How to select a Non-Inductive HF Coaxial Current Shunt or Current Viewing Resistor (CVR) for applications with high di/dt currents

About the Current Viewing Resistor (CVR) / Non-Inductive Co-Axial Shunt Resistors

 

Selecting a CVR / Co-axial shunt

1. Choose a Co-axial shunt / CVR geometry; rectangular or tubular. This is usually determined by the shape of the conductor connecting to the Co-axial shunt /CVR input. The concentric tubular geometry is slightly less inductive inherent to the design.

2. Choose your output connector: BNC (standard), SMA, TNC, N, etc. Be mindful of your frequency response.

3. Use Ohm’s Law to select your resistance value.

4. Determine the energy or power the Co-axial shunt /CVR will need to dissipate. For pulsed applications that are not rep-rated this will be the joule rating, and for continuous applications this will be the wattage rating. For rep-rated pulsed applications call to discuss energy dissipation.

5. Now consider the CVR response characteristics you require: bandpass and risetime. Note how the bandpass decreases and the risetime increases as the resistance value decreases. This is ‘skin effect’ and cannot be compensated for.

6. Now navigate through the online catalog. Renderings, dimensions, and performance specifications of the Co-axial shunt / CVRs, for each model. The resistance values in the catalog represent a range that are popular and may be in stock. We can build any resistance that is between the catalog values. Tolerances The standard tolerance of the resistance value is +/- 4%. This is the tolerance the resistive element is built to. The measurement of this resistance by our Kelvin Bridge is limited to a tolerance of +/- 0.2%. This +/- 0.2% is the uncertainty of the value on the CVR label. We provide ANSI-NCSL Z540-1-1994 traceable certification of this value in our 'Certificate of Conformance'. We offer tolerance upgrades of +/- 2% and +/- 1%. Please specify this requirement in the comments box when submitting to https://powertekuk.com/cpower. There is a price adder for these respective tolerance upgrades. Connectors The standard output connector is a 'BNC'. We can adapt your connector preference to the input and output of most Co-axial shunts / CVRs. We also offer all English and Metric threads for input studs. Please specify custom connector and stud preference in comment box. Navigating the Products Page

Click on the Co-axial series you are interested in, view spec tables. Click on the model for a dimensional drawing. When finished go to https://powertekuk.com/cpower

Our range of CVR's are non inductive high frequency coaxial shunts designed to operate over a wide bandwidth, with both AC and DC power systems, especially where phase angle is a consideration, crucial during impedance measurements and power calibration systems. Very useful for adding or extending the current range of Frequency Response Analyzers and Multimeters. CVR's can be used with phase angle meters, where phase angle between voltage-voltage, voltage-current or current-current is required, ideal for phase protection, power meters, wattmeters, energy meters and current transformer calibration.

CVR's are rugged high frequency resistors designed to sustain very high peak power and current inputs generated by capacitor banks, pulse generator systems and steady state current loads. Their linear response over a broad frequency band provides an accurate indication of current magnitude free from inductive components. Inherent in CVR design is a coupling between the major electrical parameters of resistance, bandpass, energy capacity and wattage rating. This means we provide a wide ranging series of standard units covering a broad range of specifications.

 

WATTAGE RATING

Although most CVR's are designed primarily for surge current measurements, their rugged construction and their low temperature coefficient resistive elements have made them ideally suited to a number of steady state applications. An average wattage rating applicable to continuous current loading is thus quoted for each resistor series. Care should be taken in circuitry involving either AC or high duty cycle pulse currents that this rating not be exceeded, since CVR damage by overheating can result. If requested we can supply resistors with special construction which will increase the standard wattage rating to a high value depending on the model.

ENERGY CAPACITY
A convenient criteria for selection of a CVR is provided by its pulse energy capacity. This rating is defined as: Emax = Rcvr [integral i^2dt] max Thus by definition the pulse energy capacity is the maximum recommended energy that should be dissipated in the CVR over a period so short that losses are negligible. When a square wave current pulse is utilized, the energy input (E) into the CVR is equal to Rcvr^i^2t, and any unit in which Emax greater than E can be used. Capacitor bank systems present a more difficult problem since a major fraction of the initial stored energy is dissipated external to the CVR, i.e., Ecvr = Estored (Rcvr / (Rcvr + Rexternal)) Prior to measurement, the effective system resistance is generally unknown. However, from consideration of peak current, CVR resistance, and a practical CVR output voltage, the ratio of Ecvr / Estored is about 1/10 in typical underdamped capacitor systems. Consider a system in which L = .5uh, C = 500uf, V = 20kv, and E = 25 kilojoules. Assume an effective resistance of 1/10 critical. That is, Reff = 1/5 (sqrtL/C) = .0063 IF Rcvr = .001 ohms THEN Ecvr = Estored (Rcvr / Reff) = 4 kilojoules, a value well within the capacity of our 5-kilojoule F-5000-20 model. The Emax (joule) value tabulated for each resistor model is conservative, and all CVR's will sustain limited use at energy inputs to 1.5 Emax without destruction. If a resistor is continuously operated at energy overload, its DC resistance should be checked frequently, since some permanent variations may result.

 

FREQUENCY RESPONSE

Bandpass of a CVR is essentially flat from DC to an upper limit determined primarily by skin effect in the resistive element. Associated bandpass is based on a measured 10% to 90% risetime response to a step function of current produced by a coaxial line pulse generator. The di/dt of the test pulse exceeded 10^12 amps/sec.

 

RESISTANCE VALUES

Unless otherwise specified, resistors are supplied with resistance tolerance of ±4% of nominal value. In addition, a Kelvin Bridge determination of its exact resistance, accurate to ±0.2% is supplied with each unit. The thermal coefficient of the coaxial shunts is 8-10 ppm/degC. A wide range of special resistance values for any of our standard units can be supplied.

 

MECHANICAL DESIGN

Case construction of all coaxial current viewing resistors is silverplated brass. Standard output signal connector is BNC with other connectors available. Large coaxial CVRs utilize a high current flange and coaxial threaded stud input connections. Powertek's flat configuration CVRs, the Series W, originally developed for flat plate transmission line installation, are available in a wide range of unit widths and input configurations and have been found to be particularly useful in applications requiring resistors with extreme energy and wattage ratings.

 

WORLD-WIDE TRACEABILITY

All measurements made using the CVR coaxial shunts are traceable to National and International standards; through the measurement standards of Powertek. All CVR coaxial shunts are supplied with a certificate of conformance necessary for quality assurance standards such as IEC17025/ISO9001. Independent measurement certification is possible using UKAS, A2LA or Z540/NAVLAP certificate.

   

Non inductive coaxial shunt - CVR applications

 

• GaN, Mosfet, IGBT and SCR current measurements
• Pulsed current applications
• Radar and RF
• Calibration laboratories: HF current measurement
• Current sensing for phase meters
• Single phase angle indicators V-A
• Cos Phi and PF indicators
• Power factor meters
• High Frequency Watt Meters
• Wideband Power meters
• Power analyser external current input
• Current transformer calibration
• Current probe phase delay
• Current control of automatic welders
• Measure output of automotive alternators
• Fault current detection to determine bearing wear of steam turbine generators
• Electron Beam Welding
• Current detection in detonation systems
• Three-phase fault testing in power transmission substations
• Fault detection in modulators
• Measurement of laser system currents
• IGBT Chopper current control in electric cars
• Circuit breaker testing