On Thu, 14 Jul 2005 11:32:47 -0700, Tim Wescott wrote:
Dave Platt wrote:
In article ,
Ken Scharf wrote:
snip
HV isn't the only thing that can get you. High current can give you
a nasty surprise. Like the guy replacing some batteries on a golf cart.
He was using a ratchet wrench to tighten the battery clamps and he
ended on the most positive battery terminal. The handle of the wrench
hit the chassis of the golf cart putting it between 36 volts and ground
of some VERY HEAVY DUTY batteries. Ever see a Sears ratchet wrench
glow WHITE HOT? (and melt?)


Haven't seen that myself, but I've heard of people who have had
screwdrivers, etc. literally vaporized under such circumstances.

My brother came home from a business trip once with a scar on his wrist
5/8 inches wide and all the way around -- the truck was having
electrical problems & he shorted his metal watch band through the 12V
battery (there was a wrench involved in there somehow, of course).

I felt for him, but I was very glad that _he_ was the one to learn this
first hand, rather than me...

My dad (the first W3DHJ) told me a similar story.

It was in the late 20's. He was working on _his_ dad's Ford utility
truck. (Grandpa owned a dairy in Big Bear, Calif.) Six volts here.
The truck was parked in the driveway -- just outside the garage.
When my dad caught his ring between the positive terminal and the truck
frame, my Grandpa picked him up bodily -- ran him over to the rain barrel
at the corner of the garage -- and stuffed my dad's entire left arm (and
much of his upper torso) into the barrel.

( *The worst* thing you could do in a situation like that is try to _pull_
the ring off.)

My dad was a 90-day wonder in WW II. He then spent 25+ years in the
U.S. Army Corps of Engineers (we traveled the planet....) Most of his
duty assignments were as Post Engineer -- with the added responsibity
as Post Safety Officer. I always knew him to be evangelically anal
about safety. I'm sure that incident with the old Ford truck had a
wee bit to do with it.

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK

Doug Smith W9WI wrote:

Ken Scharf wrote:
The filter caps were 10 330uf 450v computer grade units in series, each

Probably dumb (and WAY off topic) question:

What's the *intended* purpose of a 450v "computer grade" capacitor?

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com


They were used on the input side of large switching supplies for
minicomputers, and some later mainframes. I pulled about 100 of them
from the multiple switching supplies in an Amdal mainframe about 10 or
12 years ago.

--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.

Michael A. Terrell
Central Florida

Don't forget about "soak", also known as dielectric absorption or
dielectric hysteresis. In some capacitors, particularly electrolytics,
charge "soaks" into the dielectric and can take from seconds to days to
work its way out. The result is a "discharged" capacitor that seems to
spontaneously recharge itself. (One story I heard during my broadcast
days was someone who put a screwdriver across a "discharged" capacitor.
After he picked himself up off the floor, he found just the handle of
the screwdriver. He was lucky his eyes weren't hit with molten metal.
You'll often find large capacitors stored with a shorting wire across
them -- a good idea.) I recommend leaving a heavy cliplead in place
across the high voltage capacitors for the entire time you're working on
the unit. Put some tape over the on/off switch and/or a tag on the plug
to remind you to remove it before turning the power back on.

As a side note, the aquadag coating of a CRT is notorious for this. I've
gotten a healthy bite off a CRT the day after it was discharged for an
hour or more with a cliplead.

Roy Lewallen, W7EL

Mike Andrews wrote:

The canonical "Take-Your-Rings-Off" reminder in our electronics shop
in Japan, back when I was in the AF, was a color image, taken at our
base hospital, of a finger burnt to the bone all the way around: one of
the techs had got his wedding ring between ground and a high-current
low-voltage supply.

What we got was a nice photograph of a finger in a dish with the ring,
right beside the newly customized hand.

Roy Lewallen, W7EL

What's the *intended* purpose of a 450v "computer grade" capacitor?
====================
Switch mode power suppplies ??


Frank GM0CSZ / KN6WH

On Thu, 14 Jul 2005, Tim Wescott wrote:

Date: Thu, 14 Jul 2005 11:32:47 -0700
From: Tim Wescott
Newsgroups: rec.radio.amateur.homebrew
Subject: QUESTION: Fun with Svetlanas or Staying alive with kV power
supplies

Dave Platt wrote:
In article ,
Ken Scharf wrote:
snip
HV isn't the only thing that can get you. High current can give you
a nasty surprise. Like the guy replacing some batteries on a golf cart.
He was using a ratchet wrench to tighten the battery clamps and he
ended on the most positive battery terminal. The handle of the wrench hit
the chassis of the golf cart putting it between 36 volts and ground
of some VERY HEAVY DUTY batteries. Ever see a Sears ratchet wrench
glow WHITE HOT? (and melt?)


Haven't seen that myself, but I've heard of people who have had
screwdrivers, etc. literally vaporized under such circumstances.

My brother came home from a business trip once with a scar on his wrist 5/8
inches wide and all the way around -- the truck was having electrical
problems & he shorted his metal watch band through the 12V battery (there was
a wrench involved in there somehow, of course).

I felt for him, but I was very glad that _he_ was the one to learn this first
hand, rather than me...

I've got a related story to tell: I saw many years ago two guys try to
jump start another guy in the winter time. I only saw a few seconds and
surmised the following. These guys must have used the jumper cables to
connect the two twelve volt batteries in series, not parallel. Why? The
jumper cables were, themselves, literally smoking and not just a little
bit. Figure 24 volt power at, what, something like 500 amps(?) split
between heating up the interior of the two batteries and those jumper
cables (that are usually like #4 or #6 gauge?). How many seconds would it
take for on the order of 5000 watts to cause jumper cables to get up to
200-300+ degrees Farenheit? Those guys scrambled to tear those cables off
the batteries and I could tell by how they were handling the cable that it
had to be hot. They could have warped the lead plates.


--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Thu, 14 Jul 2005, Roy Lewallen wrote:

Date: Thu, 14 Jul 2005 13:27:57 -0700
From: Roy Lewallen
Newsgroups: rec.radio.amateur.homebrew
Subject: QUESTION: Fun with Svetlanas or Staying alive with kV power
supplies

Don't forget about "soak", also known as dielectric absorption or dielectric
hysteresis. In some capacitors, particularly electrolytics, charge "soaks"
into the dielectric and can take from seconds to days to work its way out.
The result is a "discharged" capacitor that seems to spontaneously recharge
itself. (One story I heard during my broadcast days was someone who put a
screwdriver across a "discharged" capacitor.

Yep, I can vouch for this effect. I've seen it, too. Short the cap with
alligator clip-tipped wire for a few seconds if you want that voltage down
to microvolts. I think it is not "soak" but simple RC time constant decay.
Just like radioactive half-life. Residual voltage on a cap _never_ goes to
true zero, only according to the decay equation. However, there is a
so-called "electret" effect which really sounds like your "soak" effect.

After he picked himself up off
the floor, he found just the handle of the screwdriver. He was lucky his eyes
weren't hit with molten metal. You'll often find large capacitors stored with
a shorting wire across them -- a good idea.)

D'arsonval meters, especially sensitive ones, are also best stored with a
shorting wire accross the terminals.

I recommend leaving a heavy
cliplead in place across the high voltage capacitors for the entire time
you're working on the unit. Put some tape over the on/off switch and/or a tag
on the plug to remind you to remove it before turning the power back on.

As a side note, the aquadag coating of a CRT is notorious for this. I've
gotten a healthy bite off a CRT the day after it was discharged for an hour
or more with a cliplead.

Roy Lewallen, W7EL

I forgot to tell my "parasitic" story..see below...

On Wed, 13 Jul 2005 wrote:

Date: 13 Jul 2005 14:05:17 -0700
From:
Newsgroups: rec.radio.amateur.homebrew
Subject: QUESTION: Fun with Svetlanas or Staying alive with kV power supplies

My knowledge of vacuum tubes and kV power supplies is limited. I have
been reading an article in the ARRL Handbook detailing the construction
of a 1kW HF Linear. I'd like to try my hand at building something like
this. I found the article a little intimidating: Ceramic insulators,
parasitic suppressors, thermal and mechanical engineering etc. Is
there some book that details this type of thing with an explanation of
the whys as well as the whats and hows. My priorities a

1) Safety. I'd like to be alive to make my first 1kW QSO
2) Avoiding equipment destruction, arc overs, black smoke, explosions
etc
3) Safetly troubleshooting this kind of equipment, loading testing
etc.
4) How to deal with tubes: warm up, care, etc...
5) Avoiding TVI (ITV), parasitic oscillations etc.

I built this two 811-a linear amplifier way long ago. Basically ave power
300 watts DC input. So, I made my own parasitic suppressors (turns of wire
over, say, a ten ohm two watt carbon resistor. Or adjust turns). This was
for a 75 meter phone band.

So, on launch day it sits there like a scared bird. Turn on fil power,
plate power (this is grounded-grid for stability and minimal if not zero
need for neutralizations), and tune (no drive) the plate capacitor.
Result: dang, RF output through the power meter AND smoke comes out the
top of the amp and I stand up, look down, and the coils of wire (made of
#22 or 24 gauge wire, can't remember) around the resistors are glowing red
hot! Too many turns of wire, so next time around I cut back to half and
all was OK. Including no self-oscillation in the two 811-As.

That's called trial and error. Or, try, smoke, fix. Like if ready, fire,
aim does not work, change the order to ready, aim, fire.

Art, W4PON


6) Longevity and Duty Cycle issues etc.
7) Costs and sources.

Hope someone can help.

Thanks,

Tim

In article g,
straydog wrote:

Don't forget about "soak", also known as dielectric absorption or dielectric
hysteresis. In some capacitors, particularly electrolytics, charge "soaks"
into the dielectric and can take from seconds to days to work its way out.
The result is a "discharged" capacitor that seems to spontaneously recharge
itself. (One story I heard during my broadcast days was someone who put a
screwdriver across a "discharged" capacitor.

Yep, I can vouch for this effect. I've seen it, too. Short the cap with
alligator clip-tipped wire for a few seconds if you want that voltage down
to microvolts. I think it is not "soak" but simple RC time constant decay.
Just like radioactive half-life. Residual voltage on a cap _never_ goes to
true zero, only according to the decay equation. However, there is a
so-called "electret" effect which really sounds like your "soak" effect.

Yup. Formally "dielectric absorbtion". In a lot of types of
capacitor, enough stored charge can come back out, over the course of
a few minutes, to raise the open-circuit terminal voltage of the cap
to a significant percentage of its fully-charged level, even if you
had previously discharged the cap all the way to a zero reading.

It can be enough to give you a nasty bite.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Highland Ham" ) writes:
What's the *intended* purpose of a 450v "computer grade" capacitor?
====================
Switch mode power suppplies ??


Frank GM0CSZ / KN6WH


I thought "computer grade" was either a marketing name, or at the
very most defined a certain design of capacitor.

In other words, they weren't designed for computers, but saw a lot of
use in them. They weren't cheap electrolytics of the type you'd
see in the average consumer equipment of thirty years ago.

You'd see them promoted in the surplus ads, and the ones I remember
were metal-cased, and had screw terminals for connections. I still
have one around I bought at a hamfest for a 12V power supply. It
was about 10,000uF (which was a fairly large size capacitor circa
1973 or so), had a voltage rating of about 16volts, and was the size
of a can of coke. Obviously a lot more impressive than the average
electrolytic of the day.

So once you had that style, it wasn't whether they were used in
computers or not, so of course you could have high voltage "computer
grade" capacitors.

I have no idea if it was a marketing ploy, "hey those are used in
computers [which were still uncommon at the time, and usually big
and very expensive], they must be good capacitors", or if there
was something about their design that made them better than the
average electrolytic of the time.

Michael VE2BVW