In article ,
Ken Scharf wrote:

I always assumed there was voltage across the caps when I worked on the
rig. First pull the plug(s). Wait till the voltmeter drops to zero.

A trick, courtesy of Bob Pease: whenever you build a power supply
circuit which carries line voltage or higher, stick an NE-2 neon bulb
and a suitable dropping resistor across the caps, and place the bulb
where it's clearly visible when the case is opened.

If the bulb is lit (at all) the caps have enough voltage in them to
give you a nasty bite.

THEN put a heavy screwdriver with a well insulated handle from ground
to the HV terminal to be sure!

I've heard cautionary notes about this... if you do this prematurely,
the arc can be scary (and dangerous in some cases - "arc flash" burns
can be severe) and I believe it's possible to damage or wreck some
caps as a side effect of a catastrophic high-current discharge.

A heavy grounding wire, with alligator clips, and with a
current-limiting resistor is another possible solution. You want a
low enough impedance to drain away any charge which may come out of
the cap's dielectric "soakage", but high enough to avoid a dangerous
arc flash when you first hook it up.


Keep on hand in the pocket if you need to
adjust anything when it's hot.

That's one of the best single pieces of advice around! Also, be aware
of what sort of grounded metal is around your workbench. Doesn't help
much to keep one hand in your pocket, if you lean your belly against a
grounded metal bench!


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.

--
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!

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...

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

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

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

Dave Platt wrote:
In article ,
Ken Scharf wrote:

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.

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.

--
Mike Andrews W5EGO 5WPM
Extra
Tired old sysadmin working on his code speed

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

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

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

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!

straydog wrote:

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.

No, it's not a simple time constant -- the slope is much different. A
reasonably good model of it is a series R, shunt C, series R, shunt C,
etc. A large number of Rs and Cs with various time constants have to be
used to imitate it reasonably well.

I don't know about an "electret effect". An electret is a dielectric
with a permanently trapped charge, so it produces a static electric
field. It's the analog of a magnet, with permanently trapped flux and
producing a permanent magnetic field. A magnet that's not moving can't
produce a current (much to the dismay of the perpetual motion crowd),
and an electret that's not moving can't produce a voltage.


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

That's for an entirely different reason -- to protect the meter from
damage, rather than the user.

Roy Lewallen, W7EL