ELM341 Low Power Thermostat 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. • • • • • • • • 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) Applications VDD 1 8 VSS R1 2 7 Out R2 3 6 MIP reset 4 5 Cap • Backup thermostats • Under or over temperature alarm circuits Block Diagram VDD reset 4 Sleep Timer Control Measurement in Progress (busy) 2 3 R1 ELM341DSB MIP 7 Out Overrange R2 5 6 Analog to Digital Converter R1 > R2 3 Consecutive Measurements Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > 1 of 4 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. 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. 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. 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. 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. 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. 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. 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. Cap (pin 5) Temperature measurements are made by determining the time to charge and discharge this VSS (pin 8) Circuit common is connected to this pin. This is the most negative point in the circuit. Ordering Information 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 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 Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > 2 of 4 ELM341 Absolute Maximum Ratings Storage Temperature....................... -65°C to +150°C Ambient Temperature with Power Applied....................................-40°C to +85°C Voltage on VDD with respect to VSS............ 0 to +7.5V Note: 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. 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 Supply Voltage, VDD 3.0 5.0 VDD rate of rise 0.05 Average Supply Current, IDD 0.008 0.002 Frequency of measurements Maximum Units 5.5 V V/ms 1.0 0.6 see note 2 mA mA VDD = 5V, see note 3 sec see note 4 600 KΩ see note 5 see note 6 2.4 2.4 2.0 VDD = 3V, see note 3 Reset pin internal pullup resistance 300 R1C or R2C time constant 500 500,000 µs Input low voltage - reset pin VSS 0.15 VDD V Input high voltage - reset pin 0.85 VDD VDD V 0.6 V Current (sink) = 8.7mA V Current (source) = 5.4mA Output low voltage Output high voltage VDD - 0.7 470 Conditions 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 R 1 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 < http://www.elmelectronics.com/ > 3 of 4 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. 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. 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. 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. 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). 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. 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 +12V 3V 12V Relay To the heating control 1N4001 see text RTEMP 10KΩ @25°C 1 8 2 7 3 6 4 5 1.5KΩ 2N3904 560Ω RSET 18KΩ 10KΩ LED 0.1µF Figure 1. Backup Heating Control Thermostat ELM341DSB Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > 4 of 4