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

REF

REF OUT

+15V

COM

–15V

18

19

17

20

22

21

FB

INPUT

1B21

50V

0.1µF

GE

VARISTOR

V27ZA1

FUSE

1/16A (S.B.)

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.

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38

RECTIFIER

OSCILLATOR

REF

+15V

COM

–15V

18

19

17

20

22

21

FB

INPUT

1B21

1%

24.3k

Ω

BECKMAN 50k

Ω R/N

NOTES

1. NODES LABELLED FOR ILLUSTRATION ONLY.

IN2

IN1

OFFSET

4mA

COM

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.

1B21

38

1

1

1B21

38

Figure 8. Multiple 1B21s with Single Loop Supply