READER-TO-READER TECHFORUM
>>> QUESTIONS
Wireless Networkable Digital
Picture Frame
I want to send digital pictures
and/or video remotely to a digital
picture frame located at my
Grandma’s house. The picture frame
should have a wireless network ability
so I can connect to it and update the
video/picture content. What circuits
or networking devices do this?
#5101 Michael Jimenez
Los Angeles, CA
Anti-theft Alarm
I would like to make a
simple/inexpensive anti-theft alarm —
like you see in the doorways of shops.
Some form of oscillator circuit that is
interrupted (disturbed) by a small
passive antenna-like circuit passing
through the field would be good.
The active range should be about
1-1.5 meters.
#5102
Claes Kamborn
Stockholm, SWEDEN
Load Controller, Micro Hydro
I built a 3,000W, 220V, micro
hydro machine and installed it for a
family of five living near a stream. I
need a load controller to offset the
extra power whenever something is
turned off. When the real load goes
down, the frequency and voltage go
up. The design should monitor the frequency ( 50 Hz) and dump some load
to resistors whenever the frequency
starts going up, and dump less of that
load when the frequency goes down.
If there is a better idea please let me
know. Is PWM a good option?
#5103 Joseph K.
La Jolla, CA
78 May 2010
Alternator Output
It is my understanding that
a standard automotive alternator
produces three phase AC before the
rectifier + (three diodes). If this is the
case, what would the voltage and
amperage output of each phase be, as
well as across two phases (tapped
directly off the windings)? I would
base measurements preferably using a
130 amp AC Delco CS130 alternator,
but any info will help. At this point, I
don't need any regulation, as I'm just
trying to find out average values.
[#12098 - December 2009]
Fuses
I have a machine that calls for a
3.15A 125V slow blow fuse. Will a
3.15A 250V slow blow fuse be wrong?
I can't find a 125V SB fuse. Do they still
make them?
>>> ANSWERS
You can use the 250V fuse.
To make it simple, fuses have
three parameters: max current before
it blows, rated in amps; response time
(fast blow vs. slow blow); and voltage
rating. The element inside the fuse will
heat up as current increases through
it, until it reaches its maximum rating.
Then, it will literally melt or "burn
open," stopping current flow — in your
case, 3. 15 amps. Your fuse being a
slow blow type, the element has a
built-in time delay to allow for current
inrush when you first turn on the
machine. Your machine may pull four
or five amps for a few milliseconds
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until it settles down to its normal
operating current. If you had a quick
blow fuse in place instead, it would
blow most every time you turned on
the machine — which would be a
nuisance.
Voltage ratings on the fuse
indicate the maximum voltage source
the fuse will clear. Cartridge fuses are
typically rated at 32, 125, 250, and
600 volts. When the element burns in
two, there is a gap. If the fuse voltage
rating is lower than the applied
voltage, current may arc across the
gap and continue to flow. Higher
voltage fuses blow larger gaps in the
element. So, if you used a 32 volt fuse
in your 120 volt application, it may not
clear and stop current flow when it
blows. However, the 250 volt fuse will
clear a large enough gap to interrupt
the 120 volt circuit.
Finally, some small cartridge fuses
have a glass body, others ceramic. In a
lot of instances, they are interchangeable (assuming electrical ratings are
the same). However, ceramic body
fuses are intended for installations
with higher heat and/or more
mechanical vibration. It's best to
replace the ceramic with ceramic, but
if not available — and the need is great
— you can get by on the glass until the
proper replacement is found.
Occasionally, a fuse will fail without an electrical overcurrent being
present. Heat or vibration can cause
the element to open, sometimes
under the endcaps where you can't
see it. If the fuseholder has dirty or
corroded contacts, they can generate
enough heat to fail the fuse. If you
keep blowing fuses and a bad fuseholder isn't present, there is probably a
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WHATSOEVER are made by the publisher.
The implementation of any answer printed
in this column may require varying degrees
of technical experience and should only be
attempted by qualified individuals.
Always use common sense and good
judgment!
