NTC temperature sensors
Technical Support Note
TEMPERATURE CONTROL OF FAT FRYERS
For equipment manufacturers needing reliable temperature
sensing and control functions, Philips has the answer.
Philips' NTC thermistors for temperature sensing are
small, accurate and offer excellent long-term stability and
reliability. With an operating temperature range between
−55 and +300 °C, the latest NTC-thermistor series provide
attractive and cost-effective alternatives to thermocouples
and electro-mechanical temperature controllers, especially in
automotive equipment, industrial systems and domestic
appliances.
Extending its interest to all aspects of temperature control,
Philips is now working with specific partners to develop
complete solutions to temperature sensing and control,
combining its extensive ceramic know-how with their
experience of housing and assembly technology.
One of the first positive results of this partnership is the
development with Energica, France, of a new temperature-
sensor for fat fryers. The sensor has already been adopted
by the French company SEB in their Universalis fat fryer.
Well-definedWell-defined fatfat temperaturetemperature controlcontrol
The figure (overleaf) shows an electronic temperature-
control network designed to sense temperatures between
150 and 200 °C in a fat fryer with an accuracy better than
5°C, and to maintain the fat temperature during cooking
with a variation of less than 0.5 °C. For this application,
Philips delivers its high-temperature NTC thermistor
(100 kΩat 25 °C) in a hermetically-sealed SOD 27 glass
encapsulation. This is built into a customized metal
housing by Energica.
The network comprises a temperature sensing circuit
controlling a relay/switch combination (K1a/K1b) that
connects the 220 V AC mains voltage across the heater of
the fat fryer. A DC voltage VCC derived from the 220 V
AC mains via a diode bridge rectifier (D1..D4) powers the
sensing circuit. This comprises a comparator (LM358) fed
from a Wheatstone bridge containing the NTC thermistor
in one of its arms. The arm also contains fixed resistors
and a potentiometer to allow external preset of the cooking
temperature. The output of the comparator (governed by
the degree of bridge imbalance and hence the temperature
of the NTC) controls the current into the coil of relay
K1a, closing or opening switch K1b to give thermostatic
control of fat temperature.
Positive feedback (via resistor R7) provides well-defined
temperature hysteresis to prevent the relay from switching
continuously when the fat temperature has reached its
specified value.
NTCNTC thermistorsthermistors offeroffer long-termlong-term stabilitystability
The NTC thermistors supplied by Philips for this
application are available with nickel-copper clad iron leads
(633 7 in bulk, 633 4 on tape) suitable for clamping or
with tinned leads (633 8 in bulk or 633 3 on tape) suitable
for soldering.
Since the ceramic chips are not soldered to the lead wires
(which can give rise to ageing effects due to recrystalization
of the solder), the components offer exceptional long-term
stability. Moreover, the hermetically-sealed environment in
which the ceramic is housed eliminates high-temperature
reaction with the surrounding air, ensuring the least
possible deviation in resistance characteristics during
operation (less than a few tenths of a per cent even after
10 000 hours operation at 200 °C). Thermal-cycling tests
have also demonstrated that the components are able to
withstand more than 105thermal shocks (between 25 and
200 °C) without noticeable change of electrical parameters.
This corresponds to more than 20 years' normal use.
The preferred electrical values at 25 °C are 100 kΩwith
aB
25/85 of 3977 K or 220 kΩwith a B25/85 of 3795 K.
Other values (10 kΩto 30 kΩ) are also available within
the Philips range but for this application (operating tem-
peratures up to 200 °C) their resistances would be too low
for good temperature discrimination.
+
D1 to D4: 1N4004
1N4004
R2
C1R1
X1
X2
X3
220 Ω
0.5 W
T
V
CC
V
CC
R3
R1 6.8 kΩ
47 kΩ
NTC
R8
V
CC
V
CC
V
CC
R5
R6
P1 100 kΩ
2
3
6
57
4
8
LM358
LM358
1
R7
24 V
D6
D5
D1 D4
D3 C2
D7 neon
CCA210
100 µF
or
47 µF
35 V
470 kΩ
150 nF
100 nF
1%
1% 1%
1%
220 V
heating
element
D2
22 kΩ
150 kΩ5.1 MΩ
K1a
not used
K1b
24 V
Deep fat-fryer temperature control circuit based on a Philips NTC thermistor in metal encapsulation
All rights are reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this
document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted
by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent- or other industrial or intellectual property
rights.
Printed in The Netherlands © Philips Electronics NV 1996 Date of release: July 1996 9398 083 31011
