ELM341
Example Application
Figure 1 shows the ELM341 in an example heating
control circuit. A closed contact output occurs
whenever the temperature measured by R
TEMP
falls to
a value less than that determined by R
SET
. 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 R
TEMP
,
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
12V Relay
1N4001
To the
heating
control
below. Take into account it’s value when determining
the setpoint, though.
For this design, R
SET
was selected to be equal to
the resistance of R
TEMP
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 de-
energized, 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
coil.
+12V
see text
1
2
3
4
8
7
6
5
1.5KΩ
2N3904
R
TEMP
10KΩ
@25°C
560Ω
R
SET
18KΩ
10KΩ
LED
0.1µF
Figure 1. Backup Heating Control Thermostat
ELM341DSB
Elm Electronics – Circuits for the Hobbyist
4 of 4
ELM [ ELM ELECTRONICS ]
