1B21
Rev. C | Page 7 of 8
APPLICATIONS INFORMATION
OUTPUT PROTECTION
In many industrial applications, it may be necessary to protect
the current output from accidental shorts to ac line voltages in
addition to high common-mode voltages and short circuits
to ground. The circuit show in Figure 6 can be used for this
purpose. The maximum permissible load resistance will be
lowered by the fuse resistance (typically 8 Ω) when protection
circuitry is utilized.
1
38
RECTIFIER
1N4002
1N4002
OSCILLATOR
REFREF OUT
+15V
COM
–15V
18
19
17
20
22
21
FB
INPUT
1B21
50V
0.1µF
R
LOAD
GE
VARISTOR
V27ZA1
FUSE
1/16A (S.B.)
LOOP
8530-004
Figure 6. Output Protection Circuitry
LOW DRIFT INPUT NETWORK
Figure 7 shows a configuration suitable for applications where
errors have to be minimized over a wide temperature range. A
temperature tracking network such as a 50 k Beckman (PN 698-
3R50KD) can be used to implement both offset and gain for
either 0 mA to 20 mA or 4 mA to 20 mA current loops. For
0 V to 10 V signals either IN1 or IN2 can be used for input.
For 0 V to 5 V signals, jumper In1 to In2. Similarly, for 4 mA
to 20 mA operation the 4 mA node should be jumpered to
OFFSET, while for 0 mA to 20 mA it should be tied to COM.
1
38
RECTIFIER
OSCILLATOR
REF
+15V
COM
–15V
18
19
17
20
22
21
FB
INPUT
1B21
1%
24.3kΩ
BECKMAN 50kΩ R/N
NOTES
1. NODES LABELLED FOR ILLUSTRATION ONLY.
IN2
IN1
OFFSET
4mA
COM
08530-005
Figure 7. Low Tempco Resistor Network Configuration
MULTILOOP ISOALTION
Multiple 1B21s can be connected to a single loop supply in
parallel as show in Figure 8. The amperage of the loop supply
should be sufficient to drive all the loops at full-scale output.
LOOP
1B21
R
L2
38
1
1
1B21
R
L1
38
08530-006
Figure 8. Multiple 1B21s with Single Loop Supply
