Transcript Relying on Relays

Seal-in ladder rung is logical
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Pre-presentation Self Assessment Activity
Your comfort level with your
responses to the following 2
question assessment tool should
indicate if the presentation that
follows will increase you
knowledge base on the topic
outlined by the questions in this
tool.
Pre-presentation Self Assessment Activity
1) What is the primary disadvantage of the following circuit.
PB 1
LB 1
MR 1
M1
CR 1
2) What is the primary disadvantage of the following circuit.
PB 1
CR 1
LB 1
MR 1
M1
Logic Circuits
for
seal in control
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
voltage
MR 1
+
NO
0
current
M1
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
NO
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
LB2
voltage
MR 1
+
CR 1
NO
0
current
M1
Ladder diagram of circuit
PB 1
CR 1
LB 1
LB2
MR 1
M1
NO
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
LB2
voltage
MR 1
+
CR 1
NO
0
NO
current
PB 1
CR 1
LB 1
LB2
Motor circuit is
energized.
M1
Ladder diagram of circuit
MR 1
M1
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
LB2
voltage
MR 1
+
CR 1
NO
0
NO
current
PB 1
CR 1
LB 1
LB2
Motor circuit
stays energized.
M1
Ladder diagram of circuit
MR 1
M1
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
voltage
MR 1
+
CR 1
NO
0
current
M1
Ladder diagram of circuit
PB 1
CR 1
LB 1
MR 1
M1
NO
Seal in review
NO
PB 1
LB 1
0
Cartoon of circuit
voltage
MR 1
+
CR 1
NO
0
NO
current
PB 1
CR 1
LB 1
Motor circuit is
energized.
M1
Ladder diagram of circuit
MR 1
M1
Seal in review
Push button is
disengaged.
NO
PB 1
LB 1
0
Cartoon of circuit
voltage
MR 1
+
CR 1
NO
0
NO
current
PB 1
CR 1
Note:
LB 1
Motor circuit
stays energized.
M1
Ladder diagram of circuit
MR 1
M1
Ladder diagram shows that the circuit can still
provide a continuous path from the left to right
through the relay.
The motor relay circuit has been “sealed in”.
a) The magnetic field closed the CR1 contacts.
b) The ladder diagram is never drawn such that a set of normally open contacts is
shown closed. It is your minds eye that sees them closed.
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
MR 1
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
CR 1
Note:
a) The magnetic field opens the CR1 contacts and closes the MR1 contacts.
b) The ladder diagram is never drawn such that a set of normally closed contacts is
shown open. It is your minds eye that sees them open.
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
MR 1
NO
current
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
CR 1
Note:
a) The magnetic field will now reopen the CR1 contacts and kept the MR1 contacts closed.
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
MR 1
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
CR 1
Note:
a) The inductor circuit is now open and the magnetic field will collapse allowing the
CR1 and MR1 contact sets to return to there normal position.
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
MR 1
NO
current
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
CR 1
Note:
a) However, now the magnetic field reopens the CR1 contacts, closes the MR1 contact
set and the oscillations continue as long as the push button is engaged.
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
CR 1
MR 1
M1
MR 1
NO
Relay oscillation
PB 1
NC
LB 1
0
voltage
Cartoon of circuit
CR 1
+
MR 1
NC
0
current
Ladder diagram of circuit
PB 1
LB 1
MR 1
M1
CR 1
Note:
a) As long as PB1 is in its normal state, the magnetic field will put CR1 and MR1 in
there non-normal state.
b) CR1 will be open and MR1 will be closed.
NO
Relay oscillation
NC
PB 1
LB 1
0
voltage
CR 1
+
0
Cartoon of circuit
current
MR 1
NC
NO
PB 1
LB 1
MR 1
M1
CR 1
Ladder diagram of circuit
Observations
When PB1 is not engaged the motor circuit associated with MR1 is energized.
The ladder diagram circuit is always drawn with the CR1 in the normal state,
which in this circuit is the closed condition.
When PB1 is engaged, the relay is unstable and will try to oscillate as long as
the the button remains engaged.
“Seal in” as logic diagram
NO
PB 1
LB 1
0
Cartoon of circuit
voltage
MR 1
+
CR 1
NO
0
current
M1
Logic diagram
PB1
CR1
MR1
NO
“Seal in” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
PB1
CR1
MR1
“Seal in” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
1
PB1
CR1
1
1
1
MR1
“Seal in” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
1
PB1
CR1
1
1
1
MR1
“Seal in” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
1
PB1
CR1
1
1
1
MR1
Light LB1 stays on now no
matter if the push button is
engaged or disengaged.
“Seal in” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
1
PB1
CR1
1
1
Relay circuit attached to MR1
also stays on no matter what
the button position.
1
MR1
Light LB1 stays on now no
matter if the push button is
engaged or disengaged.
Seal “Out” as logic diagram
PB 1
NO
LB 1
0
Cartoon of circuit
voltage
MR 1
+
CR 1
NC
0
NO
current
Ladder diagram
PB 1
CR 1
LB 1
MR 1
M1
Seal “Out” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
PB1
CR1
1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
PB1 1
CR1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
PB1
CR1
1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
PB1 1
CR1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
LB 1
MR 1
M1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
PB1
CR1
1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
LB 1
MR 1
M1
The relay contacts, MR1 are
always energized no matter
what the status of the push
button.
PB1 1
CR1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
LB 1
MR 1
M1
The relay contacts, MR1 are
always energized no matter
what the status of the push
button.
PB1
CR1
1
1
MR1
Seal “Out” as logic diagram
Ladder diagram
PB 1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
LB 1
MR 1
M1
The relay contacts, MR1 are
always energized no matter
what the status of the push
button.
PB1 1
CR1
1
MR1
As suggested earlier, by itself this
ladder rung is not very valuable, but
there are times when a modified
version of this logic is important.
Seal “Out” as logic diagram
Ladder diagram
PB 1
CR 1
Logic diagram
PB1 CR1 LB1
1
1
LB 1
MR 1
M1
The relay contacts, MR1 are
always energized no matter
what the status of the push
button.
PB1
CR1
1
1
MR1
As suggested earlier, by itself this
ladder rung is not very valuable, but
there are times when a modified
version of this logic is important.
Seal review
Ladder diagram
Seal-in
PB 1
LB 1
MR 1
LB 1
MR 1
M1
CR 1
Seal-out
PB 1
M1
CR 1
Problems
As they are now, neither of these two ideas is very comforting. The top ladder
diagram depicts a situation that does seal-in the MR1 contact related circuit but
provides no way for it to be de-energized after PB1 is pushed. While the seal-out
circuit shown isolates the MR1 circuit from anyone pushing PB1 it provides no way
to allow additional interaction with the MR1 circuit.
Post-presentation Self Assessment Activity
1) What is the primary disadvantage of the following circuit.
PB 1
LB 1
MR 1
M1
CR 1
Once the button energizes the MR1 relays, there is no way to
de-energize that circuit.
2) What is the primary disadvantage of the following circuit.
PB 1
LB 1
MR 1
M1
CR 1
As is, there is no way to affect the MR1 circuit by any action of
the push button.
End of Presentation