On Friday 14 October 2016 14:45:31 Chris Albertson wrote:

> Typically 0.1 bypass caps REALLY to have to be physically close to the
> load they are bypassing. Within a few millimeters.  Cut the leads
> short and bridge the power pins with the cap.
>
> Placing a larger cap farther away can't cause any harm but it is not
> going to do the same job.
>
> Reason:  The real world is not the same as theory.   All caps have an
> internal series resistance (ESR). Think of them as a cap and resister
> in series and model them that way too when you design the circuit.
> The  ESR of a .1uF ceramic cap is nearly zero, but for the tantalum,
> you need to look it up in the data sheet.
>
> Why physically close to the load.  Wire has non-zero impedance.
> Mainly in this case it is inductive.   You need to minimize the value
> of the inductor between the 0.1 cap and the device.  Apparently the
> manufacturer wants a low impedance power supply.
>
> Draw the circuit schematic with a the lead wire inductance and cap's
> ESR shown then you can see that needs to be minimized.  Don't worry
> about space you candy 0.1 caps that are the size of a large grain of
> sand

Anyplace of note to source them?
>
> The usual way to mount these is to make a tiny PCB that has the device
> on to and a mounting hole and the passive parts like the capacitor(s)
> and a connector for the cable.
>
> All that said, it will likely work OK if you can get that small ca
> reasonably close.   Better to get the 0.1 closer then a larger one
> farther way as the goal is to minimize the amount of wire
>
> And using a larger cap is no substitute for the .01 either because I
> bet the .1 was spec'd mainly for its low ESR, not because it needed to
> be exactly 0.1uF
>
As a CET, I am well aware of all that, including the inductance of 2" of 
wire.

As for the ESR, tantalums are generally sub ohmic too.  For half a 
century or more. Its reverse bias that nails them to the fence, even a 
few millivolts will do it eventually.

Any caps on the leads will have to be not longer than the 667 is wide, 
and not more than half its thickness.

Thanks Chris.

> On Fri, Oct 14, 2016 at 9:51 AM, Gene Heskett <ghesk...@shentel.net> 
wrote:
> > Greetings everybody;
> >
> > A bit too cold this morning to go out to the shop and saw out the
> > thicker spacers I am going to need to permanently mount the Z motor
> > and screw anchor.
> >
> > So, not wanting to waste the day waiting for it to warm, I went back
> > to work on the encoder thing this morning.
> >
> > I now have an additional tab of alu extending to the left to hold
> > one of these ATS667's as an index pulse generator, and have that one
> > s-glued in a bit beyond where the center of the glued on screw
> > passes by, hoping it won't trigger on a regular tooth.
> >
> > But, I've never used any of these before.  No clue as to their bad
> > habits.
> >
> > The spec sheet I have shows a "required" .1 bypass to ground on the
> > VCC lead, but has anyone had any problems using just one for all 3
> > devices? This thing is cramped for space for a bulky mylar .1.
> >
> > So I intend to put an even larger tantalum on the pcb, which will be
> > on the left face of this added tab as I can let it project inward
> > nearly 20 mm's in that space, lots of space before encountering
> > anything that moves there.  That means I'll have to paste some
> > wrapping wire to extend the leads about 1" to get to the pcb.  So
> > this bypass cap may be as much as 2" of wire away from the device
> > body.
> >
> > Can anyone testify that the extra lead length is a problem?
> >
> > Thanks all.
> >
> > Cheers, Gene Heskett
> > --
> > "There are four boxes to be used in defense of liberty:
> >  soap, ballot, jury, and ammo. Please use in that order."
> > -Ed Howdershelt (Author)
> > Genes Web page <http://geneslinuxbox.net:6309/gene>
> >
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Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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