Author Topic: Measuring TempCo with cold?  (Read 2377 times)

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Offline Jebnor

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Measuring TempCo with cold?
« on: July 17, 2013, 06:10:56 AM »
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 datasheet http://www.yageo.com/documents/recent/Leaded-R_MFR_2011.pdf says the resistors have an operating range from -55C.
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Offline Smokey

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Re: Measuring TempCo with cold?
« Reply #1 on: July 17, 2013, 06:28:10 AM »
You probably want a uniform steady state temp to take measurements, which you won't get with a can of freeze spray.
 

Offline ConKbot

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Re: Measuring TempCo with cold?
« Reply #2 on: July 17, 2013, 07:18:37 AM »
You probably want a uniform steady state temp to take measurements, which you won't get with a can of freeze spray.

You 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.  :o
 

Offline ddavidebor

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Measuring TempCo with cold?
« Reply #3 on: July 17, 2013, 07:38:11 AM »
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 datasheet http://www.yageo.com/documents/recent/Leaded-R_MFR_2011.pdf says the resistors have an operating range from -55C.

You can use any oil, if you verify it's completely isolatingl
Davide Bortolami,
Fermium LABS srl
 

Offline AlfBaz

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Re: Measuring TempCo with cold?
« Reply #4 on: July 17, 2013, 08:14:33 AM »
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
Hi, 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
 

Offline Jebnor

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Re: Measuring TempCo with cold?
« Reply #5 on: July 17, 2013, 08:41:06 AM »
Hi, 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

More info in another post I have.  With links to the Articles. http://www.eevblog.com/forum/projects/mini-metrology-lab-build/
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Offline AlfBaz

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Re: Measuring TempCo with cold?
« Reply #6 on: July 17, 2013, 09:02:43 AM »
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  :-//
 

Offline Rerouter

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Re: Measuring TempCo with cold?
« Reply #7 on: July 17, 2013, 12:37:05 PM »
but did it return to its normal value afterwards? otherwise you may have just slightly melted the film or windings depending on the type,
 

Offline AlfBaz

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Re: Measuring TempCo with cold?
« Reply #8 on: July 17, 2013, 01:06:40 PM »
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
 

Offline Jebnor

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Re: Measuring TempCo with cold?
« Reply #9 on: July 17, 2013, 02:06:49 PM »
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.
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