64 January 2016
READER - TO - READER TECHFORUM
All questions AND answers are submitted by Nuts &
Volts readers and are intended to promote the exchange
of ideas and provide assistance for solving technical
problems. All submissions are subject to editing and will
be published on a space available basis if deemed suitable
by the publisher. Answers are submitted by readers and
NO GUARANTEES 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!
>>> QUESTIONS
Clipping The Spike
I’m using an Arduino to
control a set of relays, and both the
microcontroller and relays share a
9V power bus. I plan to put a diode
across the relays to clip any reverse
voltage spikes. Is there anything else
I should consider to prevent a spike
from the relays from shutting down
the processor?
#1161 Ronald Miller
Kansas City, MO
Capacitor Forming
I’m trying to resurrect an old
Halicrafter’s communications receiver
from at least the ‘50s. I’m planning
to replace the electrolytic capacitors
in the power supply with capacitors I
salvaged from a more recent TV set.
However, I’ve read that
electrolytic capacitors — once formed
at a certain voltage — can take
months, if not years, to reform at a
new voltage. Until then, the capacitor
value can be significantly off from
what’s on the label.
Can anyone shed some light on
this, and any thoughts on whether I’ll
risk damaging the receiver if I use the
caps formed at the higher voltages
found in the TV circuit?
#1162 Nicolas Berger
Birmingham, AL
Switching To A Switching PS
I found a fantastic tube amp for
my stereo system at a garage sale,
What are the pros and cons? Any
advice would be appreciated.
#1163 John Ullrich
Santa Fe Springs, CA
>>> ANSWERS
[#8152 - August 2015]
Dim Bulb
What’s the difference circuit-wise
between a “dimmable” and a “
non-dimmable” 110V home LED light
bulb?
The non-dimmable variety use
a largish electrolytic capacitor in
the power supply to generate a DC
voltage, and then the LEDs are driven
with a constant current circuit. So,
because of the constant current, the
LED output is unaffected by supply
voltage, so the brightness won’t
change with voltage (or dimming).
Dimmable LEDs only have a small
film capacitor; the control circuit sets
the current according to the input
voltage (so they pulsate at mains
frequency) and when dimmed the
light output is chopped up similar
to an incandescent bulb. The above
applies to screw-in bulbs; larger LED
supplies may still use electrolytic
capacitors, but within these another
circuit measures the incoming duty
cycle and adjusts the LED current to
match.
Bob Turner
Salamander Bay, Australia
[#9152 - September 2015]
Is It Magic?
I have satellite radio (Sirius) aux
audio output connected to DLO
TransPod aux input. The TransPod is
normally used for iPod to FM radio in
a car. I am using it in my house.
The audio is transmitted on a
frequency to my FM radio ( 50 ft away)
with no power to the TransPod! How
is this possible?
It appears the output of the
Sirius unit is supplying “phantom
power” to the TransPod. IOW, there
is a small DC voltage (1-2 VDC)
present on the output signal that the
TransPod is using as its power source.
In a way, it is “magic” as phantom
power schemes are regularly used in
audio PA systems to supply power to
electret microphones without using
external batteries.
Ken Simmons
Auburn, WA
[#9153 - September 2015]
Geocache Container With
Flashing Light
I’m building a geocache
container. To make it look authentic,
I’d like the red light to flash every few
seconds. Unfortunately, I don’t have
any “Nuts & Volts” electronics skills —
what type of long-lasting battery and
low-power LED light to use — and am
hoping to get some expert advice.
Search online for “firefly circuit
7555” and you will find what you
want (or “unijunction transistor
flasher” for the old fashioned
method). The LED should be a high
brightness type with diffused lens, at
