23 Nov
2002
23 Nov
'02
10:20 a.m.
only the power harness. And assuming one couldn't
find a replacement
power harness for the bad
one, what would it take to modify/repair the bad one to
make it safe to
use? Is it just the matter >of replacing some of the existing
wire with
something of a heavier gauge?
I believe the problem is that the wires are of differing lengths
and hence resistance. Plus they were (or may have been) specced
a tad to near the limit. The end result was that the longest wire
(greatest resistance) heated up the most and eventually went phut,
leaving the other five (or so) to go pop shortly thereafter.
So heavier gauge and same length would seem to be required.
OTOH if the problem was that bad (none of mine in the lab ever exhibited
any pyrotechnic tendencies) would it not have corrected itself by now
:-)
Antonio
23 Nov
23 Nov
7:29 p.m.
On 2002.11.24 01:20 Antonio Carlini wrote:
I believe the problem is that the wires are of
differing lengths
and hence resistance.
Yes. I have seen a toasted BA23 power harness...
The end result was that the longest wire
(greatest resistance) heated up the most and eventually went phut,
No. The wires
are connected in parallel. They have different lengths and
thus different resistance - the longer the higher. "Current allways uses
the way of lowest resistance." So the shortest wire has to carry a
higher current than the other wires. The wire can carry this higher
current, but the connector can't. So the _connector_ heats up and "the
magic blue smoke escapes"...
Solution was to use equal length wires (=equal current distribution
among wires/connectors) and better connectors that can carry higher
current.
--
tsch??,
Jochen
Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/
8:13 p.m.
Jochen Kunz wrote:
On 2002.11.24 01:20 Antonio Carlini wrote:
No. The wires are connected in parallel. They have different lengths and
thus different resistance - the longer the higher. "Current allways uses
the way of lowest resistance." So the shortest wire has to carry a
higher current than the other wires. The wire can carry this higher
current, but the connector can't. So the _connector_ heats up and "the
magic blue smoke escapes"...
Solution was to use equal length wires (=equal current distribution
among wires/connectors) and better connectors that can carry higher
current.
--
Let's try to keep this in accordance with real physical laws:- the current
flowing in the wires will be inversely proportional to the resistance since
the potential difference across them is the same. The physical principle
that causes the problem is that the power dissipated is proportional to the
*square* of the current. In case of any doubters -
Ohms Law:
I = V/R
=> V=IR
Power dissipated:
P = IV
=> P = I(IR)
So, for example, if one cable/connector pair had twice the resistance of
another, the second would dissipate four times as much power as the
first. If, as other posters have indicated, the bulk of the resistance is
in
the connector then the heating effect will be quite localised, and the
home cooking lessons are on.
Cheers,
Dave.
9:41 p.m.
On Sun, Nov 24, 2002 at 10:14:31AM +0000, Dave Woodman wrote:
Let's try to keep this in accordance with real
physical laws:-
I know this laws very well, I am studying EE. :-)
I tried to keep things simple to avoid loosing my self in details.
(That nobody can understand without at least basic knowledge about EE.)
--
tschüß,
Jochen
Homepage: http://www.unixag-kl.fh-kl.de/~jkunz
24 Nov
24 Nov
4:46 a.m.
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10:03 a.m.
On 2002.11.24 19:36 Tony Duell wrote:
I can, belive me I can. ;-)
Some time ago, when I didn't have a multisync monitor, the first thing
that I did when I got a new "toy" with graphics card was to warm up the
good, old HP scope. I used it to check if the video timing would fit to
one of the several fixed frequncy monitors in my collection...
I know this
laws very well, I am studying EE. :-)
I am not convinced that one necessarily
follows from the other. I've
met plenty of so-called EE's who can't calculate (say) the voltage on
the tap of a potential divider. I have to say that anyone who attempts to work on
classic computer
hardware (or any hardware for that matter) without a basic
understanding of electrical engineering (including things like
V=I*R [1]) is making life difficult for themselves.
I second that. I know a
collector who knows nothing about hardware and
how it works. But he can tell you a lot about Lisp and how wonderful his
Symbolics machines / Genera are from a programmers point of view. I
found his machines interresting due to the bit slice CPU design with
loadable micorcode. When I told him that I builded a simple voltage
inverter (charge pump) to make the RS232 work on the Sun 4/600 MP he
looked like it was black magic to convert +12V to -12V. (The Sun 4/600
MP boards needs -12V for the RS232, even -5V are not enough. As I
mounted this board in a Sun 3/60 case that doesn't provide -12V, I could
not use a serial console.)
--
tsch??,
Jochen
Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/
25 Nov
25 Nov
10:37 a.m.
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11:29 p.m.
On Mon, Nov 25, 2002 at 11:52:20PM +0000, Tony Duell wrote:
In other words, there are EEs who know what they are
doing. There are
also ones that don't. The latter are far too common in my experience!
I know
examples for both types...
Actaully, I also have to say that any classic computer
enthusiast who
doesn't know how to program, and who doesn't understand the concepts of
machine code, is also making life difficult for himself.
What makes me afraid is
that todays CS students often don't learn
what machine code is. "All the world is Java."
I am very much hardware-biased, but I still have no
problem with
simple-ish programming....
Yes. I am also doing some Unix kernel hacking. (I try to
do. ;-) )
Does he have the tools to write his own microcode?
No. We even don't have tools / media to install the OS (Genera).
Some other friends have the same problem with there Symbolics
machines. No removable media or only QIC tape but no tapes and
no knowledge on how to build them. Dead MFM disks, ...
Has he ever tried to do it.
I don't think.
He used the machines for what they where build: Genera.
--
tsch??,
Jochen
Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/
26 Nov
26 Nov
10:09 a.m.
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12 Dec
12 Dec
8:23 p.m.
New subject: Pulling -12VDC from thin air (was Re: MicroVAX II)
--- Jochen Kunz <jkunz(a)unixag-kl.fh-kl.de> wrote:
I second that. I know a collector who knows nothing
about hardware and
how it works.... When I told him that I builded a simple voltage
inverter (charge pump) to make the RS232 work on the Sun 4/600 MP he
looked like it was black magic to convert +12V to -12V. (The Sun 4/600
MP boards needs -12V for the RS232, even -5V are not enough. As I
mounted this board in a Sun 3/60 case that doesn't provide -12V, I could
not use a serial console.)
Interesting to learn about the 4/600 board and the 3/60 case. Long ago,
I wondered if the two could work together (I _have_ the 3/60 and I almost
had the 4/600 board).
If you'd have known to ask, I could have provided you with a +5VDC to
-12VDC converter module. It's about 20mm x 30mm x 3mm. We had to put
them on the Qbus COMBOARD because the original design assumed you could
pull -12VDC off some Qbus pin somewhere, and I suspect that it is not
the case for all Qbus enclosures (don't know where it is available and
where it isn't). I have several new on the anti-static mat they shipped
in.
-ethan
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24 Nov
24 Nov
4:34 a.m.
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5:14 a.m.
Regardless if remote sensing is used or not, the power dissipated in the
harness will be P=R*I^2, so, proportional to the current squared. With
remote sensing I will be a constant as stated, even if the harness
resistance goes up, so the power dissipated within the harness then is
increased. Without remote sensing I will more than likely go down, but R
will still go up in case of an overheated cable or bad connector, provided
the bad connector is on the load end and not the source end. But since we
are only talking about defective, or inadequate cables, power dissipated in
the cable will still be P=R*I^2. Of course all the other equations given by
Tony are also correct. Low resistance cables will dissipate the most power
in the load and minimum in the cable. Regards - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell NJ, 07731
(732) 901-9193
7811
days inactive
7831
days old
11 comments
7 participants
participants (7)
-
Antonio.Carlini＠riverstonenet.com -
ard＠p850ug1.demon.co.uk -
dave＠naffnet.org.uk -
erd_6502＠yahoo.com -
jkunz＠maja.unixag-kl.fh-kl.de -
jkunz＠unixag-kl.fh-kl.de -
n4fs＠monmouth.com