COMPUTATION TOOL FOR RESISTORS/THERMISTOR NETWORK
Welcome to the VISHAY web tool for resistors/thermistors network computation. The following information will detail why and how to use the tool:
Why use this computation tool?
In a lot of applications, the user has to build a small circuitry, providing a thermal voltage (a voltage changing with temperature). This voltage is then used for different purposes:
- Input of an analog digital converter
- Contrast voltage adaptation for the wide temperature range LCD
- Correction of battery voltage (ex: Lead Acid batteries in solar installations)
- Compensation voltage for temperature variation of another sensor
- Bias of varactor capacitance for TCXO’s or VCTCXO’s applications.
In order to generate this voltage, a thermistor (non linear NTC or linear PTC) can be integrated in a network of fixed resistors in order to produce a voltage Vn which will change unidirectionally with temperature.
The circuit (left) in fig. nr1 will produce a decreasing voltage Vn with temperature (if the thermistor is an NTC) or increasing with temperature (ifthe thermistor is a linear PTC, codified as TFPT at VISHAY). The other circuit (right) will perform the opposite. Whether a linear PTC or an NTC will be used is depending on the required voltage variation is requested (linear PTC have a temperature coefficient of about 0.4 %, while NTC are 10 times more sensitive)
This web tool will identify the best values of a fixed resistor/thermistor (NTC or TFPT) network in order to fit a given requested voltage/temperature variation, and this for different purposes.
The P/N’s will be provided taking account of the chosen mechanical execution (leaded through hole or SMD for the thermistors , or exclusively SMD for the fixed resistors).
How to use this computation tool:
Well, the input of this program is typically the needed data derived from the application conditions. The user will have to input:
- The power supply voltage Vcc (maximized at 20 V)
- A so called minimum average current: As the resistance of a thermistor can change over wide range, the current flowing through the circuit can change with a factor 10 or 100. By choosing this value, you will ensure that over the whole temperature range, such average value is ensured. As example, if you deal with application where current consumption must be low, you will be able to select the lowest current possible (10 µA) in order to preserve the operational life of your device (remote battery for fire detector). At the opposite, if you are reluctant to use high resistance values, and possible induction voltages, you might then choose the max. current possible (1 mA)
- The required voltage variation in function of the temperature: 3 points minimum are needed and maximum 10 points are possible. The values of temperature (in °C) must be entered in increasing order. The associated voltage values must either rise either decrease with the temperature. The minimal voltage must be higher than 5 % of the power supply Vcc while the maximal voltage must be lower than 95 % of Vcc
- Then the required characteristics for the thermistor :
- First , its mechanical configuration : the choice is provided between leaded through hole and the different SMD cases available
- You may select the R25 tolerance at this stage but modify it later one.
Then the self heating is examined : this option is linked to the “minimum average current”: as the total current has been defined, there are already some constraints on the self heating of the thermistor . One part of the current (but not all) will pass through the thermistor and will thus heat it by Joule effect. The possible choices will automatically adapt themselves to the value of current. One possibility remains to select the option “ neglect self heating” , for example in the case where the thermistor is case in a thermoconductive grease , in a housing in contact with water , or any conditions that enable the thermositive element to dissipate a lot of power. If the chosen execution is a stand alone SMD 0402 , do not select this option.
- As last design input, the characteristics of the fixed resistors network: We have chosen to restrict the choice of fixed resistors to the SMD line high stability thin film TNPW series.Only the high precision tolerance are presented 0.1 and 0.5%. You can select the wished temperature coefficient and the choice of values in the E-series. This way instead of getting only R-values , you will get an optimized set of real P/N’s.
What is the result of the computation tool?
- The circuit configuration allowing you to generate the best approximating thermal voltage corresponding to your application.
- You get a whole set of standardized R values together with their P/N.
- If some fixed resistors are superfluous, there will be a mention that this or that fixed resistor is not needed.
Any information, computations, or results generated by the program, or otherwise available from or through the program, is provided and utilized at the customer’s discretion and risk. Vishay Intertechnology, Inc., its affiliates, agents and employees and all persons acting on its or their behalf (collectivey, “Vishay”), do not assume any responsibility or liability resulting from the customer’s use of the program or any such information, computations, or results. Vishay disclaims any and all liability for any errors, inaccuracies or incompleteness contained in any datasheet or in any other disclosure relating to any product or computation resulting from web tool usage. Customers are prohibited from modifying, reverse engineering, decompiling, or disassembling the object code portions of this software.
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