Measuring TempCo with cold? - Page 1
Forum section: electronics / beginners

author: Jebnor
author_rank: Regular Contributor
timestamp: July 16, 2013, 08:10:56 pm
content: I'm selecting my resistors for my Mini Metrology lab. In the instructions, Mr. Hoffman suggests measuring Tempco by using a warm bath of Mineral Oil. While messy, it would be quite effective. Here's the rub, I don't have mineral oil hanging around. I do however have air-duster which I can use upside down to cool the resistors under test. Since I'm looking for two resistors with the same lowest possible TempCo, would this work? Do resistors have a different TempCo going hot vs. going cold? Do I risk breaking the resistors? The datasheethttp://www.yageo.com/documents/recent/Leaded-R_MFR_2011.pdfsays the resistors have an operating range from -55C.

author: Smokey
author_rank: Super Contributor
timestamp: July 16, 2013, 08:28:10 pm
content: You probably want a uniform steady state temp to take measurements, which you won't get with a can of freeze spray.

author: ConKbot
author_rank: Super Contributor
timestamp: July 16, 2013, 09:18:37 pm
content: Quote from: Smokey on July 16, 2013, 08:28:10 pmYou will if you empty the whole can into a container and immerse the board under test into the liquid.On a related note, Ive seen a LED that was designed for 2.8A take 12A+ while it was on a MCPCB and submerged in a bath of freeze-spray/duster.

author: ddavidebor
author_rank: Super Contributor
timestamp: July 16, 2013, 09:38:11 pm
content: Quote from: Jebnor on July 16, 2013, 08:10:56 pmYou can use any oil, if you verify it's completely isolatingl

author: AlfBaz
author_rank: Super Contributor
timestamp: July 16, 2013, 10:14:33 pm
content: Quote from: Jebnor on July 16, 2013, 08:10:56 pmHi, I'm interested in what you are doing here and was wondering if you could point me to more information about this. I tried google without success

author: Jebnor
author_rank: Regular Contributor
timestamp: July 16, 2013, 10:41:06 pm
content: Quote from: AlfBaz on July 16, 2013, 10:14:33 pmMore info in another post I have. With links to the Articles.https://www.eevblog.com/forum/projects/mini-metrology-lab-build/

author: AlfBaz
author_rank: Super Contributor
timestamp: July 16, 2013, 11:02:43 pm
content: Thanks!With regards to spraying your resistors with coolant, I have recently been measuring resistor values versus temperature. Not having any means to control temperature I have been logging values over the coarse of a day's temperature variation and have found that as the temp increases so to does the resistance but there is a massive lag as temperature drops.I then applied heat from a heat gun expecting to see resistance climb dramatically but saw it drop substantially, almost as though it went from PTC to NTC. I haven't investigated further but one idea I had is that extreme temperature gradients may be causing thermoelectric voltages throwing readings off

author: Rerouter
author_rank: Super Contributor
timestamp: July 17, 2013, 02:37:05 am
content: but did it return to its normal value afterwards? otherwise you may have just slightly melted the film or windings depending on the type,

author: AlfBaz
author_rank: Super Contributor
timestamp: July 17, 2013, 03:06:40 am
content: Yes it did return to nominal value. By heat gun I meant hot air reflow station, which I dialled down and measured around 150deg C on my thermistor prior to heating the resistor.Good thought though and I'm open to any explanations

author: Jebnor
author_rank: Regular Contributor
timestamp: July 17, 2013, 04:06:49 am
content: I found some baby oil and placed some in a small glass dish in a hot water bath. The baby oil got to about 45C. In the end, I found two resistors that were dead-on ( as far as I can measure) matched to each other, and both had a zero change when going from room temperature (22C) to the oil at 44C. So I think I found a pretty good pair.Now on to final packaging.