A pulse resistor is specified to handle short electrical loads, such as capacitor discharge or inrush current, without damage or excessive resistance drift under the stated test conditions. Its pulse ratings describe how much power, energy, or voltage it can withstand for a defined waveform and repetition pattern. Compare the pulse load curve, peak voltage limit, and temperature-derated average power before selecting a resistor.

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Figure 1. An Overview of Pulse Resistors

What Is a Pulse Resistor

A pulse resistor is a fixed resistor designed or specified to withstand short bursts of electrical power under defined conditions. It has a nominal resistance and a continuous power rating like other fixed resistors, but its pulse specification adds a limit for brief events such as capacitor charging, discharge, or switching transients. Depending on the part, this limit may be expressed as allowable peak power versus duration, energy, or voltage for a defined waveform.

Pulse and surge ratings depend on the specified waveform. A surge generally describes transient overvoltage or overcurrent, while an impulse rating may refer to a defined test. Pulse-rated resistors overlap with ordinary power resistors: a power resistor with documented pulse capability may also serve in pulsed circuits. Compare the allowable pulse load, not continuous wattage alone, and check permissible resistance drift because a resistor can remain electrically connected but no longer meet the circuit’s tolerance.

How Pulse Loads Stress a Resistor

Peak Power, Pulse Energy, and Average Power

Start with the voltage v_R(t) across the resistor and the current i_R(t) through it. For an ideal resistor of fixed resistance R, the instantaneous power p(t) and the energy E_pulse over a pulse from t₁ to t₂ are:

The largest value of p(t) is the peak power P_pk. For a rectangular power pulse of duration t_p, the energy is P_pkt_p. For an ideal resistor–capacitor discharge, power decays with half the voltage time constant because it depends on voltage squared. Section 5.2 uses this relationship to obtain an equivalent rectangular duration of RC/2 for a complete discharge.

For identical pulses repeating at a start-to-start interval T, with negligible dissipation between pulses, $P_{\text{avg}}=\frac{E_{\text{pulse}}}{T}$. For bursts or a standing load, integrate total power over the repeating cycle and divide by its period. Use voltage and current at the resistor terminals because source impedance and switching behavior can change the waveform.

Figure 2. Peak Power, Pulse Energy, and Average Power

If a current pulse is superimposed on a standing current, square the total current before calculating power. The instantaneous dissipation $R{[I_0+i_p(t\left)\right]}^2$ uses the standing current I₀ and the pulse current i_p(t). Adding the powers calculated for the two currents separately would omit the cross term. This matters, for example, when a resistor already carries a load before a switching event adds current in the same direction.

Single Pulses, Repetitive Pulses, and Bursts

For a pulse short relative to the element’s thermal response, heat stays mainly in the resistive material and absorbed energy largely sets the temperature rise. As the pulse lengthens, heat spreads into the substrate and mounting, and the allowable peak power generally falls toward the continuous rating. A narrow region near a film trimming cut can concentrate current and form a hotspot. The heat path and mounting help shape the pulse curve, while pulse-voltage breakdown sets a separate limit.

• Isolated pulse. An isolated pulse begins without significant residual heating from earlier pulses. Ambient temperature and any standing load still determine the resistor’s starting temperature.

• Repetitive pulses. A steady pulse train establishes a recurring thermal cycle.

• Bursts. Closely spaced events can leave the resistor warmer at the start of each successive pulse. A long idle period can conceal this heat buildup in the average-power calculation.

For faults and restarts, record pulse width, spacing, pulses per burst, and the interval between bursts.

Key Ratings in Pulse Resistor Datasheets

Pulse Power and Energy Curves

A peak-power curve typically plots allowable peak power against pulse duration. At 10 ms, for example, follow the duration up to the line for the relevant package or resistance range, then read the power axis. Logarithmic spacing represents ratios. An energy curve instead gives the limit in joules. Match the waveform and test conditions to the rating.

Read the conditions under each pulse curve.

• Single-pulse curve. The curve may cover a limited number of well-separated events if average dissipation is negligible and peak voltage stays within its limit. Vishay specifies fewer than 1,000 events under these conditions for the CRCW-HP e3 single-pulse curve.

• Repetitive-pulse curve. The plot may give separate lines for different duty cycles or a single limit subject to the applicable average power rating. The CRCW-HP e3 continuous-pulse curve also specifies a peak voltage condition.

For rectangular pulses, duty cycle is pulse width divided by the start-to-start period. Check how the manufacturer defines pulse duration and which resistance values each curve covers. Curve definitions vary by manufacturer.

At the chosen duration, record the curve value and the conditions that make it applicable. A logarithmic plot gives an approximate limit, so avoid reporting more precision than the graph supports. If the waveform falls between published test conditions, use the manufacturer’s guidance for that series.

Figure 3. Reading Single- and Continuous-Pulse Ratings

Pulse Voltage, Temperature Derating, and Drift

Pulse voltage. Check the peak voltage across the resistor against the applicable pulse-voltage limit. A pulse can meet the wattage limit and still exceed the voltage limit, particularly at higher resistance values. A surge rating may refer to a specific waveform such as 1.2/50 µs, with a defined front time and time to half value. Use the corresponding test conditions when comparing a surge rating, including the specified resistance range and source network where provided. For sustained operation, use the separate working-voltage limit.

Working voltage. At steady state, two limits can govern the voltage across a fixed resistance. The working-voltage limit caps the electrical stress, and the thermal voltage limit is $\sqrt{P_{\text{allow}}R}$ for a temperature-adjusted power limit of P_allow. Use the lower voltage. The datasheet may also give an insulation test voltage between the part and its surroundings. That test addresses a different path from the voltage across the resistive element.

Temperature derating. Read the applicable average-power curve for ambient, terminal, or case temperature and use the stated mounting conditions. Confirm that the pulse rating covers the actual starting temperature, especially when the resistor is already dissipating power.

Resistance drift. Compare the manufacturer’s allowable resistance change with the circuit’s drift budget. For the CRCW-HP e3, the pulse curves use the permissible resistance change corresponding to 8,000 hours of operation as their reference criterion. Use the application’s service profile to determine how many pulses the resistor must withstand.

Pulse Resistor Types and Typical Applications

Pulse-rated thick-film chips, carbon-film MELF resistors, pulse-rated thin-film parts, wirewound resistors, and bulk ceramic composition resistors use different resistive structures. Element geometry and trimming influence heat distribution, while the series and resistance value determine the usable rating.

Precharge limits initial capacitor current, while discharge removes stored energy within a deadline. Snubbers dissipate switching transients, so repetition rate and parasitic inductance matter. At a surge-exposed input, check the waveform at the resistor terminals with the protection network in place.

For a snubber, the inductance of the resistor and its connection loop affects how it responds to fast edges. A wirewound construction suits many slower discharge loads, while a low-inductance construction can be a better fit for a fast switching network. Where resistance accuracy matters, compare both initial tolerance and drift after pulses. For a compact board-mounted chip, check the mounting and available copper for heat removal.

How to Select a Pulse Resistor

Start with the Circuit Requirements

1. Set the resistance. Use the required current limit, discharge time, or damping behavior.

2. Calculate the worst-case pulse. Account for the limits of supply voltage, resistance and capacitance tolerances, and repetition rate. For a capacitor discharged from a fixed voltage through a resistor, low resistance raises peak power, while high resistance extends the time to reach the target voltage. Check both corners. Include the actual starting temperature and any power dissipated before the pulse.

3. Screen the exact part. Compare the pulse with the appropriate curve and peak voltage rating, then check derated average dissipation.

For the capacitor-discharge example below, Vishay’s equivalent rectangular-pulse method supports a pulse-power comparison: an equivalent rectangle has the same peak power and energy, with duration $t_{\text{eq}}=\frac{E_{\text{pulse}}}{P_{\text{pk}}}$. Compare that point with the applicable rectangular continuous-pulse power curve under its specified conditions. For sharp surges, this conversion is not applicable. Use the manufacturer’s specified surge waveform and voltage test conditions.

Calculate and Check a Capacitor Discharge Pulse

Start with nominal values and an ideal RC discharge. A 200 µF capacitor starts at 30 V and discharges through 100 Ω, with a separate path for recharging. It must reach 3 V within 60 ms and continues discharging below that threshold. Evaluate sustained operation at one complete discharge per second. Section 5.3 adds component and supply tolerances.

The 3 V threshold defines the timing test. The energy calculation uses the full discharge. If a switch ended the pulse at 3 V, the capacitor would retain some energy, so the resistor would absorb only the change in stored energy.

Figure 4. Simplified RC Discharge Path and Capacitor Voltage Decay

The initial voltage V₀ and resistance R set the peak power. The capacitance C sets the stored energy at that voltage. For the ideal discharge,

The voltage time constant RC is 20 ms. Dividing 90 mJ by 9 W gives an equivalent rectangular pulse duration of 10 ms. At one discharge per second, the equivalent rectangular duty cycle is 1%, and the average power is 90 mW.

Use the continuous-pulse curve for sustained 1 Hz operation. Figure 3 shows where the 9 W, 10 ms operating point falls on the CRCW1206-HP e3 curves.

For the average-power check, assume an ambient temperature of 100 °C. The CRCW1206-HP e3 and CRCW1210-HP e3 are each rated at 0.75 W at 70 °C ambient and follow the same ambient derating curve, which reaches zero at 155 °C. The 1206’s 0.75 W rating requires an assembled thermal resistance of 110 K/W or less. The calculation below uses the ambient-temperature curve and assumes suitable PCB mounting.

At 100 °C ambient, the derated average-power limit is approximately 0.485 W under the stated mounting conditions. The nominal CRCW1206-HP e3 screening results are:

Revise the Selection When the Pulse Exceeds the Curve

Doubling the resistance halves peak power and doubles the equivalent pulse duration. The total energy per complete discharge stays at 90 mJ because the capacitor starts at the same voltage. The time to reach a target voltage V_target follows from the exponential voltage decay.

The nominal timing-only resistance limit is 130.3 Ω. The 200 Ω option misses the 60 ms deadline, so retain 100 Ω to preserve the original discharge time. The CRCW1210-HP e3 continuous-pulse curve reads approximately 12.5 W at 10 ms, compared with the nominal 9 W load. Check that the layout accommodates the larger footprint.

Illustrative tolerance check. Assume voltage ±5%, capacitance ±10%, and resistance ±1%. The largest initial voltage is 31.5 V. The minimum resistance sets the peak-power bound, $\frac{{\left(31.5\text{V}\right)}^2}{99\text{Ω}}\approx 10.02\text{W}$. The maximum resistance and capacitance set the equivalent duration bound, $\frac{101\text{Ω}\times 220\text{µF}}{2}=11.11\text{ms}$. Pair these bounds for a conservative comparison over this descending part of the curve. The slowest discharge reaches 3 V in:

This is within the 60 ms deadline. These bounds use the stated example tolerances. For qualification, repeat the check using the effective resistance and capacitance ranges over the operating temperature range.

The tolerance-adjusted maximum nominal resistance for the 60 ms timing requirement is:

This is a timing-only limit under the stated tolerances, before temperature effects and resistance drift. The 100 Ω nominal resistance remains below it.

At 11.11 ms, the CRCW1210-HP e3 continuous-pulse curve indicates approximately 12 W under the datasheet curve conditions, compared with the worst-case peak power of 10.02 W. As a conservative electrical screen, the 31.5 V peak is below the 200 V maximum operating-voltage specification. Pulse-power capability must be evaluated separately. At 1 Hz, the maximum average power is approximately 109 mW, below the ambient-derated limit of 0.485 W at 100 °C under the specified mounting conditions. However, satisfying the voltage and average-power screens does not establish allowable repetitive pulse power at this elevated temperature. The CRCW1210-HP e3 remains a preliminary candidate pending manufacturer confirmation or appropriate high-temperature qualification testing at the actual starting temperature.

Split the Pulse Load Without Changing Discharge Time

The discharge circuit in Section 5.2 can use two resistors while maintaining approximately the same total resistance. Figure 5 illustrates an idealized series configuration using two 50 Ω resistors, giving exactly 100 Ω. For a practical CRCW1206-HP e3 implementation, two 49.9 Ω resistors provide a nominal total resistance of 99.8 Ω. With a 200 µF capacitor initially charged to 30 V, this configuration reaches 3 V in approximately 46.0 ms, and each resistor dissipates approximately 4.51 W at the beginning of the discharge. These values are nominal and do not account for component tolerances.

Figure 5. Series and Parallel Discharge Paths with an Idealized Series Pair

Both two-resistor configurations halve the nominal peak power and energy per component. The series pair also halves the voltage across each resistor. With component tolerances, the higher-resistance part dissipates more power in series, while the lower-resistance part dissipates more in parallel. Check each part against its own repetitive-pulse curve and peak-voltage limit, using the worst-case load and the actual mounting conditions.

At one nominal discharge per second, each resistor in either pair dissipates 45 mW on average. With two CRCW1206-HP e3 resistors, the nominal load per part is 4.5 W at an equivalent duration of 10 ms, below the plotted continuous-pulse limit of approximately 8.5 W. Compared with a single 1210, the pair needs two footprints and requires a check of worst-case power sharing and board temperatures before qualification at 100 °C ambient.

Failure Modes and Bench Verification

Pulse overstress can shift resistance permanently or open the element. Localized overheating damages the resistive film, while excessive terminal voltage can cause flashover or arcing. Inspect for cracks and solder-joint damage after thermal cycling, and check inductive layouts for switching overshoot. In the discharge circuit, drift becomes a functional failure when the voltage misses its deadline.

1. Capture the baseline and waveform. Record the initial resistance and measure voltage at the resistor pads on the intended board. Capture current where needed, and use sufficient bandwidth to resolve the pulse edge and any overshoot. For measurements across floating resistor terminals, use a differential probe rated for the expected differential voltage, common-mode voltage, and bandwidth.

2. Check the measured load. Use the measured peak, duration, and energy to check the applicable ratings. If a current-sense element changes the discharge path, account for its resistance. For a near-ideal resistor, integrate voltage squared divided by the measured resistance to estimate pulse energy.

3. Reproduce the operating sequence. Test isolated events, sustained cycling, and relevant retries at the expected starting temperatures. Set the cycle count from the service profile and choose the sample count for the qualification objective. For high-voltage or stored-energy setups, follow lab isolation and discharge procedures and account for hot components.

4. Check drift and discharge time. After cycling, measure resistance at the baseline temperature and calculate drift relative to the initial reading. Apply the circuit’s drift budget and any component drift criterion that covers the test. Accept the design only if there is no damage, drift stays within the applicable limits, and the circuit reaches 3 V within 60 ms under the stated tolerances.

Record the starting temperature, pulse sequence, measured waveform, resistance change, and discharge time for each sample. Set design margin from the range of expected circuit conditions and the uncertainty in the curve reading and measurements. If the available margin is small, review the measured operating envelope with the manufacturer or choose a part with more headroom.

Conclusion

Start with the load the application imposes, whether it is precharge, discharge, snubbing, or surge protection. Select a construction and resistance that meet the circuit requirement, then check the exact part’s pulse curve, peak-voltage limit, and derated average power. Verify the measured waveform, resistance drift, and functional timing on the intended board before release.