On Monday 01 March 2010, Erik Christiansen wrote:
>On Sun, Feb 28, 2010 at 09:42:55PM -0500, Gene Heskett wrote:
>> I can't argue that Steve, having blown one.  And that is the reason the
>> output filter cap in my linear motor power supply is 75,000 uf.  Yes siree
>> Bob, gobble up them over voltage spikes. ;-)
>>
>> And because of the limited out cap size in switcher power supplies, they
>> are bound to be trouble.  The simpler the better, but build it out of
>> bulletproof stuff, pure analog.
>
>A transorb, with lower voltage tolerance a bit above the top steady
>state supply voltage, and upper tolerance a bit below the maximum
>permissible chip voltage, is a great comfort too. It might admittedly
>necessitate an additional couple of volts headroom, but even 75,000 uf
>needs headroom to absorb longish high energy spikes.
>
>A robust motor driver would have one across the supply, as a minimum, I
>suggest.
>
But probably right at the motor supply terminals of the board, not 3 feet 
away in the psu.  I've looked at mine with a 100 mhz scope, and the hum 
ripple sitting on the 27.5 volt line is in the 200mv range with all 4 motors 
plugged in for a ten amp draw, and the hf hash from the xylotex driver board 
is maybe another 100mv on top of that.  The xylotex also has its own input 
cap whose size escapes me, but it seems to do a fairly decent job of soaking 
up the surges.  

It's a pi section filter, with 75,000 uf on each side of a choke that 
probably has 3 to 4 ounces of decent silicon steel in its E core but I have 
not measured its true inductance.  The hf noise isn't much greater at the 
xylotex terminals than it is at the psu, another 50mv or so.  Barring failure 
of the caps, I think I am in pretty decent shape.  Those 75,000 uf caps are 
also beyond my ability to measure, but were top quality stuff 45 years ago, 
in humungous screw terminal cans.  Even I, as acutely aware of the ESR 
requirements for such duty as I am, was pleasantly surprised at the seemingly 
zero ESR those old caps have.

>You'd want something with tighter tolerances than the old 5Z27 devices
>floating at the bottom of my junk box, for lower headroom.

No doubt.  How are these transorbs thingies for long term stability?  Only 
true zeners are stable over time, and those stop at 4.7 volts, anything above 
that is actually an avalanche diode, and they will drift low over time, rate 
dependent on how much its average power dissipation is.  Those 150 volt, 150 
watt regulator diodes in an old GE tv transmitters aural screen regulator 
circuit will typically last 2 years if the tube doesn't sneeze, 2 
milliseconds if it does.  And 4CX5000A's can do that occasionally if not 
allowed to warm up for 5+ minutes before full voltages are applied.  IIRC 
there are 5 of them in series, for a nominal 750 volts on the screen of that 
tube.

>Erik
>
Thanks Erik.


-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)

"Nuclear war can ruin your whole compile."
                -- Karl Lehenbauer

------------------------------------------------------------------------------
Download Intel® Parallel Studio Eval
Try the new software tools for yourself. Speed compiling, find bugs
proactively, and fine-tune applications for parallel performance.
See why Intel Parallel Studio got high marks during beta.
http://p.sf.net/sfu/intel-sw-dev
_______________________________________________
Emc-users mailing list
Emc-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/emc-users

Reply via email to