Small SCRs with 1mA or less in holding current?

[[email protected]]

Not quite

You need space in between and the landing area is supposed to be 1.3mm
long plus spacing to the next part.

But he said, "There are 1mm x 1mm 2-resistor networks. No leftovers.", which
is the point. Two-resistor 0402 arrays *are* 1mm x 1mm. No, I guess you
can't put them on a 1mmx1mm board. ;-)

Using many resistors is also an
issue in some apps because they are only rated 50V. Tough this is not an
aircraft project so we may be ok.

Depending on the circuit, voltage may not be an issue. You're being quite
secretive so it's pretty hard to help with the details. OTOH, I often get
specs like this. Then when things don't turn out as _they_ later decide they
should, such complaints aren't uncommon. ;-)

 

[Joerg]

But he said, "There are 1mm x 1mm 2-resistor networks. No leftovers.", which
is the point. Two-resistor 0402 arrays *are* 1mm x 1mm. No, I guess you
can't put them on a 1mmx1mm board. ;-)


Depending on the circuit, voltage may not be an issue. You're being quite
secretive so it's pretty hard to help with the details. OTOH, I often get
specs like this. Then when things don't turn out as _they_ later decide they
should, such complaints aren't uncommon. ;-)

Nothing secretive about this one. It has to go up to 60V and all this is
used to turn off a rail when the uC says so. The same rail which also
feeds this very uC

But it's fixed, I just use a discrete flip-flop. Was just wondering
whether anyone makes <1mA SCRs but I guess nobody does.

 

[[email protected]]

Nothing secretive about this one. It has to go up to 60V and all this is
used to turn off a rail when the uC says so. The same rail which also
feeds this very uC

I wanna know where (why?) you're getting 60V uCs. ;-)

 

[Spehro Pefhany]

Those would be nice. But I am afraid they won't want anything smaller
than 0402 in terms of pitch.

1mm x 1mm is 0404 = 2x 0402 side-by-side.


Best regards,
Spehro Pefhany

 

[Joerg]

I wanna know where (why?) you're getting 60V uCs. ;-)

Here it is

http://en.wikipedia.org/wiki/File:Z3_Deutsches_Museum.JPG

In my case the uC is, of course, behind a regulator. Along with a lot of
other stuff off of some more regulators. Under certain conditions all
this has to turn itself off. Overtemp and so on. Has to be really off,
as in no supply voltage to anything.

[...]

 

[Joerg]

1mm x 1mm is 0404 = 2x 0402 side-by-side.

Then they'd be shorted out ... bzzzt ... *PHUT*

 

[[email protected]]

Here it is

http://en.wikipedia.org/wiki/File:Z3_Deutsches_Museum.JPG

You have a funny idea of what constitutes 'u'. ;-)

In my case the uC is, of course, behind a regulator. Along with a lot of
other stuff off of some more regulators. Under certain conditions all
this has to turn itself off. Overtemp and so on. Has to be really off,
as in no supply voltage to anything.

PFET?

 

[Tim Williams]

MOSFETs with a gate zener clamp will take care of the 60V...

Yeah, but that's another resistor and, worse yet, a zener on top of that.
S'pose you could use the kind of FET with gate protection zeners (assuming
they're rated for DC), but that's still a resistor, which puts it on par
with the BJT version..

Tim

 

[Joerg]

You have a funny idea of what constitutes 'u'. ;-)

There is also a "new and improved" version with tubes. A German
university is still running one, at least until the last service guy
passes away I guess.

PFET?

Yup. A big one.

 

[Joerg]

can't just use a regulator with enable?

Not really. They all have an UVLO so that could be used but there is
also some other stuff connected. We want to keep this under uC control.
Of course, there would be other ways as well such as non-volatile
digital potmeters but that starts to eat into the available real estate.

The other issue is that switcher chips do not quite drop to zero power
in UVLO shutdown and I've got half a dozen.

 

[Spehro Pefhany]

Then they'd be shorted out ... bzzzt ... *PHUT*

Now you're just being silly.

http://media.digikey.com/pdf/Catalog Drawings/Resistors/EXB24V-dim1.jpg



Best regards,
Spehro Pefhany

 

[[email protected]]

Now you're just being silly.

http://media.digikey.com/pdf/Catalog Drawings/Resistors/EXB24V-dim1.jpg

I see you referenced a Panasonic resistor array. Has anyone noticed that
Panasonic resistors are more susceptible than normal to the end-caps falling
off when de-soldering. I never seemed to have that problem before and now it
seems I can't reuse any of them.

 

[Spehro Pefhany]

I ask again, what are the resistors for?
Please give us a schematic and approximate values and reasons for
each and every resistor.
Perhaps then you will discover the original SCR equivalent...no
resistors!

Given the specs on a typical transistor data sheet, predict the
worst-case leakage current (it will be at high temperature where both
leakage and beta are high) and whether it will turn on by itself.

If there's a series gate resistor to a CMOS gate you only need one
resistor (BE on the PNP) and maybe none if you can be sure the NPN has
low leakage and the PNP has lousy gain at low current.

Nice thing about SCRs made from BJTs is that you can make an NPNP one
as easily as a PNPN type.


Best regards,
Spehro Pefhany

 

[Joerg]

I ask again, what are the resistors for?
Please give us a schematic and approximate values and reasons for each
and every resistor.
Perhaps then you will discover the original SCR equivalent...no
resistors!

Maybe this explains it:

http://www.ecelab.com/circuit-transistor-scr.htm

I cannot rely on undocumented device parameters on this one. But as I
mentioned, I now have a solution sans SCR. Still would be nice to know
if there are very low current ones somewhere.

 

[Joerg]

Now you're just being silly.

http://media.digikey.com/pdf/Catalog Drawings/Resistors/EXB24V-dim1.jpg

What I meant is that you cannot place two 0402 side by side within a 1mm
by 1mm real estate. With arrays, yes, you can get there but IME then I
run into the same bottleneck as usual: Contract mfg balking because this
is more difficult than regular 0402 reflow soldering.

 

[[email protected]]

What I meant is that you cannot place two 0402 side by side within a 1mm
by 1mm real estate. With arrays, yes, you can get there but IME then I
run into the same bottleneck as usual: Contract mfg balking because this
is more difficult than regular 0402 reflow soldering.

How is a 0404 any harder to place than an 0402? The placement accuracy is the
same and an 0404 should be easier to pick up.

 

[Spehro Pefhany]

What I meant is that you cannot place two 0402 side by side within a 1mm
by 1mm real estate. With arrays, yes, you can get there but IME then I
run into the same bottleneck as usual: Contract mfg balking because this
is more difficult than regular 0402 reflow soldering.

I doubt that. If they can't handle 0.65mm pitch, they're lower-end
than any assembly house I've dealt with this millenium.

Seems more likely to me that they're worried about tombstoning on
smaller parts. I've even seen that on 0402s (about 0.05% of units
required rework in one bad production run). Arrays would be better
than 0402s, not worse. 0201s would definitely be worse, hence the
"rule".

Here's some good info on the issue:
http://tersted.home.xs4all.nl/PDF_files/Plexus/tombstoning.pdf



Best regards,
Spehro Pefhany

 

[Joerg]

How is a 0404 any harder to place than an 0402? The placement accuracy is the
same and an 0404 should be easier to pick up.

It's not the placement, it is the reflow process. That is more difficult
because of the finer pitch in the array. It can be done, as evidenced in
cell phones. But many lower qty contract mfgs don't like it or won't
take such a job.

 

[Joerg]

I doubt that. If they can't handle 0.65mm pitch, they're lower-end
than any assembly house I've dealt with this millenium.

Oh, I could probably cajole them into it somehow

Not on this job, but on others my experience was that domestic places
are often a lot more conservative in terms of what they will take on.
Versus places in Asia. Then of course the Asian places usually won't
touch anything that's less than 10k boards/month.

Seems more likely to me that they're worried about tombstoning on
smaller parts. I've even seen that on 0402s (about 0.05% of units
required rework in one bad production run). Arrays would be better
than 0402s, not worse. 0201s would definitely be worse, hence the
"rule".

Here's some good info on the issue:
http://tersted.home.xs4all.nl/PDF_files/Plexus/tombstoning.pdf

"Stonehenging", cool. Reminds me of when the Griswolds wrere there:

 

[[email protected]]

It's not the placement, it is the reflow process. That is more difficult
because of the finer pitch in the array. It can be done, as evidenced in
cell phones. But many lower qty contract mfgs don't like it or won't
take such a job.

Cell phones use far smaller devices than 0404s (more like 01005s). I still
don't believe that anyone who can do 0402s, can't do 0404s. Nope, not buying
that one.