LM1851N/NOPB National Semiconductor, LM1851N/NOPB Datasheet - Page 5

• Manufacturer Part Number: LM1851N/NOPB
• Description: IC INTERRUPTER GRND FAULT 8-DIP
• Manufacturer: National Semiconductor

Specifications of LM1851N/NOPB

• Applications: Ground Fault Protection
• Current - Supply: 19mA
• Voltage - Supply: 22 V ~ 30 V
• Operating Temperature: -40°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1851N; *LM1851N/NOPB; LM1851; LM1851N

Figure 2 we have
Application Circuits
A typical ground fault interrupter circuit is shown in Figure 2
It is designed to operate on 120 V
normal fault sensitivity
A full-wave rectifier bridge and a 15k 2W resistor are used
to supply the DC power required by the IC A 1 F capacitor
at pin 8 used to filter the ripple of the supply voltage and is
also connected across the SCR to allow firing of the SCR on
either half-cycle When a fault causes the SCR to trigger
the circuit breaker is energized and line voltage is removed
from the load At this time no fault current flows and the IC
discharge current increases from I
Description and Block Diagram) This quickly resets both
the timing capacitor and the output latch At this time the
circuit breaker can be reset and the line voltage again sup-
plied to the load assuming the fault has been removed A
1000 1 sense transformer is used to detect the normal fault
The fault current which is basically the difference current
between the hot and neutral lines is stepped down by 1000
and fed into the input pins of the operational amplifier
through a 10
tween pin 2 and pin 3 and the 200 pF between pins 3 and 4
are added to obtain better noise immunity The normal fault
sensitivity is determined by the timing capacitor discharging
current I
At the decision point the average fault current just equals
the threshold current I
where I
amp and the factor of 2 is due to the fact that I
timing capacitor only during one half-cycle while I
charges the capacitor continuously The factor 0 91 con-
verts the rms value to an average value Combining equa-
tions (1) and (2) we have
For example to obtain 5 mA(rms) sensitivity for the circuit in
The correct value for R
characteristic curve that plots equation (3) Note that this is
an approximate calculation the exact value of R
pends on the specific sense transformer used and LM1851
tolerances Inasmuch as UL943 specifies a sensitivity ‘‘win-
dow’’ of 4 mA–6 mA provision should be made to adjust
R
Independent of setting sensitivity the desired integration
time can be obtained through proper selection of the timing
capacitor C
proper selection of C
design example then should only be used as a guideline
Assume the goal is to meet UL943 timing requirements
Also assume that worst case timing occurs during GF1
SET
I
I
R
R
TH
TH
on a per-product basis
SET
SET
f(rms)
e
e
TH
R
I
e
e
f(rms)
7V
SET
I
5 mA
t
2
f(rms) c
I
is the rms input fault current to the operational
TH
Due to the large number of variables involved
d
1000
c
F capacitor The 0 0033
7V
can be calculated by
7V
c
2
0 91
0 91
0 91
t
is best done empirically The following
TH
SET
e
1 5M
can also be determined from the
AC
TH
line voltage with 5 mA
to 3I
F capacitor be-
TH
f
charges the
(see Circuit
SET
TH
dis-
de-
(1)
(2)
(3)
(4)
5
start-up (S1 closure) with both a heavy normal fault and a
2
gramatically below
UL943 specifies
ditions Calculation of C
to normal fault only is as follows
where T
therefore
C
C
s
s
b
b
b
t
t
25 ms Specification
10 ms Maximum integration time that could be allowed
e
e
c
3 ms GFI turn-on time (15k and 1 F)
8 ms Potential loss of one half-cycle due to fault current
4 ms Time required to open a sluggish circuit breaker
8 ms Value of integration time that accommodates com-
grounded neutral fault present This situation is shown dia-
transformer
input sense
V
C
1000 turns
0 01 F
division of
120
500
I
I
t
current
sense of half-cycles only
ponent tolerances and other variables
e
e
e
e
1 turn
e
integration time
threshold voltage
average fault current into C
c
I
current generated
c
V
(swamps I
120 V
heavy fault
1 6
T
s
0 4
a
25 ms average trip time under these con-
c
0 4
R
AC(rms)
B
C
cycles only
t
TH
t
c
on half-
based upon charging currents due
charging
)
1
2
1000
17 5
1
c
c
c
1
2
conversion
t
average
c
rms to
(0 91)
fault current
(0 91)
around GFI
R
portion of
shunted
G
R
a
N
R
TL H 5177– 5
c
N
0 0008
(5)
(6)
(7)