HM169-06 Hendon Semiconductors, HM169-06 Datasheet - Page 11

• Manufacturer Part Number: HM169-06
• Description: Temperature Sensor Development Tools OM1682A demoboard
• Manufacturer: Hendon Semiconductors

Specifications of HM169-06

• Maximum Operating Temperature: + 45 C
• Minimum Operating Temperature: 0 C
• Operating Current: 7 mA
• Operating Voltage: 230 V
• For Use With/related Products: OM1682A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

provision for two positions on the
printed circuit board, and can be
chosen to give the desired direction
by placing the link in position RL2A or
RL2B. The position chosen for this
link will offer clockwise or
anticlockwise rotation for increasing
resistance.
11.8
The OM1682A IC has an inherent
minimum time cycle (set by the choice
of the input resistors to the SA and SB
pins of the OM1682A, and the filter
capacitor on CAP). This is modified
by the use of a hysteresis setting
resistor, and the effective total
hysteresis can be increased or
decreased by the addition of R3 in the
A or B position, depending on
whether the HM169 is being used for
heating or for cooling.
In thermostat applications with the
temperature feedback from the NTC
thermistor, the cycle time is largely
determined by the thermal mass
being heated or cooled, together with
the power being controlled. This
means that in a typical application,
with judicial application of hysteresis,
the minimum cycle time derived from
the difference between the SA and
SB input currents to the OM1682A
being summed in the capacitor
connected to the CAP pin makes an
insignificant contribution to the overall
cycle timing. Adjustment of the
hysteresis resistor has a much more
significant effect allowing he
response time of the system itself to
determine how fast it cycles.
In the simmerstat circuit, hysteresis is
applied in the opposite sense to that
normally used in thermostat
applications with temperature
feedback, and the CAP pin capacitor
charge and discharge rates alone set
the cycle time.
Heating: Adding a resistor to position
“R3B” increases the hysteresis and
2008 Jul 02, Revision 3.0
Hysteresis
increases the cycling time between
ON and OFF; that is the ON and OFF
points exhibit a larger difference, or
greater hysteresis. Conversely,
adding R3 to position “R3A” reduces
the hysteresis and therefore forces
more frequent cycling. EMC
standards may require the use of an
increased hysteresis figure to ensure
the cycling is not too frequent with a
high power load, causing fluctuation
of the mains supply voltage.
Cooling. The positions of R3 are
reversed for cooling. i.e. R3A is
added to increase the hysteresis and
R3B reduces it.
Hysteresis resistor values between
220 kΩ and 22 MΩ are common. The
selection of this value depends very
much on the application and is left to
the designer. This resistor must be
mains rated and suitably power rated,
i.e. Vishay VR25 or VR37 depending
on the power dissipation.
11.9
The hysteresis setting resistor can be
used to generate a cycling controller
which does not use a temperature
sensor. In this case the potentiometer
adjustment can be set up so as to
vary the ON time from fully ON with
the potentiometer at minimum
resistance, to fully OFF at maximum
resistance.
Typical applications in this simmerstat
mode are for fan control (where the
inertia of the fan permits burst mode
firing) or for heating with fixed
proportional power input. In both
cases without any temperature
feedback.
In more complex circuits some
temperature compensation can also
be applied to modify the duty cycle a
little depending on the ambient
temperature.
Cycling control
(Simmerstat)
11
Precision electronic thermostat
11.10 Choice of thermistor
It is necessary to choose a thermistor
which has a resistance of at least
2,000 Ω at the operating temperature,
otherwise the voltage across the
thermistor will be too small to provide
accurate and adequate sensitivity.
This means that a different
thermistors are usually chosen for
heating and for cooling applications.
It is also important to ensure that the
thermistor can be mounted in a way
that is electrically safe, is protected
from the access of moisture, and
which provides good thermal contact
to the intended thermal measuring
point.
11.11 Thermistor mounting
As the thermistor is electrically live to
mains voltages it must be adequately
insulated for use in each application.
A range of insulated thermistors are
available, and there are a number of
important issues which must be
considered in mounting them.
The best way in which to mount the
thermistor is to clamp along the length
of the lead for about 50 mm starting
just clear of the sensing “head” of the
lead. Ideally the plastic head of the
leaded thermistor should all be clear
of the mounting clamp by a distance
of at least 1 to 5 mm.
The sensing head of an insulated
leaded thermistor should never be
clamped.
The plastic covered thermistor does
not have a large thermal mass, and
especially in moving air, will arrive
quite quickly at a temperature very
close to that of the ambient air. The
clamping means can also allow heat
to flow to/from the copper wires
leading to the thermistor, and can
provide a significant modification to
the temperature seen by the
thermistor. If the sensing point is also
Engineering Sample Information
considerations
HM169