AN24 & AN28 Annunciators
Instruction Manual
10
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:
RELAY 1
NC
NO
C
C
R
C
R
+
-
L
O
A
D
Figure 12. AC and DC Loads Protection
Choose R and C as follows:
R: 0.5 to 1
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
annunciator
's relay
screw terminals. An RC network may also be
installed across the load. Experiment for best
results.
RELAY 1
NC
NO
C
+
-
L
O
A
D
Figure 13. Low Voltage DC Loads Protection
RC Networks (Snubbers) Available
from Dwyer
RC networks are available from Dwyer and should be
applied to each relay contact switching an inductive
load. Part number: PMA-09.
External Input Types
The external Silence (SIL), Acknowledge (ACK), and
Reset (RST) inputs may be contacts (i.e.
pushbuttons, relay contacts, etc.), open collector
inputs (OC), or TTL signals. However, throughout this
manual, the term “pushbutton” is used to describe
these external inputs. Please refer to the chart below
for signal logic.
External Input Type
Active When
Switch/Contact
Closed
Open Collector (OC)
On
TTL
Low
Multiple Unit First-Out Indication
Connections
Multiple AN28 and AN24 units can keep a single first-
out channel indicated for all connected units. An
unlimited number of units may be connected. It is
recommended that all units follow the same
sequence.
The following figure shows shared first-out indication
wiring connections for 3 units. Up to 200 units may be
connected for first-out indication.
UNIT 2
F
I
SIL
RST
ACK
COM
FO
F
I
SIL
RST
ACK
COM
FO
UNIT 1
UNIT 3
F
I
SIL
RST
ACK
COM
FO
Figure 14. System with Multiple Units Share First-Out
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.
