Last updated: 23 July 2001

Power Supply Current Requirements of the 497 Autostar

Sent:	Sunday, July 22, 2001 22:40:11
From:	marbla@naisp.net (Blais Klucznik)
Several weeks ago, after discussing the fusing value I used for the
12VDC external battery line to the ETX-125, I mentioned the possibility
of additionally fusing the Autostar 497 also.  In the long interim, the
skies have a way of reducing computer time, I have taken the necessary
current measurements and reached a personal conclusion which I would
like to share with the group.

The steady state current drain by the 497, with 9-12 VDC applied is
9.5MA or 0.0095A.  Indeed a very small current but fairly high when
thinking in terms of CMOS (Complemetary Metal Oxide Semiconductor)
devices. A change of voltage from 9 VDC to 12 VDC has a negligible
effect on the current drain of the 497 on the source battery.

The idea of whether or not one should fuse the 497 for this current
level is argumentative.  First because fuses made for this current level
having an extremely fast attack time are hard to locate and extremely
expensive.  There is also one other important issue.

The most DEVIOUS enemy of any semiconductor, and CMOS in particular, is
application of power with the wrong polarity.  No current limiting or
fusing will be of any help.  Dick Seymour has boldly stated this in
several of his notes to the ETX site and this is truly A RULE THAT
CANNOT, repeat CANNOT, be taken for granted.  There is no second chance.
Utter destruction of these devices is immediate and final.

Now, from a practical point of view, as I see it, I have mixed emotions
as to the actual value of fusing the 497.  I am of the opinion that the
only method of insuring the current from the external battery doesn't
exceed a given value is to design a constant-current source to supply
proper current to the 497.  As far as the normal user is concerned this
is most probably out of the question.  The price of obtaining a
commercial constant-current power supply that would meet the
requirements of the 497 would probably cost 4 to 5 times that of a
replacement Autostar.

So my conclusion is this.

            (1) Use a 1/2 to 6/10 AMPERE fuse in the battery DC power
            lines to protect the ETX-125.  (I use a 1/2A and find that

            (2) Check and double-check your voltage polarity if you are
            using a separate source to supply power to the Autostar 497.
             (This holds true for the 495 also.)

            (3) Keep all wires clear of any interference by any other
            equipment or personnel near the scope.

            (4) Follow a simple routine to inspect all electrical
            equipment each time before use.  It will pay dividends to
            you in a long run.

Good Evening Folks

Blais Klucznik

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