READER-TO-READER TECHFORUM
Bosch. They have what are called
popits that are a single zone unit with
addressing. These use a four conductor daisy chain wire. You can add up
to a couple hundred of them to one
line. But, this looked like a good
project to me, so I came up with a
design and built a simple setup. To
have the doors share a bus, you
need intelligence at every door.
Microcontrollers (μC) are so
cheap these days, all I believe you
need are two chips’ worth at each
door. A simple μC such as the PIC
12F509 (less than $1) could do all the
work at each door in conjunction with
a bus interface chip based on RS-485
(e.g., LTC485 at $1.75).
No other interface chips are
needed. Using Cat 5 cable (additional
design work should be done to look at
resistance for the power transmission
and max distance), make one pair for
the RS485 bus, and use the three
other pairs for the 5V power from the
central site (triple up the wires for less
resistance). The max distance also
determines max transmission speed.
RS-485 allows 32 transceivers
(doors) on the bus.
A central μC would do poll and
response of each door and then
output the status to a set of latches
wired to the LED panel.
I built a two unit system in my lab,
wrote the code to cycle through the
addresses and included a simple error
checksum (only as far as proving the
concept), and it works fine. Two bytes
are sent with the address of the door
(the poll) and a checksum. Door
returns its address, checksum, and
status. So if you want to DIY, there's
an idea.
Pete Lunt
Fairfax,VA
[#2104 - February 2010]
AC Motor Control
I would like a diagram for a three-phase AC motor control that I can
build myself. I can find all kinds for DC,
but not AC. Could someone point me
in the right direction?
#1 Microchip has several evaluation boards for this purpose using their
80 June 2010
microcontrollers and DSP chips. Look
for the MC1.
Walter Heissenberger
Hancock, NH
#2 There is a lot of info at
www.irf.com. They have some PDF
files for simple motor controls and
they also have intelligent power
modules that do most of the work.
They all need a microcontroller,
though, to generate the drive signals.
Powerex is a good source too.
Henry H.
via email
[#2105 - February 2010]
Welder Conversion
I want to enhance my Lincoln 225
amp 220 volt AC welder with a circuit
that can change the 60 Hz AC to a
variable higher frequency. Welding
aluminum with a tungsten torch (TIG)
requires a variable frequency higher
than the 60 Hz to make the best weld
due to the nature of aluminum oxide
and alloy. It is suggested in my reading
that commercial TIG welder circuits
change the sine wave to a square wave.
My concept is to add an auxiliary
box to house the power supply for the
electronics, the cooling fan, and either
the cable to a foot switch or rheostat
mounted to the torch head or surface
mounting. A selector switch will turn
the TIG feature on or off to continue to
use the original welding feature or to
TIG weld.
I have found circuits where the
frequency can be varied, but at low
voltage and amperage. One article on
converting an AC to DC MIG welder
suggested the diodes in the rectifier
have a peak invert voltage of 1,000
volts for the momentary time when
striking the arc, and be able to handle
the amperage of the original welder —
225 amps.
#1 The simple answer is that there
is no simple answer! Sorry, but the
methods of achieving what you have
and what you want are quite different.
You have (I believe) a basic
transformer welder, operating at 60
Hz, with output power selectable by
secondary tapping points. What you
desire is a high(er) frequency welder
output with effectively the same
power. Unfortunately, you cannot supply your existing welder with anything
much different from 60 Hz, or you will
probably destroy the transformer. It
just wasn't designed to work at higher
frequencies.
Sally Jelfs
Brackley, England (UK)
#2 You can weld aluminum with
your welder as-is. The high frequency
is there to strike an arc; this keeps the
work from getting contaminated from
the electrode. You can scratch-start
and weld without high frequency.
The AC setting on your welder will stir
up the weld puddle and make the
impurities come to the top. You can
get a high frequency add-on box for
your welder and be able to start an arc
easier. Reference: The Procedures
Handbook of Arc Welding, Lincoln
Electric.
Frank
via email
