Date: Sat, 6 Dec 2003 16:12:24 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Safety/Caveats Re: Making a 12v battery charger from a peecee
power supply
In-Reply-To: <BF942791-278E-11D8-AF00-000393DB2980@knology.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 08:51 PM 12/5/2003, Felder wrote:
>I've been talking with another listmemember about making a 12v batter
>charger from a pc supply according to some directions my dad found.
>Thought I'd go ahead and distribute to the group. The idea is to
>install this in my camper so when I'm plugged into 120v at a campsite,
>I'm charging my batteries automatically. My dad pointed me to a
>magazine article that detailed the process. Look for a 10 or 15 amp
>power supply to begin with.
That would be one rated to put out that amount of current at +12v. These
supplies typically have the greatest current ratings at the +5 output. The
-5 and -12 outputs are low-current.
>1. Make sure it works, if you can. My first one did not and I didn't
>know it until I had done all this (only takes 15 minutes or so, not
>counting the trip to radio shack if necessary).
Beware beware beware. Switching power supplies store remarkable amounts of
energy at significant voltages in their main energy-storage
capacitors. Depending on circumstances you could get a serious jolt --
several hundred volts -- for a long time after power is removed; so beware
of all metal on the board including heat sinks and such until you've
verified that they're safe. The caps themselves will be vertically mounted
cylindrical cans, likely black or light blue, could be a number of
half-inch-across or a pair of much fatter ones. I've got a couple of
supplies here with 1000-2000uF of capacity rated 200-250 volts. That
would give a maximum energy of about 60 watt-seconds -- call it a skinny
sixth of the jolt they use for defibrillation, the one that makes the
victim...er...patient leap so dramatically.
>2. Cut ALL the wire off at 6 inches.
>
>3. Remove the "motherboard"
>
>4. Locate the black ground wires and clip all but two close to the
>board.
Or three perhaps? Or a heavier one?
>5. Locate the +12 wires, and clip all but two close to the board.
Ditto...
>6. Locate the -5 stuff, +5 stuff, -12 stuff and anything else and cut
>it off as close to the board as you can.
>
>7. Take a soldering iron and open a hole in one of the cropped +5 wires
>and another at a ground hole.
>
>8. Now, you should be ready with two black grounds, two reds (or
>orange) +12v,
Supply I'm looking at uses red for +5, yellow for +12 and orange for the
"power good" signal to the PC motherboard.
> a hole at one ground and a hole at one +5v.
>
>Everything else should be gone.
>
>9. Now you are going to run a load between +5v and ground using the two
>open holes. The load is what the PS expects to see in a device such as
>a hard drive.
>
>10. The article suggests that you use either a 1 ohm, 25w resistor or
>an automotive light bulb type 1156 or similar to provide this load. It
>says that the light bulb is nice because you can read by it and it
>serves as a pilot light, but he actually ended up using a Radio shack
>50-ohm, 10W resistor (RS item # 271-133). That didn' t work for me, I
>went with a resistor somewhere close to the 1 ohm 25w which worked
>fine. He was worried about the inefficiency of the latter, I was not.
>The author warns that you may need to experiement as all the PSs are
>identical in many ways but subtly different in others.
Here's where I have a notion...call it a feeling...that considerable care
is called for. Basically you're taking a device that's intended to provide
a regulated output of 12.0 +/- 0.5 volts, and running it below the minimum
5v load that allows it to regulate properly for the specific purpose of
raising the +12 output voltage. That would be easy enough to take care of
if it was a linear device, but switchers are another breed altogether. I
suggest caution and thorough testing of sensitivity to input voltage,
output load, ambient and maybe fan discharge temp before trusting it out of
your sight. Scoping the output leads both +5 and +12 for signs of
instability might not be amiss. And do watch out -- they definitely bite.
david
--
David Beierl -- dbeierl@attglobal.net
