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PDF OM1895 Data sheet ( Hoja de datos )
| Número de pieza | OM1895 | |
| Descripción | Simmerstat Control IC | |
| Fabricantes | IES | |
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Draft Data Sheet
INTEGRATED CIRCUIT
2003 May 09
OM1895
ww.DataSheet4U.comSimmerstat control IC
wINTEGRATED ELECTRONIC SOLUTIONS
1BUTLER DRIVE
HENDON SA 5014
AUSTRALIA
www.DataSheet4U.com
1 page  
Integrated Electronic Solutions, Hendon, South Australia
Simmerstat control IC
Draft Data Sheet
OM1895
6 FUNCTIONAL DESCRIPTION
6.1 VCC − Common, positive DC
supply
The positive DC supply rail for the
control IC type OM1895 is used as
the common reference. This is always
connected to the T1 terminal of the
triac, and being the positive supply
rail allows negative gate drive to the
triac in both positive and negative
supply half cycles on T2. By driving
the triac in this way the insensitive
quadrant (negative T2 voltage, and
positive gate triggering signal) of
triacs is avoided.
6.2 VEE − Negative DC supply,
substrate
This pin connects to the substrate and
the internally generated and
regulated negative DC supply, and
should be bypassed to VCC (common)
by a capacitor of typically 100 µF. The
capacitor needs to be sufficiently
large to maintain the operating
voltage during the half cycle when it is
not being charged, as well as to
provide the energy to drive the triac
gate during the gate pulse.
Internal supply sensing prevents the
commencement of an ON cycle while
the voltage is too low for reliable
circuit operation. If during an ON
cycle the supply voltage falls below
this level the ON cycle will terminate
at the first opportunity consistent with
the logic cycle algorithm.
6.3 PWR − Power supply and
synchronisation from the
mains supply line
The PWR input provides both a
synchronisation signal for the logic
functions of the OM1895, as well as
the DC current used to provide the
power supply from which the OM1895
is powered. Signals are derived which
indicate the phase and magnitude of
the signal on the AC supply. Three
states, positive, zero and negative, of
this signal is recognised for
synchronisation of the triggering
times to the mains.
See Figure 3, OM1895 Power Supply
Circuit.
The PWR pin is driven by a current
limiting resistor from the mains
supply. During the positive half cycle
current flows through the upper diode
D1 to the positive common rail, while
on a negative half cycle the current
flows through the lower diode D2, and
charges the VEE power supply
capacitor.
The zero crossing is signalled by the
two comparators, the output signals
of which indicate whether the mains
voltage is above the common rail
voltage, or below the negative VEE.
There may be additional resistors in a
simple network from the AC supply
and VEE to adjust these zero-crossing
signals to provide a symmetrical
response in the positive as well as the
negative going direction.
As the AC signal passes through
zero, comparators provide control
signals Tp (when VPWR > VCC) and
Tn (when VPWR < VEE) indicating
whether the voltage on PWR pin is
greater or less than VCC or VEE
respectively. A resistor network
ensures that these switching points
correspond to equal positive or
negative thresholds about the AC
zero thus giving symmetrical
zero-crossing information to the
synchronisation and logic circuit.
Synchronisation is obtained from the
threshold comparators at the levels of
VCC and VEE on the chip.
VSS
D1
-
Tp
+
2003 May 09
PWR
-
D2 Tn
+
VEE psp1895
Fig.3 OM1895 power supply circuit
5
5 Page 
Integrated Electronic Solutions, Hendon, South Australia
Simmerstat control IC
Draft Data Sheet
OM1895
VCC
R2
R1
Rpot
R3
POT OM
1895
VEE
Fig.6 Potentiometer circuit to set a control range of less than 0 to 100%
The choice of potentiometer
resistance is limited by the loading
which can be applied to the -VEE
power supply. Choice of 470k will
load the 8 V power supply with a
current of 17 µA. The current in the
resistor network loads the wiper tap
on the potentiometer, the current
flowing in R1 needing to be small
compared to the current in the
potentiometer.
As an example, a circuits is shown in
figure 12 using a 470k potentiometer,
and values of resistors R1, R2 and R3
chosen to give control over the range
of 40% to 80%.
This has been calculated on a
spreadsheet, which also calculates
the error in the linearity caused by the
resistor loading on the potentiometer.
(Note that the maximum error may be
near the mid point on the
potentiometer if only R2 or R3 are
used alone, while if both R2 and R3
are used the error curve is S shaped
with an accurate point around the
mid-range of the pot. The equivalent
circuit load of the potentiometer at its
mid point is one quarter of the total
resistance Rpot, or 117.5 kΩ. At each
end of its travel the potentiometer
impedance is zero ohms.
100%
R1 270k
Rpot
470k
VCC
R2
220k
POT
R3
560k
OM
1895
VEE
75%
50%
25%
0
0
POTENTIOMETER POSITION
100
Fig.7 Potentiometer characteristic and circuit to set a control range of 40 to 80%
2003 May 09
11
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