LM1851 Unisonic Technologies, LM1851 Datasheet - Page 5

• Manufacturer Part Number: LM1851
• Description: Ground Fault Interrupter
• Manufacturer: Unisonic Technologies

UTC LM1851
APPLICATION CIRCUITS
5 mA normal fault sensitivity.
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
to 3I
latch. At this time the circuit breaker can be reset and the line voltage again supplied 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 10μF capacitor. The 0.0033μF capacitor between 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
charges the timing capacitor only during one half-cycle, while I
0.91 converts the rms value to an average value. Combining equations (1) and (2) we have
this is an approximate calculation; the exact value of R
LM1851 tolerances. Inasmuch as UL943 specifies a sensitivity '' window '' of 4mA ~ 6mA, provision should be made
to adjust R
timing capacitor, C
The following design example, then should only be used as a guideline.
Start-up (S1 closure) with both a heavy normal fault and a 2Ω grounded neutral fault present. This situation is
shown diagrammatically below.
UTC
A typical ground fault interrupter circuit is shown in Figure 2. It is designed to operate on 120 V
A full-wave rectifier bridge and a 15k/2W resistor are used to supply the DC power required by the IC. A 1μF
At the decision point, the average fault current just equals the threshold current, I
Where I
For example, to obtain 5mA (rms) sensitivity for the circuit in Figure 2 we have:
The correct value for R
Independent of setting sensitivity, the desired integration time can be obtained through proper selection of the
Assume the goal is to meet UL943 timing requirements. Also assume that worst-case timing occurs during GF1
R
R
I
I
TH
TH
TH
SET
SET
=
=
( see Circuit Description and Block Diagram ). This quickly resets both the timing capacitor and the output
=
=
R
I
f(rms)
f (rms)
SET
7V
I
SET
2
5mA
f (rms)
on a per-product basis.
is the rms input fault current to the operational amp and the factor of 2 is due to the fact that If
1000
7V
7V
÷2
× 0.91
× 0.91
t.
× 0.91
Due to the large number of variables involved, proper selection of C
UNISONIC TECHNOLOGIES CO., LTD.
SET
=1.5MΩ
can also be determined from the characteristic curve that plots equation (3). Note that
LINEAR INTEGRATED CIRCUIT
SET
TH
. I
depends on the specific sense transformer used and UTC
TH
can be calculated by:
TH
discharges the capacitor continuously. The factor
(4)
(1)
(3)
(2)
TH.
t
is best done empirically.
AC
line voltage with
QW-R122-007,A
5
TH