A current probe is not really a current sensor
A current probe is not really a current sensor, because it does not respond to direct current. A current probe is designed to sense and integrate the first derivative of the current – or, more precisely, the magnetic field associated with the current being measured. A current probe is a self-integrating I-dot sensor.
In contrast, an I-dot sensor can correctly be called a current sensor, because it is designed to sense the first derivative of the magnetic field associated with the current being measured. In other words, the output is proportional to the time rate of change of the aperture current.
A current probe is not really a current sensor
A current probe is not really a current sensor because it does not respond to direct current (for which the I = dot and associated B-dot = 0). A current probe is designed to sense and integrate the first derivative of the current, or more precisely the magnetic field associated with the current being measured. A current probe is a self-integrating I-dot sensor
In contrast, an I-dot sensor can correctly be called a current sensor because it is designed to sense the first derivative of the magnetic field associated with the current being measured.
The sensitivity of an I-dot sensor is determined entirely by its physical dimensions. It does not need to be calibrated, because the dimensions do not change over time. I-dot sensors are available with sensitivities ranging from 0.5nH to 10nH as standard, and a much wider range is possible if required. Rise times for these sensors range from 0.17nH to 1.8nH, which corresponds to upper 3dB frequencies from 190MHz to 2GHz. The wide range of output voltages (5mV to 5kV) and the differentiating mode of these sensors enable them to meet a wide variety of measurement requirements.
The voltage output of an I-dot sensor is limited by standoff voltage at critical points in the sensor, such as the connector or the gap connection. For most Prodyn I-dots, the limit is 5kV differential and 2.5kV to ground.
PPM Test supplies the RID-200 series of I-dots and the ID-10B.
The output voltage from an I-dot sensor is a linear function of the frequency of the current being sensed, whereas the output voltage from a current probe is independent of frequency in its useful bandwidth. In fact, the useful bandwidth is defined as the frequency range for which the output voltage is independent of frequency – the theoretical lower limit being the transition frequency.
A current probe is a current transformer: the measured current flows in a one-turn primary “winding” and the signal current flows in the N-turn secondary winding. The sensitivity (transfer impedance Zt) of a current probe depends on the number of turns (N) in the secondary winding and the load impedance of the secondary winding, usually a 50Ω cable, which may be combined in parallel with an internal resistor to adjust the transfer impedance.
The performance of a current probe is limited by standoff voltage at critical points in the probe, such as the connector or the internal shunt resistor, induction saturation in the core and resistive heating in the winding. Standoff voltage limits the output voltage to 2000V in probes without an internal resistor and to 400V in probes with an internal resistor. Induction saturation limits the measured current and the time interval during which it can be applied.
The output of an I-dot
Vout = M dI/dt
where M = sensor mutual inductance (H), I = total current through aperture (A)
The output of a current probe
Vout = Zt x Isensed
where Zt = transfer impedance
Which sensor to use?
The choice of sensor depends on the following:
- Signal processing
- Power and voltage limitations.
I-dot sensor versus current probe
|I-dot sensor||Current probe|
|Advantages||Needs no periodic calibration|
Wider operating bandwidth
Larger max output = greater dynamic range
|Provides a pre-integrated signal.|
|Disadvantages||Output must be integrated to calculate current||Narrower bandwidth
Inductive and thermal limitations
Requires periodic calibration
Current probes and I-dots from PPM Test
PPM Test is a global distributor of Prodyn Technologies Inc., which has been manufacturing EMP instrumentation for research, military and aerospace application for over 40 years in Albuquerque, New Mexico. Current probes manufactured by Prodyn include bulk injection probes, high frequency IP series probes and specialised I-dot sensors for pulsed or high power measurements. Products are designed for MIL-STD, DEF-STAN and DOE testing standards. Probes have a split core for easy installation onto cable bundles and typically exhibit a frequency response of +/- 1.5dB within the specified frequency range.
Prodyn current probes
|Model||Freq. range (useable)||Zt (Ω)||Max Output Voltage (Vom)||Saturation current (A)||CW max average current (A)||Peak current (A)||Max pulse width (μs)||Rep rate (kHz)|
|IP-2-1||100 kHz - 1.3 GHz||1||400||2.94||2.87||284||0.2||6.36|
|IP-2-5||125 kHz - 800 MHz||5||1000||1.68||13.23||180||0.02||56963|
|IP-2-10||500 kHz - 1 GHz||10||1000||2.07||7.35||100||0.01||56963|
|I-075-1B||40 kHz - 150 MHz||5||2000||27.74||35.15||360||0.22||3995|
|I-075-1C||120 kHz - 420 MHz||5||400||27.51||1.24||64||0.31||1.21|
|I-075N-10||120 kHz - 350 MHz||10||2000||9.87||9.26||120||0.04||5376|
|I-125-1A||10 kHz - 100 MHz||5||2000||1259||39.4||320||2.06||478|
|I-125-1B||50 kHz - 130 MHz||5||2000||39.45||35.15||360||0.37||2389|
|I-125-1HF||50 kHz - 1 GHz||5||400||29.93||1.29||76||0.5||0.58|
|I-125-1D||1 kHz - 110 MHz||5||2000||57.33||27.87||360||1||697|
|I-125-1E||850 Hz - 120 MHz||5||2000||9.82||44.1||360||0.52||2103|
|I-125-2A||10 kHz - 150 MHz||1||400||1125||4.27||456||9.01||0.01|
|I-125-2C||300 Hz - 25 MHz||1||2000||50.03||146||1880||5.22||697|
|I-125-2E||1 kHz - 200 MHz||1||400||9.01||2.95||400||1.14||0.048|
|I-125-2HF||50 kHz - 1 GHz||1||400||42.66||4.43||400||1.66||0.15|
|I-125-3A||1 kHz - 250 MHz||0.03||400||878.84||23.2||13397||10.3||0.01|
|I-125-4A||1 kHz - 150 MHz||0.1||400||879.75||12.85||4064||10.3||0.01|
|I-125-6A||20 Hz - 30 MHz||0.5||2000||9.82||258||4200||6.12||1055|
|I-125-7A||400 Hz - 100 MHz||1||400||20.8||11.33||389||15.55||0.06|
|I-125-9A||1 kHz - 270 MHz||0.005||400||25502||60.05||80072||5||0.01|
|I-150-1HF||1 kHz - 1 GHz||5||400||15.39||1.27||72||0.42||0.74|
|I-262-2A||10 kHz - 50 MHz||5||2000||1865||99.61||320||4.6||542|
|I-262-3A||10 kHz - 140 MHz||2||400||1690||2.32||232||11.5||0.01|
|I-262-4A||10 kHz - 100 MHz||0.06||400||1832||12.99||6707||14.36||0.01|
|I-262-5A||10 kHz - 200 MHz||1||400||1158||2.6||424||8.61||0.01|
|I-262-6A||10 kHz - 150 MHz||0.1||400||1468||9.02||4032||11.5||0.01|
|I-300B||180 kHz - 300 MHz||5||2000||3.86||30.97||400||0.02||35556|
|I-310B||50 kHz - 200 MHz||1||400||2.64||4.98||395||0.16||0.97|
|I-320B||200 kHz - 500 MHZ||1||400||0.94||2.78||352||0.05||1.35|
|I-400A||50 kHz - 450 MHz||5||2000||13.19||30.97||400||0.15||5338|
|I-410A||15 kHz - 450 MHz||1||400||22.33||4.98||395||0.73||0.22|
|I-415C||1 kHz - 350 MHz||1||2000||297.84||154.35||2800||1.36||2829|
Bulk Injection Current Probes
|Freq. range (useable)||Zt (Ω)||Max Output Voltage (Vom)||Saturation current (A)||CW max average current (A)||Peak current (A)||Max pulse width (μs)||Rep rate (kHz)|
|IT-050-1||100 kHz - 100 MHz||500||2000||38.7||30.97||4||0.79||975|
|IT-075-1||100 kHz - 100 MHz||500||2000||44.65||30.97||4||0.79||975|
|IT-125-1||100 kHz - 100 MHz||500||2000||53.58||30.97||4||0.64||1219|
|IT-125-2||50 kHz - 700 MHz||50||2000||53.58||8.6||40||0.45||4747|