On Wed, 25 Sep 2013 20:31:11 -0500, The Daring Dufas
wrote:

On 9/25/2013 6:58 PM, wrote:
On Wed, 25 Sep 2013 16:19:51 -0500, The Daring Dufas
wrote:

On 9/25/2013 3:25 PM, DerbyDad03 wrote:
The Daring Dufas wrote:
On 9/25/2013 9:09 AM, Ralph Mowery wrote:

"The Daring Dufas" wrote in
message ...
I only put the ammeter across a load that won't damage the
meter, I did mention that first. The shunt in my meter is good
for 20 amps and I certainly wouldn't put it across a circuit
that would exceed the safety limits of the meter. That's why I
only do it for low current loads. The little circuit breakers
with test leads help narrow down a problem with an overload
while saving fuses at the same time. I may be assuming that
others know enough not to do anything unsafe but I don't want
to appear patronizing. I'm only giving general information from
my own experience and I hope no one believes the same
procedures work with dangerous high voltages and currents. ^_^

Did you ever try the light bulb instead of a fuse ?

Depending on the normal load, you can use a 100 watt light bulb
(wattage to vary depending on normal current usage) in place of
the fuse. When the light bulb glows much dimmer or not at all
then you have found the problem that usually blows the fuse.


Putting the amp meter across the fuse will not depend on the
load, but the actual ammount of current that can be sourced. Say
the normal load is only 10 amps, your meter is good for 20 amps,
but the source is good for 100 amps. If you have an almost short
at the load, close to 100 amps is going to try and pass through
your 20 amp meter which is now toast , or hopefully the internal
meter fuse blows.

You should only put the amp meter in line when you get the fuse
to stop blowing. Then it should be safe to see how much current
is actually being used.



I had a 100w, 150w and 200w bulbs that I soldered test leads with
alligator clips to when I was working as a bench tech back when
appliances and TV sets used a lot more power. In the past 20
years, most of the service work I've done has been mobile and not a
good place to have glass bulbs bouncing around in a vehicle. I'm
sorry but I keep assuming others would do what I do without
thinking. The light bulb in series with a Simpson 260 was SOP when
working on two way radios to check the DC current draw. A shunt was
needed for AC current tests since those meters would only test up
to 10 amps DC. A separate AC ammeter worked for bench testing. I
had variable power supplies that indicated voltage and current
being drawn by equipment when bench testing plus those power
supplies had adjustable current limits that would drop voltage to
zero when the limit was reached. The small resettable circuit
breakers I used were put in series with the DMM when testing
current draw. I repeat, never use a DMM to check current in line if
you know it will exceed the safety limits of the meter. Test leads
can melt or have the tips burned off. It's not a problem to put a
DMM across a fuse when testing a radio being powered by a 12vdc
power supply which is rated at 3 amps and has a built in circuit
breaker like the small power supplies sold by Radio Shack for many
years. It's been a while since I've been in a Radio Shack store to
buy discrete components or batteries. If I'm working on a small AC
appliance that would draw 5 amps/600 watts,(look at the label). The
small circuit breaker in series with with a DMM having a 10 amp
range is completely safe unless you are dumb enough (like me) to
touch the exposed test clips and get a shock. Even if the small
circuit breaker is not used and the appliance has a dead short, the
20 amp breaker supplying power to the outlet you are plugged into
will trip. The small circuit breakers I once put together on my own
are now sold at supply houses with the pigtails and test clip
already on them. When repairing electrical or electronic gear, the
best test equipment are your eyes, ears and nose. ^_^

TDD

"...unless you are dumb enough (like me) to touch the exposed
test clips and get a shock."

...or the banana plug on the end of a jumper cable.

In my case it was the second week of USCG Electronics School
training. On the worktable in front of us, we each had a 400 VDC
power supply training device which was plugged into a power strip.

There was a ~4" jumper cable, with a banana plug on each end, that
connected two sections of the power supply. The jumper could be
pulled out to break the device down into smaller sections for
troubleshooting training. Basically you were removing the load. The
normal procedure was to shut the power supply down, pull the jumper
and then power it back on.

Heck, I don't need to go through all that. I'll just hooked my finger
into the loop and pull the cable out. So, with my forearm resting on
the chassis, I hooked my finger into the jumper and pulled. Too bad
one banana plug (on the output side) was a lot looser then the other
one. With input side still plugged in, the loose end flipped up and
laid against my thumb. With my arm laying on the chassis (read:
ground) I became the new load for the 400 VDC.

My arm spasmed and I couldn't pull it away from the chassis.
Instinctively, I reached out my other hand to pushed the chassis
away. All that accomplished was to cause the current to flow from one
hand to the other through my upper body. I was holding a 30 lb power
supply up off the table as if it was weightless, yelling "Turn if
off! Turn it off!" as my whole upper body spasmed.

The guy at the table in front of me turned around and grabbed the
power cord in an attempt to unplug the device. Unfortunately, the
power strip was not secured to the table so it just came up with the
cord. The guy next to me reached over and slapped the power strip
back onto the table which unplugged the device.

I dropped (actually, threw) the power supply onto the floor and they
drove me over to the infirmary for an EKG. Other than the burns on my
hand, I was apparently OK.

When I came back to class the next day all of the power strips had
been screwed down to the work surfaces and 2 other guys had quit
electronics school after witnessing my near demise. They quit ET
school and I went on to work on devices that had power supplies in
the range of 25KVDC.

Luckily (?) I still have the scars on my hand to remind me of how
stupid I was. I've been a lot more careful since then.


I remember having an arc jump from the anode cap of the horizontal
output tube in a TV set to my hand when working on a set in front of a
customer at the shop. I excused myself to go to the back room where I
yelled @#*&%*%$#@&*%^#!!!!! I then came back into the shop and resumed
work on the set in front of the customer who had an odd yet concerned
look on her face. I've been shocked, zapped and burned more times than I
can count but I never, ever let my guard down around high voltage high
current power coming into a building. When I worked as an electrician,
my superintendent got a tingle when using an old wooden hot stick while
we were connecting some 4,160 volt pad mounted transformers for the
underground electrical system we were building. If it had been 13.8kv I
doubt he would have gotten just a tingle. All my ladders and push poles
are fiberglass because one never knows what you can run into around
power systems. I learned long ago to work on everything as though it was
energized because it a good habit to get into. One thing I really hate
is when I've been working in hot weather and because I sweat like a
thunderstorm, I wind up soaking wet with all my clothing soaked and the
sweat dripping on the floor. You can tell where I've been by the wet
areas on the floor. In this condition, I've had my sweat soaked shirt
tail touch a ground while I was working on a panel. Salt water and sweat
conduct electricity very, very well. o_O

TDD
As a kid I had salvaged a power supply out of an ancient TV that
didn't use a flyback system - the main power transformer had taps
from, IIRC, 3.5 volts to 25000 volts. The high voltage was at the
opposite end from the low voltage. I needed the low voltage to test a
small motor I was working on, but I grabbed the wires on the wrong
end. I must mention the basement ceiling was something like 5 1/2 ft
to the floor decking, about 42 inches to the bottom of the joists. I
was just under six feet tall at the time. I straightened up fery
quickly and my rock-hard skull caught the end of a nail that held the
1/4" unserlay to the sub-floor, and I popped the head of that nail
through the linoleum flooring in the living room above.


OUCH!! How deep was the hole in your skull or have you always had a hard
head? ^_^

TDD
It bled a bit, but I guess I've always been a bit hard headed.
If anyone told be I had rocks in my head, I'd just take it for
granite.