Reverse polarity, Scotty.

Hi all,

Because of a miscommunication with a client's layout person, I find myself in need of a series-connected dual Schottky diode in SC70 whose pinout is opposite to the BAV54SW, i.e. I need something like this:

*------* K ==| | | |== COM A ==| | *------*

It'll be used at 20 mA, 12V, no inductive kick. I can use the BAV99RW, but that'll cost me a volt's worth of drop, which I'd rather avoid. Any suggestions?

Thanks

Phil Hobbs

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Phil Hobbs
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If you can stand the overhead, put the BAV54SW on a little 'sub' PCB and stick that down.

Reply to
RobertMacy

On a sunny day (Mon, 09 Sep 2013 16:02:25 -0400) it happened Phil Hobbs wrote in :

Mount upside down?

Reply to
Jan Panteltje

Before time runs out too far I'd consider a re-layout. The only series Schottky I ever saw in an "oops" package was SOT23, and that was a while ago and an RF part that IIRC didn't take 12V.

To get product tested and maybe even into the field (via deviation) you could have them hand-soldered reversed for cathode/anode, sort of standing at a 45 degree angle, and then have COM handwired.

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Reply to
Joerg

Or make a little "daughter" board. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

an SC70 is 2x2mm if there's room for a daughter board there's room for two discrete diodes

simple fix is like Jan suggested; mount it upside down, might not even need to bend the legs, just be little generous with the solder

-Lasse

Reply to
Lasse Langwadt Christensen

If you have to make a board anyhow, why not fix the real McCoy instead? It's less effort.

But before that, tear off a page from the "anger pad", crumple, and chuck it into a corner of the room, with gusto :-)

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For some reason I've never seen those in the US. I remember when we told our mechanical engineer at my first company that we had a "slight" change in the mold for a plastic part and at that point he really needed this pad.

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Regards, Joerg 

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Reply to
Joerg

Might be possible, but the package is a lot taller above the leads than below.

There are reverse-polarity Schottky pairs, but the ones I've found are all 4 volt RF mixer/detector things.

The application is a super high-Z front end, where I need to bootstrap the contact-to-coil capacitance of a relay. The circuit is like this. K2 is single-coil latching, and switches the actual feedback resistors, while K1 engages disengages K2 for about 30 ms after a transition of either polarity. (The coils are both about 1.3k, so 650 ohms * 47 uF ~=

30 ms.) K1 is the wrong way up, so I need to be able to invert the polarity of D1 and D2 without a board spin. *------------*------------* | | | 0 *---* D1A --- | --- | | A | D2A A -12 --* *--- | . C + | *-----* . O K1 *------*---------0 From gate | | . ..I | | driver IC | --- . L - --- | | D1B A . | . D2B A | | | . | . | | | *----.-------*----.-------* | | . . | | . . | | . K1A .K1B | | /0 . 0\ | *----0/ + K2 - . \0----* | 0---*---COIL-------*---0 | | . | | | . | CCC *--.-RRRR--||--* CCC 47uF | . | | CCC . CCC GND CCC . CCC | . | *--.-----------*---< Bootstrap . . . 50 G total *------RRRRRRR-.--RRRRRRR------* | . | | . K2A | *--------------0\ 1G | | \0----RRRRRR--* | 0 | | |\ | | | | \ *---------------*----0 Vout 0----*-------| -\ | I_in | \ | | >-----------------*

... etc....

It's interesting that the contact capacitance of K2 is large enough (0.2 pF or so) that I have to short out the 1G resistor. If I'd let it float, its Johnson noise current would have dominated at high frequency, i.e. above a few hundred hertz. The capacitance itself is noiseless, so shorting out the 1G resistor allows the frequency response to be fixed in the second stage without hurting the noise.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Could you translate, please?

I agree that it's best to just bite the bullet and lay the board out anew. Sometimes it's worthwhile to build up a first article anyway, with lots of green wires (or upside-down components), but in the end it's best to just get it right.

(What is it about diodes that they appear to be cursed? When you're not getting boards back from the assembly house with the two-lead diodes in backwards, you're getting layouts back from he layout house with three- lead diodes all cockeyed).

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Tim Wescott 
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Reply to
Tim Wescott

Thanks, I didn't see any either. However, happily it looks like the BAV99RW will work okay. I may need to make the 47 uF cap a little bigger to compensate for the extra voltage drop, but oh well.

If there needs to be a spin for other reasons we'll go back to the BAT54Ses. Right now we have to figure out how to get the board clean enough. I really miss PbSn!

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

A RoHS board? My condolences. If it's any comfort I'll have one of those coming up in fall 2014. The bureaucrats in Brussels took away the exemption for medical. Hurumph. Grumble.

Make sure there is a liability waiver in your agreement for when the lead-free stuff fails in the field.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

ANGER SHEET .. Crumple in case of anger attack and chuck into a corner.

(in German a corner in this context means the corner of a room)

Yup.

I just had one with a 2-pad SMT diode reversed because the layouter scooted the "C" mark to far. And here I thought they'd always go by the CAD file. Oh well, happens.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

At a PPoE the layout guy was always getting diodes backwards. He ASSUMEd that pin AO1 was the cathode. "It's IPC standards." Unfortunately, not all diode manufacturers got the memo. We eventually solved the problem by using 'C' and 'A' for the library pin names (for arrays that becomes "C01" and "A01").

Reply to
krw

I've been using socialist solder processes for five years. After they got the paste/stencil and temperature profiles right, I haven't seen any problems. Nothing from the field.

Reply to
krw

Use schematic symbols that match the part... complete with the "dent" marking pin 1 ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

Five years in the field is not a long time. It also depends on how it is used, harsh environments, vibrations and so on. It's not just issues that can develop with IC packages, also with passives:

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

whatever..you need Style 10:

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not available in smallish 30V VR which I assume is what your after.

Reply to
bloggs.fredbloggs.fred

Make sure they solder it in a nitrogen atmosphere. The joints will look just as good as with leaded solder.

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Reply to
Nico Coesel

We might have to go with forming gas or hydrogen atmosphere and no flux, but I $ure hope not. They've got some really gruesome leakage and 1/f noise on the first articles. What I need is a Water Pik loaded with Freon-12. ;)

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

On a sunny day (Mon, 09 Sep 2013 17:15:01 -0400) it happened Phil Hobbs wrote in :

Usually you can bend the leads once without problems. drop of solder does the rest. Else, as somebody already pointed out, make small adapter boards.

Yes that is usually small signal stuff.

It is impressive....

30ms is not that fast, is there not a normal small bridge rectifier (no Schottky) that you could solder, I had some small round ones with wires coming out, 30V 50 Hz.
Reply to
Jan Panteltje

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