ELM331 查看數據表（PDF） - Elm Electronics
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ELM331 Datasheet PDF : 4 Pages
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ELM331
Example Application
Figure 1 shows the ELM331 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 circuit is from a 12V supply, that is
reduced to 5V by the 78L05 regulator. This gives a
stable supply voltage for the ELM331, as well as
convenient voltage for use with a standard relay coil.
The type of relay is not important, as long as
consideration is given to its coil requirements, and the
capabilities of the ELM331. In this example, a relay
with a 400Ω coil resistance was chosen so that a
2N3904 could drive it directly.
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 ELM331, 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 (220 Ω)
resistor physically close to the ELM331 as shown
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.
Variations on this circuit could easily be made…
Simply by reversing RSET and RTEMP, one obtains a
cooling control thermostat… Rather than a relay output,
the circuit could have been connected directly to other
logic circuits. The measurement in progress pin could
then be used either as an interrupt, or as a busy flag
that can clock in new results on it’s falling edge…
Battery backup is another option that could be
added to this circuit, but then consideration should be
given to using the ELM341 Low Power Thermostat…
+12V
0.1µF
78L05
0.1µF
RTEMP
10KΩ
@25°C
1
2
see text
3
4
RSET
18KΩ
8
7
6
5
10KΩ
0.1µF
1N4001
2.2KΩ
+12V
12V Relay
To the
heating
control
2N3904
560Ω
LED
ELM331DSB
Figure 1. Backup Heating Control Thermostat
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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