beads and zero ohm resistors

My reply here does not answer your question. However, given the more general topic of "0 ohm resistors," I want to pass on some that I found.

An 0402 is less than 0.5 milli-Ohm and takes 20 A.

They are not inexpensive (cheap), especially the 0402.

I found some parts from Stackpole that I thought were "equivalent," but the more I looked at them, I thought the metallic Rohm might be different in important ways.

As noted, I measured a cheap 0805 0r resistor and got around 6 milliohms with a positive TC around 3000 PPM/K.

One of my guys measured a ferrite bead:

The background is that we are first-article testing a new resistor/RTD simulator box. The 5-to-50 ohm range was expected to have a positive tempco from some range-switching SSRs, and there's software correction for that, but the measured tempco is almost twice the expected value. I wanted to just frob the correction factor, but some people wanted to understand why. Turns out that about half the error is in PCB traces and the other half is in the ferrite bead. We could replace the bead with a zero-ohm jumper, but we may as well leave it in (EMI garlic-vampire mode) and correct for everything.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

And SMT capacitors can crack, too. For instance, try not to place them next to the edge of a board... Place them such that if the PCB is going to flex, the cap can flex the least.

SMT capacitor size seems to matter also, Smaller is usually better we find. 1206 is getting too big at times. At least for stiff ceramic types like X7R. Film is better maybe.

Then again, if you don't mind paying a bit more, there are these semi-flexible SMT caps we heard about but would rather not spend the extra money if possible.

All these things that are found out over time. We try to learn from past mistakes and experiences but sometimes we have to learn again.

boB

Hello John, ...

No.

Have a look at the video and the pics here:

to get an idea. I don't now how the produce the helix and how they fill the gaps with ferrite. Some are at least produced as multilayer components, just like you would build a multilayer transformer coil or antenne on a PCB.

Cheers

Robert

IPC recommends 1210 at the largest. I'll use up to 1812 (or is there any

1818ish?), but carefully.

They're not too expensive IIRC, being used in automotive quantities.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

There are X7Rs that are designed with some flex and others that are designed to open, rather than short.

Yes, in X7Rs, too. In some uses, we're required to put two caps in series so in case one shorts the magic smoke doesn't ruin our day. That's not cheap, either (4x the capacitor).

That's how the above edicts come about, too. Strike your thumb with a hammer and you tend to learn a lesson.

I just got a quote for a 1uF 25V cap for about a nickel. I think it was for a million a year but I'd have to check that. I didn't have too much time to look at it before I left yesterday.

I have an inrush problem with some of my high-voltage pulser designs, which have electrolytics to deliver pulses up to 20A. Large external capacitors are also allowed. In my prototypes I hold off the HV until 200ms after the low-V supplies settle. This means the switching MOSFET must dissipate a substantial amount of energy, 1/2 C V^2, or up to 120J for the onboard caps, and much more for any outboard caps.

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 Thanks, 
    - Win