P
RO
V
U
™ PD6100 Strain Gauge, Load Cell, and mV Meter
Instruction Manual
16
Shunt Calibration
The PD6100 is equipped with a means of simulating
strain in a strain gauge bridge circuit, via a shunt
resistor included inside the meter. This technique is
performed by enabling the “shunt resistor” switch
(switch 5), which in turn shunts one leg of a
connected strain bridge with a predetermined resistive
load (60.4k). This technique can be used as a means
of verifying instrumentation by simulating a physical
input. With no load connected, the enabling of the
shunt resistor (switch 5) will simulate approximately a
70% F.S. load in the case of a 350Ω Strain Bridge.
Figure 19. Shunt Resistor
Modbus RTU Serial
Communications
Serial communications connection is made to an
RJ45 connector labeled M-LINK. Use the PDA1232
for RS-232 interfacing or the PDA1485 for RS-485
interfacing. The same port is used for interfacing with
all expansion modules (
e.g.
external relays, digital
I/O).
Relay Connections
Relay connections are made to two six-terminal
connectors labeled RELAY1
– RELAY4. Each relay’s
C terminal is common only to the normally open (NO)
and normally closed (NC) contacts of the
corresponding relay. The relays’ C terminals should
not be confused with the COM (common) terminal of
the INPUT SIGNAL connector.
Figure 20. Relay Connections
Switching Inductive Loads
The use of snubbers to suppress electrical noise is
strongly recommended when switching inductive
loads to prevent disrupting the microprocessor’s
operation. The snubbers also prolong the life of the
relay contacts. Suppression can be obtained with
resistor-capacitor (RC) networks assembled by the
user or purchased as complete assemblies. Refer to
the following circuits for RC network assembly and
installation:
Figure 21. AC and DC Loads Protection
Choose R and C as follows:
R: 0.5 to 1
Ω for each volt across the contacts
C: 0.5 to 1 µF for each amp through closed contacts
Notes:
1. Use capacitors rated for 250 VAC.
2. RC networks may affect load release time of
solenoid loads. Check to confirm proper
operation.
3. Install the RC network at the meter's relay screw
terminals. An RC network may also be installed
across the load. Experiment for best results.
Use a diode with a reverse breakdown voltage two to three
times the circuit voltage and forward current at least as large
as the load current.
Figure 22. Low Voltage DC Loads Protection
RC Networks
(Snubbers)
Available
from Precision Digital
RC networks are available from Precision Digital and
should be applied to each relay contact switching an
inductive load. Part number:
Note: Relays are de-rated to 1/14th HP (50 watts)
with an inductive load.
EX+
EX-
SG-
SG+
C
NO
NO
NC
NC
C
RELAY4
RELAY3
4
3
6
5
2
1
C
NO
NO
NC
NC
C
RELAY2
RELAY1
4
3
6
5
2
1
C
R
C
R
