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ELM341

Low Power Thermostat

Elm Electronics 

ELM341

Example Application

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

RTEMP

10KΩ

@25°C

see text

1

2

3

4

RSET

18KΩ

8

7

6

5

10KΩ

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 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.

1N4001

1.5KΩ

+12V

12V Relay

To the

heating

control

2N3904

560Ω

LED

0.1µF

Figure 1. Backup Heating Control Thermostat

ELM341DSB

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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