LM355 Replacement?

[Andy514452]

Hi everybody!

I'm looking for a part that is a direct replacement for an LM355 Temperature sensor. The reason we need to replace it is because it's too big, the part is fitted into an aluminum tube, which is 4.5mm in diameter and the SMT part is too big also(we've been using these for 10 years and now the tube has shrunk!!! and we are stuck with $10k worth!!

I've had a look and can find a couple of temperature sensors but they have different output characteristics. I was thinking of a SOT23 part fitted on a PCB but I'm really struggling to find one.

Can anybody else??

Thanks

Andy

 

[itsmoked]

You say the SMT part is too big too but that you'd like a SOT23.. An SMT part!

So what is the ID you're battling with?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Have you considered a chip-on-board or thick film hybrid solution? I'm not sure of the LM355 die geometry but I expect with a good layout you could get the size down far enough. Might work out pricey for small volumes but would be relatively cheap for high volume. Someone like TT-Welwyn could probably do it if they are still in business.

http://www.welwyn-tt.co.uk/products/microelectronics.asp

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[btrueblood]

Do you mean an LM35? I don't remember a 355 temp sensor...and searching the various supplier websites confirms.

In any case, presumably you are referring to the LM35 in TO-92 configuration, with its nominal .18 inch o.d. plastic shell (LM35D or similar): your options are to bore out the end of the tube, or rebore the whole tube, or create a lathe chuck to hold the leads of the sensor whilst machining away some of the plastic overmold, until the parts fit. Nobody makes an equivalent temp. sensor in a different package.

I've made tubular sensors using these or their sister (the LM34 sensor) before, and have always had to allow for variations in the actual package o.d. It looks like, from the National website, that the package underwent a redesign in 2000...funny that you didn't catch the change until now.

 

[Andy514452]

Yes I mean the LM35

The datasheet shows an S0-8 SMT package which is too big.

This tube we use is 4.5mm inner diameter and the length is 100mm, so a small pcb could fit in there, If there isn't a direct replacement then maybe a small op-amp and a transistor(the base emitter voltage reacts to temperature(I think)) will acheive what I need?????

Thanks for the replys people

 

[IRstuff]

I don't understand. National makes 4 other sensors, at least 2 of them in microSMD packages that are less than 1 mm across. The other 2 are SC70 packages with about 2.7 mm max footprint.

The LM35 can be purchased in die form.

TTFN

FAQ731-376

 

[Andy514452]

IR stuff,

These packages are for LM35?

Can you post the link please

Thanks

Andy

 

[Andy514452]

Just had a look on the National website and indeed there are other sensor, but none with the same temp response characteristics.

When you say die form what do you mean?

 

[IRstuff]

That means a bare chip. It's the actual circuitry on silicon sans packaging of any sort.

And, perhaps you should tell us what characteristics you actually need.

TTFN

FAQ731-376

 

[ScottyUK]

Bare dice are used with the technologies I mentioned earlier. It is likely to give you the physically smallest result solution, but cost will be volume-dependent. CoB will definitely be cheaper than a hybrid.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[btrueblood]

"Yes I mean the LM35"

Thought so.

"The datasheet shows an S0-8 SMT package which is too big."

Yes. So that package won't fit, never could fit, would be the completely wrong package to try and stuff into a 4.5 mm tube. ... But, the TO-92 package is also an option for the LM35 sensor, and that package has an o.d. not much bigger than .19 inches, or 4.8mm. When I said no other package is available, I meant that no package with a dimension smaller than the TO-92 is available for the LM35, and no equivalent sensor is available either, in any package.

"This tube we use is 4.5mm inner diameter and the length is 100mm, so a small pcb could fit in there, "<snip>

Yes. So, you bought a tube that's too small on the i.d. to fit the new TO-92 package dimensions for the LM-35. 4.8mm id for the tube would've worked nicely I'd bet. So, drill out the end of the tubes to .20 (4.9mm) i.d. for a depth of about .25 inches (6.4mm), solder leads > 100mm to the sensor, and feed the leads thru the tube (starting from the bored-out end), then stuff the sensor body into the bored section. Pot with sticky stuff of your choice, and march onwards. That's option #1, and what I would consider the cheapest option (but that's just my opinion). Option 2 is buy 4.9 mm i.d. tubing, and scrap the stuff you have. Option 3 is whittling the o.d. of the TO-92 package to fit 4.5mm i.d. tubing (expect to scrap a fair number of sensors that you cut into the underlying silicon). Option 4 is to buy bare die as suggested by others, and the production equipment to solder leads onto the tiny, tiny, lead pads on those bare chips, and hold the chips in place during potting/overmolding...

I once considered option #4, but the designer of the chip at National convinced me otherwise. If you don't believe me, call or write to Mr. Pease yourself, he's quite a nice gentleman and likes to support dumb geeks like us. He's also as nice/funny in person (via phone) as he is in his articles in the trade pubs.

Your choice.

 

[ScottyUK]

You don't solder to a bare die of that size, you need a ultrasonic wire bonding machine. Soldering is pretty much limited to bigger power devices, and even then only to one electrode. Conductive epoxy is sometimes used in lieu of solder. The electrodes on the top of the die still need a bonder. Bonders are expensive pieces of hardware and take a lot of setting up - I used to work on them a long time ago.

http://www.wirebonders.com/images/2460.jpg

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[IRstuff]

An alternative to classical wirebonds is TAB, which is more like a conventional lead frame, but the leads are bonded directly to the bond pads on the die. Still ultrasonic, with perhaps a bit of heat applied. The advantage is that the outer perimeter of the TAB structure can be directly soldered to a PC board.

Nonetheless, the OP hasn't really indicated why the other National temp sensors wouldn't work in his application. Given that the other devices require zero post-procurement work, I'd seriously revisit the requirements and see if there's a different way to skin that cat, rather than going into the wirebonding and/or hybridization business.

TTFN

FAQ731-376

 

[ScottyUK]

...a different way to skin that cat

Electric planer. Works every time.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[DHambley]

It seems that changing your device to a LM50 (SOT23 - 2.5mm x 3.0mm) would be the easiest way. change your sensing circuit to comply with the output of the LM50. Make your new PCB less than 4.5mm wide.

D.H.