Resistors Pulse Load, Power and Voltage Derating

The article discusses key electrical resistor pulse load, power and voltage derating parameters that are important for the proper selection guide and reliable operation of these passive components.

Pulse Loads

In many applications, the resistor is subjected to pulse loads. We make distinctions between periodical/repetitive loads and pulse trains: on the one hand, where the pulse is repeated at a certain frequency, and on the other, separate pulses where a possible repetition will occur after such a long time that the resistor has recovered its original ambient temperature.

Periodic pulse loads

Figures 1. and 2. show two typical types of pulses: rectangular ones and exponential ones. The theoretical square pulse has, in practice, sloping fronts and rear edges. The definitions of times and voltages of a square pulse are shown in Figure 3. With definitions from Figures 1., 2. and 3., the following general conditions apply.

here:

P_T = permissible power at the ambient temperature T
P¯= mean power in the pulse train.
1/t_p = pulse repetition frequency.
t_i  = pulse width t₂ – t₁.
R  = rated resistance.
V  = instantaneous voltage.
t_r = rise time (Fig 3.)

Figure 3. Rectangular pulse loads definitions.

For rectangular pulses the following equation applies:

rectangular pulse loads power equation [2]

and for exponential pulses, according to Figure 2.;

One talks in this connection about

Figure 4. shows the overload factors c_p and c_u for carbon film resistors.

Figure 4. The overload factors cp and cu versus ti/τw for carbon film resistors (according to IEC 60115).

Pulse loads in single pulses or pulse trains, with a permissible mean power as shown in Equation [2] or [3], always have to take into consideration the voltage strength.

A possible pulse load of 100 P_R for 10 ms does not permit us to apply 1000 PR for 1 ms directly. The latter pulse implies √10 times higher voltage since V=√( P x R). At a certain resistance value, internal spark-overs may occur. In recommendations for pulse powers, the quotient Pˆ/PR stops growing in principle the same way as cp in Figure 4. does, when the pulse length decreases. Note also that a voltage quotient is chosen for the power overload factor, c_p, which means that the power quotient Pˆ/P_R can be obtained as (c_p)².

The overload factors in Equations [4], [5] and [6] are confined by the resistor material and construction. For example, wirewound resistors have much higher resistance against overloads than film resistors, which, in turn, may differ greatly in reciprocity. For example, carbon film has a considerably higher pulse capability than metal film, which depends on the film thickness. A carbon film is thicker than a metal film of corresponding resistance and thus contains a more heat-absorbing substance. Hence, the current density will be too high in the metal film if it is loaded with a maximum pulse for carbon film. Equation [6].

In thick film/metal glaze resistors, other conditions apply. The film’s conductive paths constitute a series-parallel network of an infinite number of tracks consisting of conductive particles reciprocally connected in weak contact points. The pulse load capability is generally poorer for these resistors than metal films. It may be otherwise for low resistance values with higher metal content.

To verify the pulse capability certain standard tests are outlined by the IEC 60115, for example, the following suggested data:

Single Pulses

Sometimes, it is necessary to load the resistor with a single pulse (that possibly may be repeated after a long time that a possible heating can be neglected). Certain standardized tests for film resistors, as outlined in appendices to IEC 60115, give us some useful basics for comparison. Two pulse shapes are defined: 1.2/50 and 10/700. The figures mean rise time t_r/pulse time t_i according to the definition in Figure 3. and are expressed in microseconds.

The test is performed with a successively increased voltage expressed in multipliers of the limiting voltage V_g until the resistance changes exceed specified values, usually the maximum change during a 1000-hour life test. (The limiting voltage Vg corresponds to the maximum field strength that the resistor construction can withstand. See figure and legend under 2.) The test follows the description in Table 1.

Table 1. Summary and excerpts of resistor pulse tests in the standard IEC 60115.

The following comparative figure for single power pulses may serve as a guide for the comparison of standard designs in different materials, provided the resistance value doesn’t cause the voltage in the overload factor to exceed cu (Equation [4]). We choose as reference power pulses with a duration of 10 ms. Comparison between materials gives

However, note that in the lower resistance range, thick films may be more resistant to pulses than corresponding metal films.

If we compare the effect from power pulses of different magnitude applied on a certain design, we get

We repeat that the stated ratios are approximate. What recommended pulse power, Pˆ, the resistor design can stand has to be read from the manufacturer’s technical information and diagrams and “possible” pulse power has to be verified by tests on the chosen resistor type and manufacture.

Power and Voltage Derating

Figure 5. Voltage and power versus resistance.
Vg = limiting voltage. PR = rated power

In Figure 5. we have plotted the voltage, V, and the power, P, versus resistance. In the left half of the diagram, the power is constant and equal to P_R (dotted line). Because P = V2/R, we get V ∼√R . With a linear scale on the resistance; we get the bent, unbroken voltage curve right on to the point where the construction doesn’t stand any higher field strength. We risk having a flash-over internally or to earth, which may happen above the so-called limiting voltage, V_g. The corresponding resistance value is called critical resistance. The limiting voltages are standardized in the IEC and CECC systems.

In the right half of the diagram, the voltage is constant, i.e., V = V_g. The power curve varies in accordance with the proportionality P ~ 1/R.

A resistor that works at rated power at an increasing ambient temperature eventually becomes so warm that the materials can’t stand the temperature. If we still want to use the component at a higher ambient temperature, we can lower the self-heating, i.e., the load, as shown in Figure R1-12. This kind of reduced load is also called derating. Thus, eventually, we attain the point on the temperature scale where, once more, the materials, despite zero power, tell us we have reached the temperature limit.

The different temperatures and powers defined in Figure 6. are obtained from the IEC standards. However, the curve’s shape is commonly accepted in all standard systems. Sometimes, we can find variants with broken curves, but the basic principle is the same. Where on the temperature scale, the standardized breakpoint varies. The most common is 70°C. The upper category temperature, T_uc, depends on the resistor type and is specified according to standard temperatures.

Figure 6. Permissible resistor power loads versus ambient temperature

Maybe the alert reader wonders if we can’t load the resistor harder below the rated temperature T_R. Well, there are even older specifications that state such possibilities. Now, however, for reliability reasons we go in the opposite direction. The rated power is reduced, i.e., derated. Then, we get diagrams that, in principle, look like the one in Figure 7. There a typical case of derating is described according to a US military standard.

Figure 7. Resistor power derating according to MIL-STD-1547 of a metal film resistor as specified in MIL-R-87254

Resource: EPCI Blog