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'[EE:] SMT vs. TO-92 temp sensors'
2004\08\18@210436 by Charles Craft

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buy.microchip.com/chart.aspx?branchID=2001&mid=11

Microchip temp sensors come in PDIP and SMT packages.

The "great when they work" Dallas/Maxim parts come in a TO-92 thru-hole package.

Is there a big difference in temp response for board mounted versus "up in the air" sensors.
Does the actual chip package dampen the response more than the PCB a SMT a part is attached to?

thanks
chuckc

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2004\08\18@211719 by Bob Axtell

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Actually, I was wondering the same thing... I have an application where
I need to measure product ambient temp.
It it more accurate if mounted on the PCB, or in the air? Did anybody do
that kind of testing before?

--Bob

Charles Craft wrote:

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2004\08\18@212340 by Martin Klingensmith

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At first thought, the TO-92 would be better because it is not thermally
linked to the board, until you realize that it IS, with 3 leads. I do
believe that the SMT part would be more strongly coupled to the thermal
mass of the PCB though. Experimentation would be necessary.
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http://nnytech.net/


Bob Axtell wrote:
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2004\08\18@213211 by Spehro Pefhany

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At 06:16 PM 8/18/2004 -0700, you wrote:
>Actually, I was wondering the same thing... I have an application where
>I need to measure product ambient temp.
>It it more accurate if mounted on the PCB, or in the air? Did anybody do
>that kind of testing before?

I think it depends on whether you want to measure the temperature of the
PCB or the temperature of the air...

Best regards,

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2004\08\18@222159 by Ken Pergola

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I saw this written somewhere, and have always gotten a kick out of it -- I
hope I did not butcher the saying too much:

"The only temperature that a temperature sensor measures is its own
temperature."

Sounds like it might have been a Bob Pease saying? I can't find the original
article at the moment where I remember reading this humorous and pointed
remark...

Best regards,

Ken Pergola

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2004\08\18@222406 by Robert Young

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All things being equal, both would give the same steady-state reading.  How
long it takes the sensor to reach steady-state and how it reacts to
temperature transients is another story.  The smaller the thermal mass, the
quicker it can respond but it may also make the sensor more sensitive to
self-heating issues.

It has been my experience that board mounted sensors tend to react a bit
slower due to the thermal mass of the circuit board.  Package type doesn't
matter except for the fact that some packages allow you to have more of the
package in thermal contact with the board than others.  If you want to
measure the temperature of an air stream and the air stream temperature can
change faster than the temperature of a hunk of copper and FR-4, then I'd
definitely want the sensor mounted up, off the board and in the air stream
(or water, oil bath, whatever).  I have found that it is generally easier to
mount TO-92 packaged sensors in odd places than SO-8 but if you are careful
you can wire up the SO-8 sensor then encapsulate it in a little bit of epoxy
and have a blob-on-wire device only a little bigger than a TO-92.  If you
use a SOT-23 you may end up with something smaller than a TO-92.

And as a side note, don't put your temperature sensor near other things that
you know to be heat sources.  For example, don't put it next to the linear
voltage regulator that is trying to dissipate 5W.  Also don't make your
wires long (inches, feet, miles depends on the sensor) and expect good
results either.

Robert Young
YR Consulting
RemoveMErwyoungTakeThisOuTspamieee.org
{Original Message removed}

2004\08\18@222407 by Dave VanHorn

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At 09:22 PM 8/18/2004, Ken Pergola wrote:

>I saw this written somewhere, and have always gotten a kick out of it -- I
>hope I did not butcher the saying too much:
>
>"The only temperature that a temperature sensor measures is its own
>temperature."
>
>Sounds like it might have been a Bob Pease saying? I can't find the original
>article at the moment where I remember reading this humorous and pointed
>remark...

It does sound like bob, and very much worth remembering!

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2004\08\18@231636 by Chetan Bhargava

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What kind of epoxy can be use to encapsulate the sensor?
Wouldn't epoxy make the temp transition slower?



On Wed, 18 Aug 2004 21:21:32 -0500, Robert Young <TakeThisOuTrwyoungEraseMEspamspam_OUTieee.org> wrote:
{Quote hidden}

> {Original Message removed}

2004\08\19@050448 by Alan B. Pearce

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>At first thought, the TO-92 would be better because
>it is not thermally linked to the board, until you
>realize that it IS, with 3 leads. I do believe that
>the SMT part would be more strongly coupled to the
>thermal mass of the PCB though. Experimentation would
>be necessary.

I guess it depends exactly what you want to measure.

1. The TO92 will measure mostly the ambient air temperature, as the leads
will have a reasonable thermal resistance, and the PCB itself would not
normally be getting warm. Mounted like a normal transistor on the PCB would
have minimal difference in functionality to an SMT one for most uses.

2. The SMT will measure an average (roughly) of the PCB and air
temperatures. If the PCB is being used as a heatsink for a hot component
such as a regulator or high dissipation chip (e.g. video generator or CPU
chip) than having an SMT sensor close by is probably good enough to see that
temperature stays within limits.

3. If a more accurate temperature sensing is required then a TO92 glued to
the sense point in question, with the leads then fed into nearby holes in
the PCB would be the way to go.

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2004\08\19@072827 by Gerhard Fiedler

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> What kind of epoxy can be use to encapsulate the sensor?
> Wouldn't epoxy make the temp transition slower?

I don't have any datasheets handy, but a lot of epoxy has a thermal
conductivity that's (much) better than (still) air. For example, if you
have problems getting heat off the board in a small enclosure, potting it
may be one way to help it -- especially if the enclosure is made of metal.

Of course, the thermal mass of the sensor increases. So depending on the
specific circumstances, the low pass may get slower (because of the
increased mass = C) or faster (because of a reduced thermal resistance = R)
-- especially if you glue it with the epoxy to the surface you want to
measure.

Gerhard

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2004\08\19@094551 by Rob Young

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From: "Chetan Bhargava" <cbhargavaEraseMEspam.....GMAIL.COM>
To: <EraseMEPICLISTspamMITVMA.MIT.EDU>
Sent: Wednesday, August 18, 2004 10:14 PM
Subject: Re: [EE:] SMT vs. TO-92 temp sensors


> What kind of epoxy can be use to encapsulate the sensor?
> Wouldn't epoxy make the temp transition slower?
>
>
---SNIP---
>
> Chetan Bhargava
> http://www.bhargavaz.net

I have used several different types of epoxy.  Depends on the environment
the sensor will be exposed to.  Pick one that won't come apart or
expand/contract so much the sensor is damaged.  MasterBond Inc makes all
kinds of good things.

And yes, it will slow the transition a bit.  But if you use just enough to
protect the sensor or bond it down to the target item (motor housing for
example) its affect will be minimized.  Also depends on the composition of
the epoxy.  Something with a similar compositon to the plastic used on the
IC case would probably be best.  I've got some that is a 2-part that is ment
for bonding temperature sensors but it has been so long since I used it I
don't remember where it came from, probably MasterBond.

Rob Young
RemoveMErwyoungEraseMEspamEraseMEieee.org

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2004\08\19@130753 by Chetan Bhargava

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Thanks,

I'll be working with TMP121 from TI. It is packaged in a SOT23
package. To make it fluid proof I'll try some epoxy on that.

Regards,

Chetan


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