Sea-Bird Electronics SBE 45 MicroTSG, User Manual

Page: 21 / 143

Section 2: Description of MicroTSG SBE 45
13
Power and Cable Length
There are two issues to consider:
•
Limiting the IR loss to 1 volt; higher IR loss will prevent the
instrument from transmitting real-time data because of the difference in
ground potential.
•
Supplying enough power at the power source so that sufficient power is
available at the instrument after considering IR loss.
Looking at each issue separately:
Limiting IR Loss to 1 Volt
The limit to cable length is typically reached when the maximum current times
the power common wire resistance is more than 1 volt, because the difference
in ground potential of the MicroTSG and ground controller prevents the
MicroTSG from transmitting real-time data.
V
limit
= 1 volt = IR
limit
Where I is the current required by the MicroTSG (34 milliamps at 8 VDC;
see Specifications above).
Maximum cable length = R
limit
/ wire resistance per foot
Supplying Enough Power to MicroTSG
Another consideration in determining maximum cable length is supplying
enough power at the power source so that sufficient voltage is available, after
the IR loss in the cable, to power the MicroTSG.
Example – For 18 gauge wire, what is the maximum distance to transmit power to the MicroTSG?
If this the controlling factor for maximum cable length if wanting to transmit at 1200 baud?
For 34 milliamp current, R
limit
= V
limit
/ I = 1 volt / 0.034 amps = 29 ohms
For 18 gauge wire, resistance is 0.0064 ohms/foot.
Therefore, maximum cable length is 29 ohms / 0.0064 ohms/foot = 4531 feet = 1381 meters.
Note that 1381 meters > 800 meters (maximum distance MicroTSG can transmit data at 1200 baud), so IR loss is not
controlling factor for this example.
Example 2 – Same as above, but there are 4 MicroTSGs powered from the same power supply.
R
limit
= V
limit
/ I = 1 volt / (0.034 amps * 4 MicroTSGs) = 7.35 ohms
Therefore, maximum cable length is 7.35 ohms / 0.0064 ohms/foot = 1148 feet = 350 meters (this is cable length to
MicroTSG furthest from power source).
Note:
Common wire resistances:
Gauge Resistance (ohms/foot)
12 0.0016
14 0.0025
16 0.0040
18 0.0064
19 0.0081
20 0.0107
22 0.0162
24 0.0257
26 0.0410
28 0.0653
Example – For 18 gauge wire, what is the maximum distance to transmit power to the MicroTSG if using a 8.5 volt power
source to supply power?
Is this the controlling factor for maximum cable length if wanting to transmit at 1200 baud?
MicroTSG’s input power specification is 8 – 30 volts. Therefore, a 0.5 volt IR drop (8.5 volts – 8 volts) would still provide
enough power to MicroTSG.
V = IR 0.5 volts = (.034 amps) * (0.0064 ohms/foot * cable length) Cable length = 2297 ft = 700 meters
Note that 700 meters < 800 meters (maximum distance MicroTSG can transmit data at 1200 baud), so IR drop in power
is controlling factor for this example. Using a higher voltage power supply or a different wire gauge (12, 14, or 16 gauge)
would provide sufficient power at MicroTSG to allow an 800 meter cable length.
Example 2 – Same as above, but there are 4 MicroTSGs powered from the same power supply.
V = IR 0.5 volts = (.034 amps * 4 MicroTSGs) *(0.0064 ohms/foot * cable length)
Cable length = 574 ft = 175 meters (this is cable length to MicroTSG furthest from power source).
18