Page 152 - Programmable Logic Controllers, Fifth Edition - Mobile version

On-delay symbols O -delay symbols
or or or or

Normally open, timed Normally closed, timed Normally open, timed Normally closed, timed
closed contact (NOTC). open contact (NCTO). open contact (NOTO). closed contact (NCTC).
Contact is open when Contact is closed when Contact is normally Contact is normally
relay coil is de-energized. relay coil is de-energized. open when relay coil closed when relay coil
is de-energized. is de-energized.
When relay is energized, When relay is energized,
there is a time delay in there is a time delay in When relay coil is When relay coil is
closing. opening. energized, contact energized, contact
closes instantly. opens instantly.
When relay coil is When relay coil is
de-energized, there is de-energized, there is
a time delay before the a time delay before the
contact opens. contact closes.

Figure 7-3 Timed contact symbols.

Timing diagram L1 L2
L1 L2 10 s Timing diagram
S1 S1 10 s
TD
TD 10 s
Input Input
L1 (S1) On TD1 L1 (S1)
TD1 On
Output O Output
(L1) (L1) O

Figure 7-4 On-delay timer circuit that uses a normally Figure 7-5 On-delay timer circuit that uses a normally
open, timed closed (NOTC) contact. closed, timed open (NCTO) contact.

• When S1 is closed, TD coil is energized and the Figure 7-6 shows an off-delay timer circuit that uses a
timing period starts. TD1 contacts are delayed from normally open, timed open (NOTO) contact. The opera-
closing so L1 remains off. tion of the circuit can be summarized as follows:
• After the 10 s time-delay period has elapsed, TD1 • With S1 initially open, TD coil is de-energized so
contacts close and L1 is switched on. TD1 contacts are open and light L1 will be off.
• When S1 is opened, TD coil is de-energized and • When S1 is closed, TD coil is energized and TD1
TD1 contacts open instantly to switch L1 off. contacts close instantly to switch light L1 on.
Figure 7-5 shows an on-delay timer circuit that uses a • When S1 is opened, TD coil is de-energized and the
normally closed, timed open (NCTO) contact. The opera- timing period starts.
tion of the circuit can be summarized as follows: • After the 10 s time-delay period has elapsed, TD1
• With S1 initially open, TD coil is de-energized contacts open to switch the light off.
so TD1 contacts are closed and light L1 will
be on. Timing diagram
L1 L2
• When S1 is closed, TD coil is energized and the S1 10 s
timing period starts. TD1 contacts are delayed from TD
opening so L1 remains on. 10 s Input
L1 (S1) On
• After the 10 s time-delay period has elapsed, TD1 TD1
contacts open and L1 is switched off. Output O
(L1)
• When S1 is opened, TD coil is de-energized
and TD1 contacts close instantly to switch Figure 7-6 Off-delay timer circuit that uses a normally
L1 on. open, timed open (NOTO) contact.

Programming Timers Chapter 7 133

pet73842_ch07_131-155.indd 133 05/11/15 4:21 PM