READER-TO-READER TECHFORUM
>>> QUESTIONS
3D Printer Materials Reference
I’m designing an RC dune buggy
using a 3D printer, but I can't decide
which material to use. I want it to be as
light and strong as possible, but I need
a reference that shows (for example)
the strength of an I-beam made of ABS
vs PLA. Does anyone know of such an
engineering reference?
#5141 Peter Murphy
Pensacola, FL
Thermocouple Switcher
I want to read and log temperatures from multiple points in my
(unnecessarily complicated) central
heating system. I want to use thermocouples because: 1) they are easy to
attach to my hot water cylinder
(stabbed under the insulation); and 2)
I already have them — from IKEA meat
thermometers.
I would like to select each thermocouple in turn, use a single amplifier
/analog-to-digital stage (Max31855 ??)
to obtain the reading, and move on to
the next thermocouple.
I need ideas for switching
between the different thermocouples
without introducing unwanted switch
resistances, etc. Would an array of
reed switches driven by a shift register
be a sensible solution? Is there a
better solid-state solution?
#5142 Mike O'Hagan
Aberdeen, Scotland, UK
Dimmable LED Bulbs
I've seen replacement LED light
bulbs in the hardware stores that claim
they are 'dimmable.' Is this a real
feature that has a different internal
design or is it just a way to get me to
pay extra for an LED bulb? If there is a
difference, what are they doing circuit-wise to make them dimmable?
#5143 Robert Parsons
Detroit, MI
What Wall Wart?
While on vacation, I managed to
lose the power supply wall wart to a
cheap vintage portable no-name
brand shortwave radio. On the back of
the radio, the power jack says 9 VDC
and has a C shaped circle and a dot in
the center with a plus sign on the dot.
I’m pretty sure this means nine volts;
positive tip. However, what it doesn't
state is the milliamp rating. If I use a
power supply that has too high or low
an amperage rating, am I in danger
of damaging the radio? How can I
determine the right supply?
#5144 Lyle Gardner
Vista, CA
Pot Cleaning
I recently purchased a small "pig-nose" guitar amplifier at a neighborhood garage sale. I put batteries in it
and it works, but the volume knob is
very "scratchy" and at certain places
in its rotation, the sound cuts out
entirely. Is this something I could fix by
cleaning the potentiometer and if so,
what would I use? If it's time for a
replacement, does anyone know
where I could find a schematic for this
unit?
#5145 Emanuel Estrada
Denver, CO
[#3141 - March 2014]
Motor Kickback
I am using an Ametek 965922-101
brushed DC motor (rated 38V nominal,
12A peak) in a project that runs it as
both a drive and a brake. (Interestingly,
the 38V rating is a "bus rating," i.e.,
with the motor stalled at 38V, it will
use 12A and get nothing done.) In
drive mode, the controller typically
runs it up to 60V; current limited to
10A. Works great.
For the brake, I am using an
IRFB260 MOSFET with a 0.6 ohm
spring in series to limit the surge
current. The PWM rate is 120 Hz.
(NOTE: When sinking 1/2 HP of
energy, the motor generates 30V.)
However, each time the MOSFET turns
off (at 120 Hz), the motor provides a
huge kickback, and the MOSFET's
reverse-protection zener diode probably
will not survive that for very long.
At first, I paralleled the motor with
an Elite 100 µF 400V capacitor [marked
PM 105°C, (M 9305)] which calmed
down the splash nicely. Then, one day
after some generous usage, a huge
cloud of smoke boiled out of a
severely melted capacitor. It was a very
abrupt introduction to the (unmarked)
ripple current rating on capacitors. At
the moment, I'm just using the same
4,700 µF 100V bolt-mounted huge
capacitor that filters the drive circuit.
(When using the 100 µF capacitor, one
of the relay contacts would switch the
4,700 µF capacitor out of the circuit.)
However, the 0.64 ohm brake circuit
discharges the capacitor so quickly
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>>> ANSWERS
76 May 2014