Re: Power supply protection networks

On Wed, 17 Jun 2009 17:36:36 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 17 Jun 2009 18:47:19 -0500, krw <krw@xxxxxxxxxxxxxxxxx> wrote:

On Wed, 17 Jun 2009 13:45:36 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 17 Jun 2009 09:38:21 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Wed, 17 Jun 2009 08:17:13 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Tue, 16 Jun 2009 19:55:53 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:

[...]

You're probably right. (It's partly an exercise.) OTOH, after doing
some math to get the transzorb/polyfuse solution to work at moderate
overvoltage without the transzorb unsoldering itself, it's a bit more
subtle than it first appeared.

Well, at some point the user blew it up. I assume that any abusive
voltage will be applied suddenly and will (usually) open the polyfuse
before the transzorb melts. But it could be carefully teased to fry
stuff. That's improbable. Worst case, the transzorb shorts and
protects the expensive stuff downstream.

Or the polyfuse smokes out. I am not a great believer in those things
except for protecting control lines that can't realistically be
protected any other way.

The surface-mount polyfuses have low voltage ratings and they are
serious... a few volts over rating and they evaporate. The bigger
leaded parts are better.

The trip current of a surfmount polyfuse is also very layout
dependant; pcb traces/pours/vias dominate heat transfer to ambient.
2:1 at least.


One other comment about your circuit: it still has an all-silicon path
from the outside world into the guts. A good ESD zot could take out
the fet or the schottky. I like to avoid any all-silicon zap paths.
The polyfuse+transzorb+capacitor is rugged and is a first-level RFI
filter too.

Does it really cost a quarter to put a resistor on a board?

That's the number we use for purchasing/kitting/placement/inspection.
That really changes your perspective on design.

Why are so many companies still do kitting? It never made sense to me.

I had a hard time convincing companies to quit doing that. Even when I
ran a division and was da boss this proposition was met with great
skepticism. Then I lucked out, found and hired a production manager who
believed in Kanban and now there was more "convincing power". After we
ditched kitting, costs dropped quite noticeably and the skeptics became
less skeptic ...

How can you stuff a board without pulling the parts?

We have two semi-auto p-n-p machines with automated carousels that
present the correct parts to the operator. Each carousel bin has to be
filled with the correct parts. That's "kitting" to us. A limited
number of reel feeders are also available, for bypass caps and such.
For those, we can just pull the reel from stock, use it, and maybe put
it back.

If we send boards out to a contractor to stuff, again we have to
furnish the correct parts in about the correct quantities, in labeled
bagggies. Kits again.


Ok, that's not the typical meaning of kitting. The typical scenario that
I found is this:

Production wants to build 40 machines of type XYZ. So they start pulling
40 transformers, 40 fuse assemblies, 160 casters, and so on. When there
are only 38 transformers in the stock room a mid-size problem situation
arises because the kit order can't be completed. A gigantic waste of
time and inventory Dollars. What we changed that to is this:

Whenever a production guy feels his floor stock for certain parts is
uncomfortably low he or she is at liberty to get as many more from the
stock room as desired. The next step in our change process was that many
parts were no longer stored in the stock room, they went right from
loading dock to floor. To my surprise a lot of higher level folks felt a
bit uncomfortable with this idea. Until they saw that it worked.

We don't have "floor stock" except a carousel thing full of
nuts/bolts/washers, which are still technically in the stockroom.

The stock room is open and unguarded and is along one wall of the
production area. Anybody can grab a part if they need it (and fill out
a line on the signout form of course.) But each production batch has a
work order that says to build N assemblies, and we pull a kit for that
batch, and account for the parts used. We know at all times how many
parts are in each stock bin, fairly accurately, and we know what we
need to buy for anticipated production needs.

Out stock room is locked and only a few have access. We have to fill
out a req to get any parts. Yes, it's a PITA to find our admin to
fill out a req (she has all the charge numbers) to "buy" a $.0015 0603
resistor. If we're working after hours, it's time to go home.

Does it have a drop ceiling? I always found them to be effective.

No ceiling at all.

I can also ask my computer to show me the status of any part, where it
is used, what it costs, and usually see the datasheets. And I can
examine and price any BOM.

Our Admin can look at costs and inventory. I don't have access to
that system. All the rest of that information is available.

How can you pick parts without knowing how many are in stock, or what
they actually cost?

That's a problem, huh? I know pretty much what the parts cost that
I've specified. Other than that, it's a crap shoot.

I have worked for companies that lived in Kit Hell, where the
stockroom floor was covered with partially-complete kits that had
shortages, and parts were constantly being stolen from some incomplete
kits to try to finish other incomplete kits.

We only have one SMT line. When a board is run, they check out the
reels they need and return them to stock when they're done. "Kitting"
isn't required.

Our semi-auto machines allow about 10 reels, and a lot of loose parts
in carousel bins. And we may buy 25 or 50 parts at a time, not enough
to wrap around a reel.

Our pick-n-place machine has 135 "slots" (components are up to 5 or
six slots wide for an audio transformer), which limits our component
diversity to that per side. One board pushed that limit so I had to
go back and choose resistors more wisely.

Some parts, like bypass caps, they can pull off a reel. Most parts
have to be kitted into the carousel bins.

We have almost 5000 different parts in stock.

The issue is how many you stuff at once. We take full reels to the
floor for the board being run and return them after. Sometimes they
get intercepted for the next board, but that's really no difference.
Most inventory is serialized by pick-n-place.
.

Relevant Pages

  • Re: Power supply protection networks
    ... before the transzorb melts. ... For those, we can just pull the reel from stock, use it, and maybe put ... Kits again. ... Production wants to build 40 machines of type XYZ. ...
    (sci.electronics.design)
  • Re: Power supply protection networks
    ... before the transzorb melts. ... For those, we can just pull the reel from stock, use it, and maybe put ... Kits again. ... Production wants to build 40 machines of type XYZ. ...
    (sci.electronics.design)
  • Re: Power supply protection networks
    ... before the transzorb melts. ... For those, we can just pull the reel from stock, use it, and maybe put ... Kits again. ...
    (sci.electronics.design)
  • Re: Patience is a virtue...so is stubbornness! A GT500 at last. (kinda long)
    ... I would think 700-800 rwhp should be a snap with one of their kits. ... I think the wire controlled throttle can be adjusted through software to give much better response than the stock setup. ... unless you do serious rear end modifications anything over 550-600 rwhp will just go up in smoke so a KB blower should more than get you enough usable horsepower. ...
    (rec.autos.makers.ford.mustang)