Background:
Compliance West USA offers the BNCI option for connection to an oscilloscope to montior the current waveform while a pulse is being delivered. However, this option can only be used on a MegaPulse with a grounded return. Grounded return MegaPulses can only be used when the DUT is not energized. If a MegaPulse is designed to be used with a CDN; that is, the MegaPulse will inject the pulse onto a mains voltage to test an energized DUT, then the MegaPulse will have a floating output. There cannot be a connection to ground while using a CDN.
The BNCI can still be ordered on a MegaPulse with floating outputs. It can only be used in the following conditions:
If the BNCI is ordered on a MegaPulse with floating outputs, it must never be used to measure the current waveform fron Neutral to Line, or serious damage to the oscilloscope could result.
Methods to read Pulse current output from line to neutral while using a CDN
Use a current probe: If a current probe is used, it must have sufficient bandwidth and current carrying capability to support the pulse. Generally, a probe with a bandwidth of 250kHz will be sufficient to accurately show risetimes of as short as 2uSec. Current carrying capability is a problem on many MegaPulses, as current outputs of 1000A or more are not uncommon. For lower current MegaPulses, this could be a solution but the cost is high.
The point of the Compliance West USA BNCI option is it inexpensively shows the operator that a pulse has been delivered. The BNCI option is reasonably accurate when displaying peak current but is not accurate in the time domain. For most users, this is acceptable. However, since the BNCI cannot be used on floating output MegaPulses for use with CDNs. if the user is forced to use a current probe he gains accuracy in the time domain but at high cost.
Use a current shunt: A current shunt consisting of a known low resistance in series with the (floating) MegaPulse return is a cost-effective way to read the current output. Care must be taken when designing the current shunt to make sure it is capable of handling the current flow without heating. In addition, care must be taken to ensure the value of the shunt is vanishingly small with respect to the output impedance of the MegaPulse tester itself. Further, checks must be made during the test sequence to make sure the current shunt stays cool, as resistance changes as the resistor rises in temperature. If sequential tests are conducted with a shunt that is rising in temperature over the course of the tests, the current output waveform shown on the oscilloscope will appear to have a lower and lower output value, due to the heating of the shunt resistor, even though the current output is constant over the test sequence.
Compliance West USA has used a current shunt consisting of ten matched 1 ohm cement type resistors assembled in parallel with good results. This external solution must be used with care, and is not acceptable for all MegaPulses. If a low impedance MegaPulse is considered, 0.1 ohm may be too high a percentage of the overall impedance and could affect the output. In this case, a 0.01 ohm shunt may be better.
The current shunt is placed in series with the ungrounded return lead, then the voltage across the shunt is measured with a high frequency differential probe, such as the Tektronix P5200/P5205/P5210 (http://www.tek.com/site/ps/0,,51-11195-INTRO_EN,00.html) connected across it. The output can be measured with a common grounded BNC oscilloscope.Care must be taken to select the correct probe range to keep the BNC voltage within the capabilities of the oscilloscope (around 30V).
This method will work when the MegaPulse with the floating output is connected to the CDN, and the DUT is energized with the line voltage from the CDN.
If a MegaPulse with a floating output is used without a CDN and connected to the DUT in an unenergized state, then the output of the MegaPulse must be connected to ground with an external jumper. This jumper must be removed when a CDN is connected to the test setup.
Order the CS option: Compliance West USA offers a floating current shunt built into select MegaPulses with floating outputs designed to be used with CDNs. This shunt output is conditioned and will have the accuracy of the BNCI option. It must be used with a high frequency differential probe, and can be used in any operating mode (no CDN, CDN L-N, CDN L-Ground, CDN N-Ground). The CS option may not be available on all MegaPulses, due to real estate concerns. The part number of the built-in current shunt is 00-CSXX, where XX is the shunt resistance (determined by the factory). Call Compliance West USA or contact your local representative for more information about the CS option.. For further assistance regarding reading current output on a MegaPulse, please contact the factory.
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