On 17 Sep 2013 04:57:44 GMT, "DoN. Nichols"
wrote:
On 2013-09-15, Larry Jaques wrote:
On Sun, 15 Sep 2013 10:12:52 -0400, "Jim Wilkins"
wrote:
[ ... ]
When I was learning the intricacies of op-amp measurement circuits OJT
I realized that voltage can just sit there but current HAS to be
accounted for, and began thinking in terms of it, and how to control
and measure it. External meters don't work as well as a power supply
that shows current directly without a voltage drop across the meter
and jumper wires to consider. The big advantage of a lab supply is its
adjustable current limit which protects the load, and sometimes the
user.
Yeah, current limiting seems like the way to go for safety. I don't
recall seeing that feature on any of the old supplies I ever worked
with, but it's been awhile. I could have forgotten.
My favorites were made by Power Designs, and I got them for good
prices at hamfests, because I knew them from having worked with them at
work. Two styles.
2005 0-20V 0-0.5 A adjustable in steps of 1 mV. Just dial
in the desired voltage from 0-10V, and a switch adds
another 10V if you need it. It has a nice current
limit. They even have the zener reference in a crystal
oven, so the voltage does not drift with room
temperature variations. Plug them in and the ovens
starts working, and once the front panel light for
the oven starts cycling, it will be stable.
3650 0-36V, 0-5A, with a variac built in so the difference
between the voltage out of the transformer and the
voltage desired out of the front panel terminals is not
too different, so you don't have to drop too much
voltage (and thus heat in the regulator.
I'll keep eyeballs peeled for those, Don. Thanks. Speaking of crystal
ovens, I have one from the '50s which I use as a fire safe. It's made
with that horrid OHMIGODWEREALLGONNADIE asbestos stuff.
I got two of the 2005s in a rack-mount frame for practically
nothing. The seller said that he could not get them to work, and I
glanced at the rear panels, and found the jumper blocks where the
external sense could be wired had some wires coming from it going off to
nowhere. Just replace jumpers between the "+" and "+sense" terminals,
and likewise between the "-" and "-sense" terminals and they worked just
like they were supposed to. :-)
I love it when I find things like that. My neighbor gave his golf
cart to me when it wouldn't charge or run. I had to replace a $5
cable between batteries and sold the thing for $900. I gave him $100
for the thing after the fact, and told him "it cost under $50 to
repair."
and this has the capability of the 4-quadrant V-I supply that is the
heart of an automated analog semiconductor tester:
http://www.keithley.com/products/dca...rpose/?mn=2400
http://en.wikipedia.org/wiki/Automatic_test_equipment
Nice.
That would be nice. And I've liked Keithly equipment in the
past.
[ ... ]
Cheap import lab supplies are good enough for my home experimenting,
such as measuring the resistance of a motor or antenna by forcing 1
Amp through it and reading milliOhms as milliVolts. Even a $4 HF DVM
will resolve tenths of a milliOhm that way. The power supply's output
voltage can be turned down very low so an open shouldn't damage the
circuit being checked, for example the wiring harness of an old
vehicle.
Huh? How can an open harm a circuit? (Other than arcing, and its
resultant damage and chaos?)
The short which should be there is holding the voltage low. If
the open occurs, the voltage shoots up to whatever the supply is set to,
and this may be too much for something else connected to the same wires
-- or not too much, but the wrong polarity.
OK, I'll almost buy that. But what happened to those components
-after- the incident but -before- the short dropped the voltage, hmm?
I used 9v batteries to check for opens and shorts in wiring harnesses
when I wrenched for the body shop. Lots of fenders were driven into
the looms when the bodies crumpled. I repaired and retaped (miles)
many a harness, since it was usually cheaper to repair than it would
be to replace it. In most instances, it was only one or two wires
which were damaged (usually severed.)
The 9V batteries are probably no problem for most things in
older cars, but there may be things in the newer ones which don't like
even that much voltage. Just like an old analog multimeter which on the
Ohms range can provide enough voltage to zap a logic chip, while a
Digital one limits the maximum voltage to perhaps 1V or so.
Yeah, I keep forgetting that I stopped wrenching before most of the
cars went to digital systems. I haven't had to troubleshoot any of
the newer ones since I've owned only brand new trucks (2) since then.
It's sure nice not having to get my fingernails dirty any more. I wish
I'd remembered to wear latex gloves during house painting this last
week, though. Oh, those new white 3mil nitrile gloves at HF are TITS,
guys. They're quite a bit like latex only they're 5 times tougher,
resistant to more solvents, and much easier to install/remove. I love
those things enough that I have a dozen in my BOB.
It won't do both at once. I was looking for parts at a flea market to
complete a less ambitious one when I found the salvaged front panel,
with a 10A Variac and 40V and 20A meters, so I used the other
components I had already collected to resurrect it. It's not regulated
(yet) but good enough to quickly charge a dead battery or bench-test
heater and wiper motors that need more than the 5A limit of my lab
supplies. I don't like to connect sickly, sluggish DC motors directly
to a battery. It's safer to turn up the voltage on them slowly while
watching the current.
Aw, ya wuss!
At a recent hamfest, I got a motor-generator -- 28 VDC input,
115 VAC three phase 400 Hz output. I could not run it from the total of
the power supplies on my bench, but I recently got a dainty little
transformer from a friend -- NOS. Dual primaries for either 120 VAC or
240 VAC input, and two center-tapped secondaries, each capable of 20 A
at 12V (or at 6V using the center taps). With the two secondaries in
series I get 24VAC at 20 Amps -- and that through a bridge rectifier
and with a big filter cap runs that motor-generator nicely.
Cool! Was your "dainty" above facetious? 20A usually isn't schmall.
Now that I own a ham radio (Baofeng UV-5R portable) I need to get my
tickee and go to those ham fests so I can find your treasures here.
What is such an inverter used for -- to power certain aircraft
instruments, including to spin up gyros built into instruments.
I've measured the DC current or AC power most of my antique
electronics consume and written it on them for when they need work
later. Yesterday I extended the wiring to my 1950's TV antenna rotator
and then watched the power the controller drew when turned back on to
confirm I hadn't miswired it. Now I can unplug it where it comes into
the basement instead of going out in the rain when lightning is
predicted.
You live in that kind of area, eh?
Doesn't everyone? We get a lot of it during the summer. :-)
AFAIK, I've never lived where lighting struck wires or antennas. It
has never affected me personally during my 60 years so far. (Me glad,
BTW)
--
Try not to become a man of success but
rather try to become a man of value.
--Albert Einstein