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'power supply [OT]'
1999\05\25@120448 by Bernhard Kraft

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Hi,

A few days ago someone of you posted that he had built a power supplay
which is able to supply +12V, -12V +5V, -5V from a car power supply (12V).
Unfortunatly I deleted this message. But I need such a supply so I would
be happy if you post this message again with the URL for the schematics (I
think there was a URL)

thx,
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'power supply [OT]'
1999\11\01@080719 by Michael Rigby-Jones
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part 0 2124 bytes
<P><FONT SIZE=2 FACE="Arial">I usually use DC 'wall warts' - Interesting!&nbsp; I hadn't measured an AC</FONT>
<BR><FONT SIZE=2 FACE="Arial">one, I just let someone else use those, usually.&nbsp; I didn't anticipate</FONT>
<BR><FONT SIZE=2 FACE="Arial">any real drop in voltage - guess I forgot about internal resistive</FONT>
<BR><FONT SIZE=2 FACE="Arial">losses inside the transformer, which is already part of why a DC</FONT>
<BR><FONT SIZE=2 FACE="Arial">plugpack &quot;peaks&quot; sans load.&nbsp; Need a vacation here, obviously &lt;G&gt;</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">I used to use a DC Lump, put it through a reverse protection diode for</FONT>
<BR><FONT SIZE=2 FACE="Arial">safety &lt;G&gt;, then a fuse &amp; Zener for over-voltage protection (cheaper:</FONT>
<BR><FONT SIZE=2 FACE="Arial">Use the reverse protection diode AS a fuse, not ideal but cheap!</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">&lt;snip&gt;</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">I used to repair car audio equipment, and most of the cheaper head units used a 1N400X wired accross the supplies for reverse polarity protection, relying on the in-line fuse to blow.&nbsp; Of course, lots of people don't bother with the fuse, or wrap it in tin foil, or put a 6 inch nail in.&nbsp; Under these conditions I have seen many of these little rectifiers fail to a short cicuit mode, the PCB traces then (usualy) take the place of the fuse.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Regards</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Mike Rigby-Jones</FONT>
</P>

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1999\11\01@134035 by bill

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When attempting to use that same technique to protect a high-powered
automotive accessory from reverse polarity, we (meaning myself and two
other engineers working on the project) discovered that big fuses are
horribly inaccurate, usually erring on the high side.  These were 100A
fuses, and in our test setup would usually survive 180 to 300A for many
seconds. They varied all over the place, even among fuses from the same
manufacturer.  Diodes capable of reliably blowing the fuses were too large
and expensive for this application.

So, we tested the device to see what would happen when someone
connected it backwards. The main concern was several large-ish electrolytic
caps.  Placing these in a metal bucket and connecting a car battery
backwards yielded a pretty strong bang with lots of debris in the bucket.
The 100A fuse was unharmed.  Definitely not the kind of thing we wanted to
happen to someone who's installing the thing in their car.  However, once
sealed in the product's extruded aluminum enclosure and epoxy
encapsulant, the results were not as bad.  It made a pop, cracked the
epoxy, and emitted a little smoke, but no projectiles or flames in any of the
tests using fully assembled devices.  Still not pretty, but much less likely to
cause injury.  The product was otherwise quite robust and could survive just
about any other kind of wiring error, short circuit, or other abuse.

The final decision was to leave the reverse polarity protection out and apply
really obvious warning labels. The connections were clearly marked with "+"
and "-". There's only so much idiot proofing you can reasonably do.

{Quote hidden}

---
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                                       William Kitchen
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The future is ours to create.

1999\11\01@134653 by paulb

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Michael Rigby-Jones wrote:

> Of course, lots of people don't bother with the fuse, or wrap it in
> tin foil, or put a 6 inch nail in.  Under these conditions I have seen
> many of these little rectifiers fail to a short cicuit mode, the PCB
> traces then (usualy) take the place of the fuse.

 One learns not to pay money for second-hand automotive electronics.
Something to be said for putting the fuse internally - a blown fuse
invalidates the warranty.

 But then that's equally true of the PCB traces, so I suppose the
external fuse is as good as any.  It gives the user one chance to get it
right.
--
 Cheers,
       Paul B.

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