General Information Vishay Rated Resistance Resistance value indicated upon the resistor Critical Resistance Resistance value at which the rated voltage is equal to the limiting element voltage Resistance Tolerance Permitted variation of the nominal resistance value expressed as a percentage of that value Nominal Dissipation Maximum permitted load at a defined ambient temperature e.g. T@ = 70°C, which ensures that resistance stability limits in the relevant specification are not exceeded. Limiting Element Voltage Maximum d.c. or a.c. effective voltage which can be applied continuously to the resistor Thermal Resistance Under electrical load a film resistor generates heat which increases the film temperature. At the same time heat is dissipated to the environment, so that with constant electric load and constant convection a thermal balance appears between the heat, generated by the electrical load and the heat lost by convection. These proportions are characterized by the thermal resistance. The thermal resistance is defined by the mechanical dimensions of a resistor, the heat dissipation by the wire leads as well as the convection, radiation and the mounting of the resistor. The thermal resistance Rth is defined as follows: Rth = (Js -JU)) / P = JÜ / P Js = film temperature in °C Ju = ambient temperature in °C Temperature Coefficient The permissible change of the resistance value depending on temperature and can be described by the following equation: Jü = temperature rise P = load in Watts TC (10-6/K) = (RJ - RTREF.) / (RTREF * DJ) * 10-6 The thermal resistance measurement is made under defined conditions according to DIN. DJ is the difference between Reference Temperature (TREF) and the corresponding ambient temperature. The maximum permissible increase of the resistance value by the TC, in case of electric load can be determined by way of the maximum permissible film temperature. The change of resistance value is calculated by: The maximum power rating can be calculated using the following equation: RJmax = RN[(1+(Jsmax-20°C)*TCmax)] Consequently the maximum permissible current for the voltage for P70 can be calculated by RJmax Insulation Voltage Maximum peak voltage which may be applied under continous operating conditions between the resistor terminations and any conducting mounting surface. Insulation Resistance Electrical resistance value of the encapsulent measured between the terminations of the resistor and applied V-block according to IEC60115-1 Derating Boundary curve of maximum allowable dissipation at T@ between upper and lower category temperatures. Document Number: 20000 Revision: 16-Oct-03 Pmax = (Js - Ju) / Rth Thus, the maximum permissible power rating Pmax is dependant on the maximum permissible film temperature, the ambient temperature Ju and the thermal resistance. Current Noise The current noise voltage expressed in µV, is that portion of noise voltage that arises from d.c. current in a resistor in addition to the thermal noise voltage. The relative noise voltage, expressed in µV/V is independent of the applied dc-voltage U= Non linearity A3 The harmonic index and the voltage coefficient of resistors are a criteria for the non-linearity of the current voltage characteristic. The harmonic index is defined as the logarithm of the ratio of the fundamental U1 to the 3rd harmonic E3. It is specified in dB: A3 = 20 lg ( U1 / E3 ) in dB Measurements are according to IEC 60440 For Technical questions in Europe, contact: [email protected] For Technical questions in Americas/Asia, contact: [email protected] www.vishay.com 5 General Information Vishay Stability The change of resistance values at certain loads and ambient temperatures can be obtained from the Stability Nomogram which consists of 4 diagrams; these can also be used independently. The stability nomogram for different products can be seen on the relevant data sheets. Additionally the limiting values stated in the data sheets such as maximum load, surface temperature etc., have to be observed.The following examples show how to use a nomogram: Example 1: Known: size D R= 1KW, P = 0.5W, ULEV= 350V, t = 5000h, Ju = 70°C Example 2: Known: size F; R = 1M, P70 = 1.5W, ULEV= 500V, t = 2000h at Ju = 50°C Unknown: DR /R after 5000h From Diagram A we see a temperature rise of Jü = 65°C for size D at P = 0.5W Unknown: DR /R after 2000h For R = 1M the following equation applies: P = (U2LEV / R = 0.25W as U = P* R > ULEV (see the dotted line in the nomogram). From Diagram B a surface temperature of 135°C can be obtained for JU = 70°C From Diagram D a DR /R after 5000h of 0.4% can be obtained for a surface temperature of 135°C of a 1KW resistor (see solid line in nomogram) Diagram B Diagram A 120 ze Si D 100 80 Size 60 F 40 20 J = f [P] Parameter: size 1.2 Load in W Temperature rise Jü in ∞C 140 0.8 0.6 0.4 0 Jü ∞C 40 ∞C 60 80 ∞C 80 C 0∞ 60 10 C 0∞ 40 12 C 0∞ 20 14 40 80 60 100 120 140 160 180 Film temperature J s in ∞C Diagram C Diagram D 0.01 h 0 t= 10 0 00 0.1 0h h 00 00h 00h 00 10 50 20 10 1 10 Resistance change DR/R in % Stability nomogram typical values 0.1 1 0.01 0.01 Resistance change DR/R in % after 1000h DR/R (t) = f [DR/R (t = 1000h)] Parameter: time www.vishay.com 6 C 0∞ =2 100 0 20 0 1.0 140 J ü = f [Jü] 120 Parameter:Jü 10 W 10 0W 1K 10 33 K K 0.1 1M DR/R [t = 1000h] = f [Js ] Parameter: resistance value 1 20 40 60 80 For Technical questions in Europe, contact: [email protected] For Technical questions in Americas/Asia, contact: [email protected] 100 120 140 160 180 Film temperature Js in ∞C Document Number: 20000 Revision: 16-Oct-03 General Information Vishay 120 Jü [∞C] Packaging density The temperature rise in respect of the surface temperature of the hottest SMD component on the board can be obtained from the nomogram below. It is necessary that the components are distributed uniformly over the whole circuit board. 100 JU 0.4W 80 JU A 60 0.25W JU 40 0.125W JU 0.0625W =2 =4 =6 JU 1 2 3 5 7 10 2 3 5 7 100 2 3 5 7 Example 1: Known: 9 resistors each rated at 0.25W Ju = 60°C Unknown: temp.rise Jü, surface temp. Js of the hottest component Ju = 65°C (A), Js = 125°C (B), see solid line Pulse Load When a resistor is subjected to impulses the following points have to be observed: 1. The maximum pulse load permissible P^max depends on the pulse duration ti This also applies to the maximum permissible pulse voltage U^max 2. The average load P may not exceed the corresponding nominal load. For resistors with resistance values greater than the critical value the nominal value is determined by the critical value and the maximum operating voltage permissible. Required P = 1 tp ∑ R t2 Ú U2 ( t ) dtP J t1 Explanations: R = nominal value tp = period of time U (t) = pulse voltage PJ = nominal load of the resistor for the ambient temperature J t2 – t1 = pulse duration ti Document Number: 20000 Revision: 16-Oct-03 60 80 0∞C 0∞C =8 20 0∞C 0∞C =1 00 ∞C 100 JU =1 20 ∞C B 140 120 Example 2: Load of each resistor = 0.0625W, Ju = 50°C maximum admissible surface temperature 90°C How many components may be mounted? - 33 pcs, see dotted line 3. Differences arise when resistors are subject to single shot (switching-on processes) or repetitive pulses. Approximate values for the load with rectangular pulses for each model are stated in the appropriate sections of the catalog. All other pulses have to be converted to rectangular pulses which show the same energy content and the same pulse voltage. Example: Exponential pulse t Us2 / 2R = ti Us2 / R e.g. ti = t / 2 Explanations: t = time constant of the exponential pulse ti = pulse duration or the rectangular pulse Ûs = peak voltage R = nominal value of the resistor The maximum permissible pulse voltages Ûmax are also stated. The permissible pulse loads have been fixed in such way that the changes which appear in resistance values are comparable to those stated for the electrical long time load according to IEC 60115-1. For Technical questions in Europe, contact: [email protected] For Technical questions in Americas/Asia, contact: [email protected] www.vishay.com 7