| About static electricity... |
| If you live in a dry (low humidity) environment, static electricity is a fact of life - especially in Winter. Putting on or taking off a sweater, walking across carpet or linoleum can generate 35 -50000 volts of static. This can easily zap a sensor, or the microcontroller inputs. When handling sensors, cables, or the
circuit board, it's best to wear a grounded wrist strap or, touch a ground point (eg: a
metal water pipe, or the ground point of an electrical outlet) before handling, and to minimze physical movement as much as possible.
This will bleed off any static you're carrying. If walking on carpet or lino, lift your feet, rather than sliding them. |
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| About electrical interference... |
|
Heavy duty electric motors have large start up currents, which can cause large
emi (electromagnetic interference) around them. If a sensor or sensor cable is
in the vicinity, it may be impacted by the emi. I know this can cause "noise" on the data signal, if the cable isn't properly shielded. But I'm not sure if it could actually stop a data signal.
See this article about data cabling and shielding. |
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| About relay life, and load capabilities... |
|
1. The small relays used on these boards may have a typical switch life of
100,000 operations (see here for an example of a
relay datasheet). If your project involves turning a pump on/off every few
hours, that's quite a long lifetime. If you're driving a flashing signboard that
blinks on and off every few seconds, that will be a short lifetime. It's a good
idea to do a bit of math to see what the expected life might be. This will help
you decide whether the relay board you've chosen is suitable for the intended
application.
2. The relays used in the kits vary from circuit
to circuit, some are rated at 15 amps, some less. The K108 board uses 15 amp
relays, but the relay switching circuits are actually rated for only 5
amps.
The limiting factor is more often the copper traces on
the circuit board, rather than the relay contacts themselves. The traces are not
heavy enough to carry a great amount of current. It may be possible to either
build up the solder along a trace or solder a heavy jumper wire on top of a
trace. But this will extend current capabilities only so far. It's better
to use the onboard relays to switch a second set of relays with heavy duty
contacts.
3. Relay contacts usually have 2 load ratings -- resistive
and inductive. A resistive load would be something like an
incandescent light bulb or an electric heater. An electric motor would be an
example of an inductive load.
In a general way, you can say an inductor is a storage
device (similar but opposite to a capacitor), whereas a resistor is not (doesn't store
energy). When power is removed from an inductor (eg: motor), the stored energy
reacts by creating what's called back emf. This back emf (aka voltage surge) can
cause arcing across the relay contacts, which will eventually lead to contact burnout.
Since resistors don't store energy, there is no
"reaction" when power is removed from a resistive load.
Relay contacts usually have a lower inductive load
rating because of this reactive effect. If you are going to be controlling a
pump or compressor motor, keep this in mind.
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| USB
- RS232 Adapters (Feb/09) |
| A user has reported
successful connection to a K145 with the Aten brand (www.aten.com)
of USB-Serial converter. |
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| Clock
Time shift (Feb/09) |
| If your system is
configured to periodically update the system clock from the Internet, system
time may jump by a few seconds, depending on the stability and accuracy of the
system clock. This time shift may appear in log timestamps for the K145,
K190, and VK011.
Note that Daylight Savings
time change will also affect log timestamps.
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| Put
a Capacitor at the Sensor on Long Cables (Feb/09) |
| Quozl's Community
Project page has a good discussion of the benefits of placing a capacitor
across the power connections at the temperature sensor. This is especially
important with the K190 because of the low resistance value of the pullup
resistors used. The low resistance means higher current levels being
switched by the temperature sensor when sending data. The combination of
higher current draw and resistance in the cable can cause enough of a voltage
drop so that the sensor might be starved for power, resulting in data errors. As
Quozl noted, a capacitor at the sensor acts as a reservoir to provide extra oomph
when the sensor is sending data.
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| Lightning
Protection (Mar/09) |
If you have sensors
(or K108 Inputs) on cables running outside, there may be the possibility of
damage to electronics from lightning strikes. One fellow has suggested
using fiber optics as a coupler between the board and the cable. The fiber
would act as a super insulator, preventing the high voltage charge from zapping
the electronics. The fiber needs driver electronics to function properly,
so this gets fairly complicated. You can see his circuit here:
http://home.hetnet.nl/~thomas_7/1wire/1wire_isolating.html
A simpler alternative is perhaps to disconnect all external cables when a storm
is approaching. |
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| K145
interrupted by new USB connection (Mar/09) |
| A user has reported
that plugging in a USB stick caused the K145 to disconnect. The K145
reconnected when the stick was removed. |
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| K190
as Temperature Alarm system (May/09) |
| Some alarm dialers
can be triggered by a relay contact closing. Such a dialer could be used
in conjunction with the K190 to send a high- or low-temperature alarm message by
phone. Rather than using the relay contacts to turn a heater on/off,
connect the contacts to the alarm dialer. To function as a
high-temperature alarm, set the channel for Cool mode, and the opposite for a
low alarm. Some dialers are multi-channel, allowing a different message to
be programmed for each one. This can be useful for temperature monitoring
at remote locations, where it's important to be notified if there's an alarm
condition. Note that channels used for alarms couldn't be used for
control, though. (Thanks to Greg Mitchell for the idea.) |
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| K190
Remote Disconnect Alarm (Dec/09) |
| The K190TCLite
program includes an alarm function that will trigger if the serial connection is
broken. When triggered, the alarm flashes in the main program window, and
a recurring chime sound is generated (assuming there's a sound card in the
system).
An FM wireless microphone could
be used to help create a remote alarm. For example, let's say you're
controlling incubator temperature out in the barn. You wont be able to
hear the alarm back in the house. What you can do is put an FM mic in
front of one of the computer speakers, and tune an FM receiver in the house to
the mic frequency. Wireless FM microphones are available in kit form from
several suppliers. A "Baby Monitor" would probably also work for
this.
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