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
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