[ale] [OT]: one for the EE's...

[Calvin Harrigan]

On Mon, 2003-04-07 at 14:01, jemarasco at bellsouth.net wrote:
> Robert,
> 
> IIRC
> 
> Batteries have an internal resistance.  As such, even if you got the voltage right (which you 
> may not have as the short battery life might suggest a parallel arrangement) the amperage drawn 
> could still differ.  The device you connected probably had a much lower internal resistance than 
> those 4 AA batteries.  As a result the light drew way too much current, burned a little 
> brighter/hotter and burned out quicker.  See howstuffwork.com (howthingswork.com?) for details.
> 
> As a quick exercise, you can take a 1.5V AA battery and attach a 0.01 ohm (1 amp) fuse.  
> Theoretically the fuse would see 150 amps and blow.  The fuse will not blow because of the 
> batteries internal resistance (cross my fingers).  Car batteries have very low internal 
> resistance and that is why a 12 Volt car batter will start your car but 8 AA batteries won't 
> even move the starter. You could measure the voltage across the light with the four AA batteries 
> attached.  You could then buy a small potentiometer at Radio Shack (a few dollars), solder it 
> inline with your power source and then solder it to your night light and adjust the potentiometer 
> for the same voltage across the night light as measured earlier.  

I don't think a potentiometer alone will work to dim an incandesant
bulb.  It won't have the size to dissipate the heat that will build up
across it.  Say we have a 5 volt adapter at 1 amp, if it's a good
adapter the rectified voltage will be around 7 volts (5 x 1.4, the 1.4
is the peak-peak AC voltage through the bridge rectifiers with filter
caps).  This supports your idea of maybe having the bulb burn too
bright/hot.  The problem with a potentiometer from radio shack is that
it will probably be too small (physically) unless it's one of those
rheostats type wire wound ones.  

If the bulb draws 500ma - 1A at 6 volts the potentiometer will have to
dissipate:
 ~ (7-6)Volts * .5 amps = .5 watts to about (7-6)Volts * 1amp = 1 watt.

Most pots are used in very low current circuits where there current flow
is in high micro to low milliamps.

Note those adjustable gizmos to control fan speed for your PC, they tend
to be a pot and some supporting electronics (usually a transistor) to
control the fan speed by lowering the voltage.

I think the LED replacement might be a good idea, just wire 4 of them in
series and off you go.

Disclaimer:
All numbers used above were guesses (though educated), they can be
wrong, take with a grain of salt.  :-)

Calvin

> This should work without 
> sacrificing bulb life or luminocity. If it was my children's room I would buy one of those plug 
> in LED night lights (UL approved).  This would be more expensive though and wouldn't have the 
> stars, moon and a space shuttle.

> 
> Good Luck!
> 
> > 
> > From: "Robert L. Harris" <Robert.L.Harris at rdlg.net>
> > Date: 2003/04/07 Mon AM 10:03:15 EDT
> > To: Atlanta Linux Enthusiasts <ale at ale.org>
> > Subject: [ale] [OT]: one for the EE's...
> > 
> > 
> > 
> >   To stave off the fear of the dark I got two of those little push
> > lights for my kids.  Blue with stars, moon and a space shuttle, they're
> > pretty cool.  The only problem is the suckers will eat 4 AA batteries in
> > a single night if left on.  When the kids wake up in the middle of the
> > night and turn it back on, simply turning it off doesn't help.
> > 
> >   Last night durring the thunderstorm I cut the end off a 5vold, 1amp
> > power supply and soldered it to the battery leads.  It ran great for
> > atleast 3 hours before I went to sleep (GFCI outlet) but was quite dead
> > when we got up this morning.
> > 
> >   I had used the 5volt as I figured I needed 6 volts (4xAA) and that was
> > the closest, the next being 9.5volt.  As per the amperage I hoped it was
> > within tollerances, but it was $4 for a new light if I was wrong (oh
> > well).
> > 
> >   Any suggestions on the Volt/Amperage power adapter I should look for
> > when I go to wally world to replace this push light and get a proper
> > power supply?  Ideally I'd rather not pay $4 for a light, $20 for a
> > power supply (multi-voltage) that'll burn out overnight again.
> > 
> >   The lights are actually pretty cool...
> > 
> > Robert
> > 
> > 
> > :wq!
> > ---------------------------------------------------------------------------
> > Robert L. Harris                     | PGP Key ID: E344DA3B
> >                                          @ x-hkp://pgp.mit.edu 
> > DISCLAIMER:
> >       These are MY OPINIONS ALONE.  I speak for no-one else.
> > 
> > Diagnosis: witzelsucht  	
> > 
> > IPv6 = robert at ipv6.rdlg.net	http://ipv6.rdlg.net
> > IPv4 = robert at mail.rdlg.net	http://www.rdlg.net
> > 
> > 


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