1) Section 1

Carbon composition usage has been declining over the past decade due to poor environmental performance and increasing prices. The main advantages of CC resistors is in their ability to withstand high energy/high voltage surges as well as their ability to operate at high frequencies due to noninductive construction. Conventional wirewound and film resistors (carbon film, metal film, metal oxide, metal glaze) are generally incapable of withstanding equivalent energy levels. The PR Series was developed to offer overload capabilities approaching or exceeding those of carbon composition resistors, particularly in values below 1000 Ohms. If your application does not involve a sizable surge, then please consult with our engineering group since it is likely that a lower cost film or wirewound resistor will suffice. Customers are requested to fill out RCD's Surge Questionnaire" form in order to ensure proper resistor selection.

2) Section 2

The "bulk" nature of the carbon slug enables CC resistors to withstand high energy levels regardless of the resistance value. The pulse withstand performance of RCD's PR Series is value dependent. Low values offer increased overload capabilities due to the increased mass of resistance material. Consult the Multiple Surge Capability Charts to determine if the PR series is appropriate for your application. The charts give the multiple pulse surge rating of 1/8W to 2W hotmolded carbon composition resistors in comparison to 1/8W to 2W Series PR resistors. The information is based on short duration pulses (less than 500mS). For longer overloads, extrapolate from the following...

RCD's PR series are rated the same for single or multi surges as long as average power level doesn’t exceed the resistor power rating. Carbon comp resistors are generally capable of much higher single pulse levels than indicated on the multiple surge capability chart so this must be considered when replacing CC resistors.

3) Section 3

Carbon composition resistors are essentially non-inductive. RCD's standard PR resistors are inductive, specify option "X" if non-inductive version is required. Maximum series inductance level of PR1/8X, PR1/4X, and PR1/2X shall be .2 microH <= 50 ohm, .37microH > 50ohm when measured at 500KHz. Maximum series inductance of PR1X and PR2X shall be .3 microH <= 50 ohm and .6 microH > 50 ohm

4) Section 4

In order to use PR series resistors in a pulse application, 3 criteria must be met simultaneously...

• A) ENERGY: Pulse must not exceed the rated energy level (refer to surge capability charts)
• B) VOLTAGE: Peak pulse voltage must not exceed the following levels... (PR1/8 = 3KV, PR1/4 = 5KV, PR1/2 = 7KV, PR1 = 12KV, PR2 = 18KV)
• C) AVERAGE POWER: :The average power level in equally spaced repetitive pulse applications must not exceed the rated wattage... (PR1/8 = 1/8W, PR1/4 = 1/4W, PR1/2 = 1/2W, PR1 = 1W, PR2 = 2W)

Example #1:

Circuit requires a 200 ohms resistor that can handle a square wave pulse of 2000V for 800mS. Determine the energy requirement. Single square wave pulse energy is calculated as follows:

E = Pt, P = Vsquared /R

• E= Energy (Watt-seconds or Joules)
• P = Pulse Power (Watts)
• V = Pulse Voltage
• R = Resistance (Ohms)
• t = Pulse Duration (Seconds)
• P = Vsquared /R = 2000squared /200 = 20,000 Watts
• E = Pt = (20,000) (.0008) = 16 joules

By reviewing the Multiple Surge Capability Charts, it is determined that type PR2 200 Ohms is capable of 21 joules. The peak pulse voltage is within the 18KV rating, and since this is a single non-repetitive pulse, the average poser requirement doesn’t apply. Type PR2 would be the best choice for the application.

Example #2:

A 10 MicroF capacitor charged to 1KV is being discharged into a 10 Ohms resistor. Determine whether 1/2-watt resistor will suffice.

Capacitor discharge energy is calculated as follows:

E = CVsquared /2

• C = Capacitance (Farads)
• V = Peak Voltage
• E = Energy (Wattseconds or Joules)

E = CVsquared /2= (.00001)(1000)squared /2 = 5 joules

Surge capability chart indicates that 10 Ohms PR1/2 is capable of 4.5 joules and therefore isn't satisfactory for the application. PR1 10 Ohmsis capable of 18 joules, and its peak pulse rating of 12KV is well above the 1KV surge. PR1 is therefore the correct choice.

Example #3:

Circuit requires a 10 Ohms resistor that can withstand a 1KW square-wave surge with 1mS duration which repeats every .5 seconds.

• Determine the energy... E= Pt = (1000)(.001) = 1 joule
• Determine peak pulse voltage... V=Sqroot(PR) = Sqroot(1000)(10) = 100V
• Determine average power...

  

  • Average Power = Pt/T = E/T
  • P= Peak Power (Watts)
  • t= Pulse duration (Seconds)
  • T= Cycle Time

  Average Power - E/T = 1/.5 = 2W

  Type PR2 meets all 3 requirements of energy, voltage, and average power.

  5) Section 5

  RCD offers a variety of design modifications to enable increased power, voltage, energy, etc. It is recommended that the factory be consulted to ensure optimum reliability and lowest cost selection.

  In addition to resistor families specifically designed for highsurge applications, we offer modified versions of most standard resistor families which are typically capable of pulses 50% to 200% above those of standard parts. Increased surge capability is achieved by utilizing specialty materials, construction, and/or processing techniques. RCD offers the industry's widest range of surge resistors with pulse ratings up to 30,000 joules; 450,000 volts; 1000 Amps.