COMPUTER CONTROL and
by David A. Ward
Part 3: Matching Computer Signals to
Real World Signals
From the previous two articles, you learned how
to output and input digital signals using National
Instruments LabVIEW software and their
USB-6008( 9) hardware. However, the signals
have only been in the TTL range of voltages (0V
and +5V) and with fairly low current levels, 8. 5
mA maximum per digital terminal. In the real
world outside of the PC and the USB-6008( 9)
units, voltage and current levels are typically
much higher and also include AC signals. This
article will demonstrate some of the devices that
can be used to match these real world signals
with the USB-6008( 9) signal levels.
On the output side of things — where the PC and
the USB-6008( 9) units will be controlling real world
voltages and currents — solid-state relays or SSRs are
often used. SSRs take TTL types of inputs and use these to
control much larger AC and DC voltages with much higher
current demands. SSRs are usually built to control either AC
or DC voltages and currents; most cannot handle both.
SSRs offer an important safety feature for your interfacing
circuitry and your PC, as well. Their inputs from the control
circuit typically employ optical isolators.
Optical isolators separate the output portion of the SSR
from the input side with light as the only connection between
them. The light connecting the input and output sides of
the SSR cannot transfer harmful voltage spikes from the
output back into the input side, and SSRs offer voltage
isolation into the kilovolts.
When selecting SSRs
several specifications must
be considered. First, will the
SSR be controlling an AC or
a DC load? Second, at what
voltage will the load be
operating? Third, how
much current will the load
draw? And finally, what are
the input signal levels for
control? In this article, two
SSRs will be demonstrated:
a DC SSR and an AC SSR. Let’s look at the DC SSR first.
On the left side of Photo 1, you can see the Kyotto
KF0602D is a DC SSR that can control up to a two amp load
from 3 VDC up to 60 VDC. Its input signal can be from 3 VDC
up to 32 VDC which is within the capabilities of the USB-6008( 9)
digital outputs. Not listed on the package is its isolation voltage
from the output side to the input side which is 4,000 VRMS.
To demonstrate how this SSR operates, we’ll connect a
12 VDC 15W incandescent lamp as the load. This will draw
15W/12V or 1.25A which is below the SSR’s maximum
amperage. Connect the circuit as shown in Figure 1; notice
that the USB-6008( 9) terminal will be “sinking” rather than
“sourcing” the SSR inputs. That is when the digital terminal
goes low or to ground, the SSR will turn ON and when the
digital terminal is high or +5V, the SSR will be OFF. This is
the opposite of what the user would usually expect; we
merely need to invert the digital outputs in LabVIEW in the
DAQ Assistant setup to correct this.
Notice too that there are no electrical connections
between the input side of the SSR and the output side to
insure maximum electrical isolation. We won’t go into
building a VI to control the SSR at this time; you can use
the demonstration VI that was presented in the previous
article to test the SSR’s operation.
Make A Switch
To control AC loads, I have constructed a box containing
two AC SSRs to control two AC receptacles (see Photo 2).