the source current and read the voltage drop of the resistor. You would repeat that for every measurement. That is effectively offset compensated ohms.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 14, 2013, 02:03:48 am
content: Quote from: robrenz on March 13, 2013, 10:26:15 pmI was thinking along the lines of this implementation,http://www.analog.com/library/analogDialogue/archives/34-05/sensor/index.html, i never looked through the datasheetIt uses AC stimulation in order to cancel out noise. I wonder how well it would work for much lower resistances though, they specify a 100 ohm measurement (near bottom of page).I am interested in mass testing of resistances <10 ohms, I feel that in order to obtain useful results on low resistance measurement I would need to buy a meter similar to yours, I doubt a homebrew solution can beat it. I mean to work the multiplexing with a 34401a.I am starting to think that maybe a home made knife switch for this job. Something like variable air-capacitor with 4 plates which move between many plates. Readings from the multimeter could be taken into excel, perhaps triggered by a foot switch. I feel that maybe a electronic solution to this problem is a pointless endeavor as the majority of the time consumed by this procedure would be inserting the resistors into the apparatus and saving yourself the button pushing and switch rotation is kind of silly as it would take minutes off an hour.Or a slider of some sort, wooden rail system with the meter being connected to the slider which would consist of those ball thingies which are sometimes found on screw driver bits which would press into plates separated for each resistor.Do you have any ideas on a switch design like this which could be made of sheet metal and basic tools, you seem to be the expert here for these types of affairs. Since its 4 wire measurement then good contact resistance is unnecessary a non swiss-made solution should work.Or maybe this type of switch is commercially available for cheap? I feel like if I wanna test 50 resistors i gotta make my own thing.Right now I got something in my mind made of 4 strips of wood, sheet metal and 4 screw driver bits with wire soldered to them lol.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 14, 2013, 02:33:53 am
content: Maybe it could be mounted to one of those slide hinges that keyboard trays use so it does not operate like a piece of shit.the black thing on top is a screw driver bit btw. (I bet something better exists but its the only thing that comes to mind, you would know)a ball-version of those retracting probes that dave jones used in order to make a test jig for the board tester comes to mind but I don't know if their durable for this kind of motion.This design in my mind seems easier to implement then a "air variable capacitor type switch", or the titanic's propeller control.OR maybe the meter could be connected to a delicious cream filled waffle like clock hand and the conductors could be there instead of numbers. That actually sounds easier to design then a linear switch. A stepper motor could even control it so the process is automated.

author: robrenz
author_rank: Super Contributor
timestamp: March 14, 2013, 02:43:02 am
content: I would look for surplus rotary gang switch like the (SWR) MS20B2 part way downthis pagethat is a 4 pole 19 throw rotary switch. That would switch all 4 kelvin leads to each of 19 resistors.Are you looking for absolute accuracy or just matching of the resistor values and tempco?

author: ftransform
author_rank: Frequent Contributor
timestamp: March 14, 2013, 02:45:58 am
content: Quote from: robrenz on March 14, 2013, 02:43:02 am150$!!!~!!~!!!!! I wanna make this thing out of junk I have around.Do you think that a poor quality switch could cause problems despite it being a kelvin connection as you are asking that question?Absolute accuracy would be nice.Or I just realized that they are on ebay for like 10$.\I did not think they made such a thing commonly.What if I wanted something insane like 100 throws? Is making a huge ass switch from an old desk fan just not going to do the business here?The resistors could be spaced around the fan into groves melted into the frame *using a heated resistor assuming it could take the heat needed to melt the plastic* and then 2 of those plunger probes could be fixed to a fan blade. This could eliminate the need for 200 clips. Do you think reliable contact could be made in this case?

author: robrenz
author_rank: Super Contributor
timestamp: March 14, 2013, 03:00:36 am
content: The one I mentioned was $85.00 But I get your point. Even with all the equipment I have I would work some extra hours and buy the switch. If you are going to make your own like the design you showed I would think about using a circuit board and etching or dremeling the contact pattern and make a fingered slider like a linear pot uses to make contact with the circuit board pads.Absolute accuracy, nice or necessary? Good absolute accuracy below 10 ohms is very expensive.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 14, 2013, 03:02:29 am
content: I'm really digging the old fan idea... it seems perfect.. why go linear when the whole assembly is practically made complete with bearing? A "hot stamp" could be made (clay with the resistor embedded in it) for making the slots for the resistors. The fan I have has a inside which is like section cut out of a cone, not a cylinder, so when laid down flat the resistors could probably stay in their slots due to the tilt. I could probably do like 300 resistors on this thing lol.And screw the low resistance values. I don't want to deal with that shit.Do you think my design could offer good absolute accuracy and tempco for 50ohm + ?The whole fan could be put into a Styrofoam box with a crank that comes out of it.Something could be made to click in order to reassure you that proper contact has been made.does the idea of a repurposed house fan at least make you chuckle?and yes your idea for etching is really good you cannot beat the precision without some serious equipment/workI feel like a "ass shaped" plunger probe would be appropriate in this situation.