Products / Quality

QUALITY ASSURANCE

At HTR we are convinced that quality of a product can only be assured in its true sense if "QUALITY IS BUILT IN AT SOURCE".

Towards this goal we have established the HTR reliability testing facility, which can carry out the following tests as per JSS/IEC/MIL specifications and also carry out certain key qualitative tests on essential raw materials.

[All equipment accuracies are calibrated on standards which are traceable to nationally recognised standards]

Customer assurance

HTR's reliability testing facility is fully alive to the fact that it's work in assuring quality can only be judged to be successful if it's work in quality assurance is converted to customer assurance.

Customer confidence will always be retained if the correct resistor is used in the way it is designed to be used. Towards meeting this objective the quality assurance team of HTR has put together a small guide, -

"USE AND SELECTION OF RESISTORS".

1) Improper heat dissipation is the predominant contributing cause of failure for any resistor type; consequently, the lowest possible resistor surface temperature should be maintained. The chart given illustrates the manner in which heat is dissipated from fixed resistors in free air.

Resistors above 2 watts	Resistors, 2 watts and below
Heat Dissipation of Resistors Under Room Conditions

The intensity of radiated heat varies inversely with the square of the distance from the resistor. Hence maintaining maximum distance between heat generating components serves to reduce cross - radiation heating effects and promotes better convection by increasing air flow.

2) Towards promoting better use of resistors, given below is A GUIDE FOR RESISTOR MOUNTING.

• Maintain lead length to a minimum. [Note :Where low temperatures are present, leads should be offset (bent slightly) to allow for thermal contraction]

• The mounting of close tolerance and low value resistors require special precautions (i.e., short leads and good soldering techniques) since the resistance of the leads and the wiring may be as much as several percent of the resistance of the resistors.

• Maintain maximum spacing between resistors.

• For resistors mounted in series, consider the heat being conducted through the leads to the next resistor.

• Large power units should not be mounted directly on to the PCB - try to select physical configurations which provide a standoff.

• Large power units should be mounted to chassis wherever possible - mainly applicable in the case of 35W and above in tubular style.

• To provide for the most efficient operation and even heat distribution, power resistors should be mounted in a horizontal position.

• Consider levels of shock and vibration to be encountered. Where large resistors are concerned, the body must be restrained from movement.

3) Wire wounds and their limitations when used in a high frequency circuit - Wire wound resistors have inductive and capacitive effects and are unsuited for use above 50 kHz even when specially wound by the Aryton-Perry method. Wire wounds also usually exhibit an increase in resistance with high frequencies because of the 'skin' effect.

4) The selection of a particular resistor for a particular circuit is often the result of a tradeoff between the actual parameters required and the realities of the product being designed in terms of size or costs.

In order to help the designer make a cost effective practical decision the following "Rating versus Life" material being offered may be appreciated.

A good rule to follow when choosing a resistor size for equipment that must operate for many thousands of hours is to derate it to one half of its nominal power rating. Thus, if the self generated heat in the resistor is 1/3 watt, do not use a 1/2 watt resistor, but rather a 1 watt size. This will automatically keep the resistor cool, will reduce the long term drift, and will reduce the effect of the temperature co-efficient. In equipment that need not live so long and must be small in size this rule may be impractical and the designer might have to compromise to some extent. In short, a "cool" resistor shall generally last longer than a "hot" one, and can absorb transient overloads that might permanently damage a "hot" resistor.

CAUTION

1] The types and specifications given in this site are general in nature and apply largely to our standard types and hence if the resistors are to be used in special applications or are to be used in areas that need extremely high reliability such as life maintenance equipment, atomic energy, aircrafts and man-made satellites, it would be advisable that samples are procured from HTR and evaluated for suitability in real life conditions/simulation.

2] During soldering operations, care should be taken to keep recommended temperatures in mind and prevent unnecessary stress on the component, which can lead to discolouring, resistance change and opening, if it is subject to abnormal high temperatures for a long time.

3] If the component is to be used in a circuit where pulse wave current (single pulse / repetitive pulses) and surge current run, it is suggested that samples are obtained from HTR and tested in the circuit itself for their suitability Please specifically mention the application and the pulse stresses that the resistor could be subjected to, while asking for samples.

4] When ambient temperatures exceed the rated ambient temperature, it is advisable that the resistor is utilised after proper derating as per the derating curve provided.

5] Under overload conditions, general resistors are likely to emit flame/fire, gas/smoke; however flame retardent resistors under overload conditions emit smoke and even glow red hot under extreme overload conditions but do not emit fire / flame which even if it occurs, is extinguished within seconds due to the flame retardent properties of the coating / filling.