I'm not sure that "jumpering across" the 7805 will be healthy for the 7805.
Of course, if the input doesn't drop below the output during shutdown this
may not be a concern. However, if there's a significant off-board load on
the input switching supply, and significant capacitance (bypass, for
example) on the board, it's likely to cause problems.
Dick
-----Original Message-----
From: CLASSICCMP_at_trailing-edge.com <CLASSICCMP_at_trailing-edge.com>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Tuesday, August 17, 1999 4:20 PM
Subject: Re: imsai 2
>>What puzzles me is why the IMSAI folks decided to use a switching power
>>supply when the box and everything else already supported the needs of the
>>S-100 with the previously available and now quite inexpensive unregulated
>>supplies of yesteryear.
>
>Probably an issue of economy, using PC-clone power supplies at about
>$25 each vs using a custom-wound transformer at $200 or so a pop.
>
>> One of the main benefits of the S-100 was that it
>>had on-board regulation, so that if you didn't need a given supply, you
>>didn't have to bring it on board and regulate it, dissipating power as you
>>went. If the new box is capable of running the original boards, it must
>>provide the raw 8 and +/- 16-volt supplies. Where's the benefit in having
a
>>switching regulator sitting in the back of the box? I suppose it creates
a
>>market for a power distribution module to put +5 and +/- 12 on each board
>>which needs it rather than using the on-board regulators, but that opens
>>another can of worms. What's the "right" way to distribute it without
>>tampering with a historically correct board?
>
>According to what I read from the web page, there's a PC-clone switching
>power supply followed by a "boost" switching circuit to get back to
>+8 and +/- 16. That's not completely unreasonable, but it sounds a
>little bit fishy to me in the sketchiness of the details.
>
>Several S-100 manufacturers who were still at it in the mid-80's had
>regulated +5V on the S-100 backplane. Those who wanted to use their
>older S-100 cards in such a machine just jumpered across the 7805's and
>viola!
>
>--
> Tim Shoppa Email: shoppa_at_trailing-edge.com
> Trailing Edge Technology WWW: http://www.trailing-edge.com/
> 7328 Bradley Blvd Voice: 301-767-5917
> Bethesda, MD, USA 20817 Fax: 301-767-5927
Received on Tue Aug 17 1999 - 17:35:15 BST