Subject: Re: [linux-audio-dev] OT: Electronic advice for PC.
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Tue Apr 30 2002 - 16:01:52 EEST
On Tuesday 30 April 2002 02:39 pm, Patrick Shirkey wrote:
> >1.) Required voltage. What voltage is needed? If this is going to be
> > driving an inverter, 12V is typical.
> Why would I need an inverter if the power level from the batteres are
> correct in the first place?
> From my research so far I have estimated that the machine would draw about
> 12v maximum.
> Can anyone confirm or deny whether this is correct?
> The machine is:
> possible pci soundcard for pro recording.
Is this a typical motherboard (ATX) or is this something oddball? My notebook
requires 19.5V at the DC input -- but there are inside-the-case DC-DC
inverters that transform to the required internal voltages.
A standard MB takes +12V, -12V, 5V, and 3.3V, plus a signal back to the power
supply called 'power good' (and then there's the turn-on lead). Standard
desktop-style hard drives, floppies, and CD's take 12V and 5V. Notebook HD's
take 5V only. Newer AGP stuff takes 1.5V.
Do you have an existing AC power supply? If so, you need to actually measure
the current consumed (and the voltage at which that current draw occurs)
before you can size a battery. How do you plan to charge the battery?
> >2.) Required current. At the required voltage, how much current does the
> > PC draw? This is necessary for the next criterion:
> Does anyone know what this would be based on the above hardware?
You have to measure this with a multimeter. In case you have never done
current measurements before, the basic concept is to place the ammeter in
series with the power supply. This can be difficult, depending upon the
supply. But, if you can get to one leg of the supply by itself, clamp-on DC
ammeters are available that will allow you to get an accurate reading without
messing with the wiring.
Typically, if the machine is designed to run off AC, it's easier to buy an
inverter and take the measurements on the DC going to the inverter. As a
data point, Best Buy and others are selling a cute little 75W inverter made
by APC as an 'automotive and airborne notebook power supply' -- just plug
your AC supply into the handy 120V outlet, plug the cigarette lighter socket
(or the airplane-compatible pigtail, which is included) into 12V, and start
computing. The APC inverter is much less expensive than the DC-input
supplies made by the notebook manufacturers.
> >3.) Required battery run time. How long do you want the battery to provide
> I would say 6 hours would be about right. Most performances don't last much
> longer than that. Of course more would be better but this thing has to be
> easily lifted - eg shoulder strap - as I expect people will want to use it
> in the field.
Ok, once we settle the current draw and operating voltage issues, you're well
on your way.
Supposing your machine draws 2A_AT_12V, you would need a 12AH battery -- two of
those 'Power Wheels' batteries would be just about right, in parallel. Quite
totable, too. As another aside, I carry such an outfit with me all the time
-- two Power Wheels 7AH 12V batteries plus a 120W inverter, all in a military
surplus ammo case (7.62mm NATO, the small case). I'm an amateur radio
operator (KF4MYT), and often need to run my Yaesu FT90 mobile radio outside
the car. The ammo-pack power-pack does the trick without the inverter. It
can also run my notebook with the inverter.
You may find that the current draw of a typical desktop MB is very high.
> Another question that I'm unsure of is whether having a battery that is too
> powerful for the machine is going to cause problems or is that not possible
> because the machine will just run for longer? Or is that where an inverter
> comes in?
A battery that has too much ampacity for the charger used is the biggest
concern. As long as the voltage(s) isn't too high, you should be OK.
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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