electrolytic cans

I'm doing the synchro/resolver board and the top will be paved over with transformers. So a lot of parts will have to go on the bottom of the board. We usually space these boards off the the aluminum box with

0.25" PEMs.

This is the sort of box we'll use:

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The tallest parts will be electrolytic caps. So does it matter if the can touches the box?

I ohmed out a bunch of caps, both leads against the case. They seem to run in the roughly 10K to 100K range, wet lytics and polymers. I guess I should use longer PEMs.

I have a nice 1000uF 2.5V surface-mount polymer in stock, that is

0.31" high. So maybe use 0.375" PEMs. We could add some insulating tape if we get into trouble.
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John Larkin         Highland Technology, Inc 
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John Larkin
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doesn't matter if you put a sheet of plastic film down.

NT

Reply to
tabbypurr

torsdag den 20. september 2018 kl. 23.51.41 UTC+2 skrev John Larkin:

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Reply to
Lasse Langwadt Christensen

Nice packaging. I like the way the locking ears on the BNC connectors are visible when the connector is inserted so one can see if they're securely locked in place. Also, methinks you could have spelled out ERR -> ERROR, PWR -> POWER, and BIST -> SELF TEST.

Probably doesn't matter, but if you want to be safe, some kind of additional insulation is easy enough. Either clear shrink tubing over the capacitor can, or a sheet of thin plastic glued to the inside of the cover. I've seen the vinyl insulation on big electrolytics get abraided where it hits the cover in a mobile (vibration) environment.

The end of the electrolytic with the stripe is the outside foil and usually (not always) the negative terminal. If the negative terminal is grounded to the case, it doesn't matter if it hits. If the negative is isolated from the case, it's probably worth adding some additional insulation to avoid surprises.

Tape falls off when it gets too hot.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

What about using a blob of that white goop they often use to mechanicalyl secure inductors and caps on PSU boards? It provides some mechanical reinforcement but it's not super-adhesive, shouldn't prevent removing the board if it needs to be serviced.

Reply to
bitrex

AFAICT the mark is always on the negative terminal except with tantalums. Are there aluminum electros that are marked the other way?

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Somehow I can't see JL and Co. using hot snot.

Cheers

Phil Hobbs

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Phil Hobbs

Not if the box has a bit of fish paper inserted between... as long as you're using surface mount, the capacitor height is controlled, and even if it DID touch the paper, it'd only do some capacitive coupling. Best plan would be to have the high parts near the standoffs, because the case (and board) can flex slightly.

One of the worst kludges I ever saw, was a commercial gizmo that tried to mount screw-top capacitors to a circuit board, and wave-soldered the screw head side. Also, they populated the capacitor footprint with components. Power diode leads in contact with the acetate wrap on the capacitor melted through, and when I saw it, there was melted-copper spray around those spots.

Maybe they DID have enough clearance with the screw-top posts on the capacitor, but there was no tolerance when the solder-wave hit the loose diodes.

Reply to
whit3rd

Well la di dah!

Reply to
bitrex

No, both the top and the bottom of the box need to be easily removable for test access.

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John Larkin         Highland Technology, Inc 
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John Larkin

But the aluminum cans seem to be somewhere in-between the + and - terminals.

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John Larkin         Highland Technology, Inc 
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John Larkin

They don't make cheap stereos, you see. ;)

(Neither do we.)

Cheers

Phil Hobbs

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Phil Hobbs

What's a stereo

Reply to
bitrex

Something for people who listen. ;)

Cheers

Phil Hobbs

Reply to
pcdhobbs

Todays aluminum capacitors have a stripe marking both the outside foil and the negative terminal. There's usually a negative bar somewhere near the stripe denoting the negative terminal. However, it wasn't always that way:

"Are Your Capacitors Installed Backwards? Build this and find out" (49:35)

"Polarity of foil capacitors"

I've also seen some confusion on NP (non-polarized) electrolytics that have no polarity markers, but sometimes have a stripe to mark the outside foil.

Of course, I can usually find a way to get it wrong: Normally, the PCB silk screen marks the negative stripe for electrolytics. However, that's not the way Dell did it on the Optiplex GX520. Despite the nearby + sign on the PCB, I managed to install 4 caps backwards.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

John Larkin showed off

The question is now 'what does that do?'

Would not do that for 3 reasons:

1) there is sometimes some conduction to the case. 2) some electrolytic caps do swell up when going bad and may push your PCB up. 3) tolerances?
Reply to
<698839253X6D445TD

That box is an 8-channel arbitrary waveform generator. It plays back waveforms acquired from running jet engines, to test FADECs and such. We call it The Wayback Machine.

This was a figure in our original proposal to a giant aerospace company:

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That was my original question, what is the case connected to? The answer seems to be "some sort of conductive goop inside."

That's not a concern. There won't be a lot of power available.

Sure.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

John Larkin wrote

There are connectors on the back to tell it what sort of wave form to output I suppose?

eeh they would not 'appen to build some F35 ?

Snoopy, I have some nice pictures of that dog...

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OK, that brings me to an other point, Johnny

Johnny is doing some training to stay in condition for his job. If he sees a connector like those BNCs, then he will use his full strength to tighten it (few hundred Newton at least).

I sure hope your BNCs are on a at least 5 mm thick front plate hidden behind that front, and not those solder on the board types. Also I do not quite see, but that could be arranged on the back panel, a good connection between the case and the BNC grounds.

?
Reply to
<698839253X6D445TD

Ethernet and USB. The user loads waveforms that he acquired wayback in time. The box can trim amplitudes and phases and offset and mix/modulate waveforms for more fun. The DACs are clocked at a constant high rate, and we interpolate between user data points. That works really well; no DAC steps are visible.

Parts of F35s.

There is a PC board behind the panel, and the connectors are soldered to it. Nobody has broken any so far. They make Corvettes out of fiberglass.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

"Outside foil" is the aluminum can!

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John Larkin         Highland Technology, Inc 
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