Subject: Re: Running PCs on 12v

From: "Richard A. Cini" <[email-masked]>
Date: Fri May 16 20:26:00 2003

Joe:

>>
>>  Any chance of using 48VDC? There's plenty of surplus industrial PCs (and
>>PSUs) that run off of that.  I've often wondered if you couldn't run a PC
>>directly off of 12VDC (with a regulator to drop one line to 5VDC).

	I saw a lot of these in my usual surplus catalogs but producing 48v from
batteries of a sufficient amperage is a bulky proposition. I envision a
small cordura camera bag filled with two surplus 12v 7ah lead acid batteries
(in parallel) and some switching gear for the scope and the PC.

	I also looked at some of the mobile MP3 projects but the most suitable
solution was $150 and didn't have the right amperage ratings.

	I don't know...maybe I'm looking for something that really doesn't exist. I
really wanted to avoid building my own supply, though.

Rich
Collector of classic computers
Build Master for the Altair32 Emulation Project
Web site: http://highgate.comm.sfu.ca/~rcini/classiccmp/
/************************************************************/

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From: "John Allain" <[email-masked]>
Date: Fri May 16 20:39:01 2003

Here's a pretty damn dumb question.
   Can 5v regulators just be stacked up to get
      more than 100mA?
   One possibility is that the weakest one would
      go first, then the rest after it's dead.

John A.

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From: "J.C. Wren" <[email-masked]>
Date: Fri May 16 20:41:00 2003

This looks interesting. The current version is pretty much limited to a
12V input, but they're saying they'll have a 9-18V with ignition sense
input in May/June 2003. Suddenly, vehicle-based PCs are looking a little
more doable. And at $50, it's not a bad deal.

<URL: http://www.mini-box.com/pw-60.htm >

	--John

On Friday 16 May 2003 21:25 pm, Richard A. Cini wrote:
> Joe:
> >>  Any chance of using 48VDC? There's plenty of surplus industrial PCs
> >> (and PSUs) that run off of that.  I've often wondered if you couldn't
> >> run a PC directly off of 12VDC (with a regulator to drop one line to
> >> 5VDC).
>
> small cordura camera bag filled with two surplus 12v 7ah lead acid
> batteries (in parallel) and some switching gear for the scope and the PC.
>
> 	I don't know...maybe I'm looking for something that really doesn't exist.
> I really wanted to avoid building my own supply, though.
>
> Rich

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From: Joe <[email-masked]>
Date: Sat May 17 07:08:21 2003

>   Can 5v regulators just be stacked up to get

   Huh? a standard 7805 is good for 1 amp and there are other 5V regulators
that can handle more current than that. And yes they can be ganged together
but it's best to put a resistor in series with each one to try and prevent
current hogging. Don Lancaster shows this in his TTL Cookbook but I'm sure
there are lots of other sources for info too.

    Joe

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From: Kevin Handy <[email-masked]>
Date: Sat May 17 15:24:00 2003

>      go first, then the rest after it's dead.
Much better to use one of the circuits that come with most
data sheets that show how to increase power output using
additional components (power transistors).

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From: ben franchuk <[email-masked]>
Date: Sat May 17 17:49:00 2003

> additional components (power transistors).

A switching regulator is better for 12 volts to 5 or less volts.
Ben.

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From: [email-masked] (Tony Duell)
Date: Sat May 17 18:32:01 2003

>    Can 5v regulators just be stacked up to get

It's not generally a good idea to run regulators in parallel to increase 
the current rating, unless they're designed for this. If they are, then 
the data sheet will indicate this (and will probably give an example 
circuit). 

However, (a) 3-terminal 5V 5A regualtors exist and (b) the data sheet of 
many 3 terminal regulators shows how to increase the current capability 
using external transistors.

That siad, remember that linear regulators are very inefficient (power 
equal to the load current mulitiplied by the different between the input 
and output voltages goes out as heat), and that you might be better 
considering a swtiching regulator for such currents.

-tony

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From: "John Allain" <[email-masked]>
Date: Sat May 17 19:47:00 2003

> the data sheet of many 3 terminal 
> regulators shows how...

FWIW all the 7085's (and 7905's, whatever they
are) that I get came w/o datasheets.
Guess I picked them up too cheap.

John A.

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From: Joe <[email-masked]>
Date: Sun May 18 10:01:00 2003

At 08:44 PM 5/17/03 -0400, you wrote:
>+AD4- the data sheet of many 3 terminal 
>+AD4- regulators shows how...
>
>are)

   The 7905s are minus 5 V regulators. It uses - voltage input (say -12VDC)
on the first pin, return for both the input V and output V on the second
pin and outputs a regulated minus 5VDC output on the third pin. IF your
power input is floating (no common ground) you can use a 7805 (+5 VDC)
rgulator to do the same thing. Connect -Vin to pin 2, +Vin to pin 1, +Vout
to pin 3 and -Vin to pin 2.  The important thing to remember with this
circuit is that the input and out sides do not have a common ground. That's
not a problenm in some applications but it is in others.

 that I get came w/o datasheets.
>Guess I picked them up too cheap.

   You hardly need a data sheet for a 78xx or 79xx. They're extremely easy
to use. Once you'd used them once you should need to look a a data sheet
again.

  Joe

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From: [email-masked] (Tony Duell)
Date: Sun May 18 17:01:00 2003

> FWIW all the 7085's (and 7905's, whatever they

The 7905 is the -ve version -- the +ve rail is the common ground between 
input and output (with the 7805 it's the -ve rail that's common). Be 
warned that the pinout is different between the 7805 and 7905...

> are) that I get came w/o datasheets.

So do mine -- I use plenty of 7805s, but I only need one data sheet :-). 
I bought the National Semiconductor power devices databook, which 
includes this datasheet. I suspect you can also get the datasheet as a 
.pdf file on the web somewhere.

-tony