Page 1
ELM341
1234876
5
Low Power Thermostat
23467
5
Description
Features
The ELM341 is a complete temperature
measurement and control system in an 8 pin
package. It automatically reverts to a very low power
sleep mode between measurements, making it
suitable for battery operation.
This integrated circuit is designed to compare
two resistances and drive an output pin depending
on the relative value of each. Typically, one of the
resistors will be an NTC thermistor, and the other
one will be a temperature independent resistor
(whether fixed or variable). When the magnitude of
the resistance connected to pin 2 exceeds the value
of the resistance connected to pin 3, the output pin
will be driven to a high state. Hysteresis maintains
the output in that state until the relative values differ
by approximately 8% (or typically 2°C for a 10KΩ
thermistor).
To reduce the possibility of sporadic outputs, a
condition must exist for three successive cycles, or 6
seconds, before the output pin can change state.
Applications
• Backup thermostats
• Under or over temperature alarm circuits
• Low power CMOS design
• Wide supply range - 3.0 to 5.5 volt operation
• Built-in proportional hysteresis
• Measurement in progress output
• Time delay on operate improves noise immunity
• Internal pullup resistor on the reset input
• High current drive outputs - up to 25 mA
• Sleep mode reduces power requirements
Connection Diagram
PDIP and SOIC
(top view)
VDD VSS
OutR1
R2
reset
MIP
Cap
Block Diagram
VDD
Control
Analog to
Digital
Converter
Measurement in Progress (busy)
Overrange
R1 > R2
R1
ELM341DSB
reset
R2
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
Sleep
Timer
MIP
Out
3 Consecutive
Measurements
1 of 4
Page 2
ELM341
Pin Descriptions
VDD (pin 1)
This pin is the positive supply pin. Internal
circuitry connected to this pin is used to provide
power on reset of the microprocessor, so an
external reset signal is normally not required.
Refer to the Electrical Characteristics section for
further information.
R1 (pin 2)
One of the two resistance input pins. A
temperature dependent resistance is usually
connected to this input for heating or undertemperature alarm type applications. When the
value of this resistor is greater than the value of
the resistance connected to pin 3 (for three
successive measurements) the output will be
driven high.
R2 (pin 3)
The reference resistance is connected to this pin
for heating applications (and the temperature
dependent resistance is connected here for
cooling applications). The other end of this
resistor is connected to the integrating capacitor.
reset (pin 4)
The active low reset input. An internal pullup
resistor is provided for convenience. If unused,
this pin may be connected to VDD or left open.
integrating capacitor. Pin 5 forces the capacitor to
a known voltage for these measurements though,
resulting in large current flows. To limit these
capacitor currents, and protect the ELM341, a
series resistor must be connected to this pin. The
value of the resistance, and of the capacitance, is
not critical to the measurements.
MIP (pin 6)
This pin provides a logic high level output while
the ELM341 is busy (measurements are in
progress). It is suitable for directly driving an LED
through a current limiting resistor. As a warning,
this output pulses rapidly if either resistor input is
found to be open circuited.
Out (pin 7)
The output pin assumes a logic high state once
the resistance of R1 exceeds that of R2 for three
successive measurement cycles. The output is
maintained until R1 is less than R2 by the
hysteresis amount for an additional three counts.
Note: Consideration must be given to the fact that
this output is in a tri-state (open circuit) mode
each time the circuit wakes from sleep to take a
measurement. This mode lasts for approximately
300µs, which is not generally long enough to
affect a relay output, but certainly long enough to
be seen by high-speed logic circuits.
Ordering Information
All rights reserved. Copyright ©1999 Elm Electronics.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
ELM341DSB
Cap (pin 5)
Temperature measurements are made by
determining the time to charge and discharge this
These integrated circuits are available in either the 300 mil plastic DIP format, or in the 200 mil SOIC surface
mount type of package. To order, add the appropriate suffix to the part number:
300 mil Plastic DIP............................... ELM341P 200 mil SOIC.....................................ELM341SM
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
2 of 4
Page 3
ELM341
Absolute Maximum Ratings
Stresses beyond those listed here will likely damage
the device. These values are given as a design
guideline only. The ability to operate to these levels
is neither inferred nor recommended.
Storage Temperature.......................-65°C to +150°C
Note:
Ambient Temperature with
Power Applied....................................-40°C to +85°C
Voltage on VDD with respect to VSS............0 to +7.5V
Voltage on any other pin with
respect to VSS........................... -0.6V to (VDD + 0.6V)
Electrical Characteristics
All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Characteristic Minimum Typical Maximum ConditionsUnits
Supply Voltage, VDD 3.0 5.0 5.5 V
VDD rate of rise 0.05 V/ms
Average Supply Current, IDD 1.0 2.4 mA
Frequency of measurements 2.0 sec see note 4
Reset pin internal pullup resistance 300 470 600 KΩ see note 5
0.008
0.002
see note 2
VDD = 5V, see note 3
VDD = 3V, see note 30.6 2.4 mA
R1C or R2C time constant 500 500,000 µs see note 6
Input low voltage - reset pin VSS 0.15 VDD V
Input high voltage - reset pin VDD V0.85 VDD
Output low voltage 0.6 V
Output high voltage VVDD - 0.7
Current (sink) = 8.7mA
Current (source) = 5.4mA
Notes: 1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX as the core embedded
microcontroller. For further device specifications, and possibly clarification of those given, please refer to the
appropriate Microchip documentation.
2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved
using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as
may be obtained through direct connection to solar cells, or some charge pump circuits.
3. Integrated circuit only. Does not include any LED or drive currents. Minimum currents represent those which
are typically found between measurements when in the low power sleep mode.
4. If a measured resistance is determined to be out of limits, the frequency of measurements is increased to
provide visual feedback as well as a faster recovery.
5. The value of the pullup resistance is supply and temperature dependent.
6. One should also maintain R1 and R2 to not less than about 5KΩ. When C is chosen, select the pin 5 current
limiting resistance so that RLIMC is less than 1msec, and RLIM is greater than 1KΩ.
ELM341DSB
Elm Electronics – Circuits for the Hobbyist
3 of 4
< http://www.elmelectronics.com/ >
Page 4
Example Application
coil.
ELM341
1234876
5
Figure 1 shows the ELM341 in an example heating
control circuit. A closed contact output occurs
whenever the temperature measured by RTEMP falls to
a value less than that determined by RSET. It is
anticipated that this type of circuit could possibly be
used to control temperatures over the range of -40°C
to +40°C.
Power for the control circuit is from a 3V battery,
while the output relay is powered from a 12V supply.
The output relay type is not important, as long as
consideration is given to the coil drive requirements,
and the capabilities of the ELM341. In this example, a
relay with a 400Ω coil resistance was chosen so that a
2N3904 could drive it directly. For further reductions in
current requirements, consideration could be given to
using a power MOSFET transistor instead of this
bipolar one.
Temperature measuring is performed by RTEMP,
which is a negative temperature coefficient type
thermistor. It has a resistance of 10KΩ at 25°C, and
this value decreases with increasing temperature. This
value was chosen both because it is commonly
available, and because it limits the 0.1µF integrating
capacitor currents to less than 1mA over the typical
range of operation (keeping the thermistor self-heating
to a minimum).
If the thermistor is mounted any appreciable
distance from the ELM341, consideration must be
given to cabling effects such as capacitive and induced
currents. Generally the integrated circuit can be
adequately protected by mounting a small value (220Ω)
resistor physically close to the ELM341 as shown
3V
below. Take into account it’s value when determining
the setpoint, though.
For this design, RSET was selected to be equal to
the resistance of RTEMP at 10°C, so that the relay
contact closes for any measured temperatures less
than 10°C. The resistance value was determined from
specs given by the manufacturer, but could have been
determined experimentally as well.
An LED has been provided for visual feedback of
the circuit operation. It is connected to the
‘measurement in progress’ output, so that it is
energized each time a measurement is being made.
Typically, this would be for about 25mS every 2
seconds.
Current requirements for this entire circuit have
been measured to be about 2µA minimum and 23µA
average, at room temperature and the relay deenergized, even with the LED blinking. Measurement
times, and thus current consumption, vary with
temperature, so this should only be used as a
guideline. With a 5V supply, these values escalate to
about 8µA and 70µA, respectively.
Just a reminder that consideration must be given
to the fact that the pin 7 output is in a tri-state (open
circuit) mode just prior to making a measurement. This
mode lasts for approximately 300µs, sufficient to
possibly affect some output circuits. It is generally not
sufficient for an electromechanical relay to drop out,
however, especially with a ‘kick-back’ diode across the
+12V
12V Relay
1N4001
To the
heating
control
see text
RTEMP
10KΩ
@25°C
ELM341DSB Elm Electronics – Circuits for the Hobbyist
RSET
18KΩ
0.1µF
10KΩ
Figure 1. Backup Heating Control Thermostat
1.5KΩ
560Ω
LED
< http://www.elmelectronics.com/ >
2N3904
4 of 4
Loading...