pole single throw; two
Since the contacts are
they can operate either
an AC or a DC load.
Note the high current
and voltage ratings as
well: 12 amps for up to
240V AC 60 Hz loads,
and 12 amps for up to
30V DC loads.
They are very
reliable, and the contacts
are very forgiving
for brief periods
of time when
compared to the
used in SSRs.
Relays are quite
compared to SSRs
that can handle
the same currents and voltages. Figure 7 shows how an
EM relay might be connected to a 120V AC load and
controlled with an SSR.
As far as matching real world input voltage levels and
types to the MyRIO, there are devices called input
modules which are like SSRs in reverse; see Photos 13
and 14. Like SSRs, because of the circuitry inside of them,
they are designed to handle either a DC voltage or an AC
voltage, but not both. These items are available from
www.alliedelec.com for just under $30 each. The yellow
AC input module, IAC5, can take from 90V AC up to
140V AC and convert it down to a 5V TTL
level signal. It also uses optical isolation to
prevent high voltage levels from getting back
into the output (computer) side of things.
The white module, IDC5, can take from
3.3V DC up to 32V DC and convert that
down to a 5V TTL level signal as well. Figure 8
demonstrates how these input modules can be
connected to convert real world voltages
down to levels that the MyRIO can tolerate.
Please exercise extreme caution when
dealing with the high level voltages and
currents demonstrated in this article. However,
if you really want to control something, you will
need to learn how to safely interact with them.
SSRs will need to be used in the final
temperature control project that will be
demonstrated in the final article of this series;
see Photo 15. A 120V AC 60 watt
incandescent bulb will be used as a heater and
a 120V AC fan will be used as the cooling device.
The fourth article next month will examine inputting
analog voltages and converting them into digital numbers:
analog-to-digital conversion (ADC), and also converting
digital numbers back into analog voltage levels (
digital-to-analog conversions; DAC). ADC will be needed in the
final temperature control project to detect the analog
temperature, so decisions can be made as to when to turn
the heater and fan on and off. NV
February 2017 49
PHOTO 13. PHOTO 14.