Current probing techniques
AC current probe Philips PM 9355
This is a highly useful probe, but not seen as often as similar AC models from Tektronix, HP/Agilent/Keysight, or perhaps Yokogawa. With a frequency range of 12 Hz to 70 MHz (45 MHz in the two most sensitive ranges) you got plenty of bandwidth to cover measurements of switch mode power supplies, LED drivers, etc. A note of caution: The amplifier unit contains the reed-relays that have been observed to fail on the PM519x series of function generators. If you get a malfunctioning unit you should definitely check the relays. Should you need to replace a relay or two with alternatives this is not a huge deal. For checks and adjustments the manual calls for the Tektronix 067-0559-00 calibration fixture (see below), an item frequently referred to when it comes to current probe testing, though hard to come across.
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Tektronix P6042 Current Probe
A classic combination of probe and amplifier dating back to the 1960's, and having been out of production for ages, but still highly valued and being sold at rather elevated prices. The combination of a loop and a Hall device extends the reponse to DC. A weak point is the cable where the individual conductors may break from simple wear and tear. The unit I have had broken wires at two locations, close to the entry of the strain relief bushing on the front panel of the amplifier, and close to the entry of the cable strain relief for the probe assembly. The solution was to shorten the cable at both ends and resolder the wires (the dismantling of the probe shell and removing the crimp mantle required a steady hand and some patience)...
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Tektronix 067-0559-00 Calibration Fixture
The fixture was designed for the calibration and servicing of current probes such as the P6042 or models with a similar form factor. The fixture includes a 50 Ohm termination in series with the shorted center conductor, and mates directly with 50 Ohm gear equipped with a GR-874 connector. Obviously, an adaptor for BNC or N helps to put the fixture into use. You may spend a while trying to find the fixture, but there's an alternative: Make your own! It requires a couple of connectors (of your own choice), a piece of metal profile, and a termination. |
Tektronix CT-1 and CT-2 AC Current Probes
A small form factor and a wide bandwidth are important features of these neat current probes. Together with the CT-6 these current probes cover different frequency spans, max. current and sensitivities. The cousin CT-6 has the widest bandwidth of the three, but its cable cannot be removed. You should consult the data sheet of the probes to decide which one to put into use. Note that the core of these probes is solid, not split, which requires some soldering to do a measurement, but the advantage is robustness and repeatability. The connector for the CT-1 and CT-2 is an Sub-Minax #27-3 from Amphenol according to the manual. Perhaps for that reason, the CT-1 and CT-2 are used almost exclusively together with the P6041 Probe Cable. However, according to Tektronix the P6041 cable is also used for the trig and signal connections on TLA7xx series logic analyzer controllers, and in the documentation for these, the connector is claimed to be SMB. The mounting stud allows the CT-1 and CT-2 to be used for in-circuit installations, but need to be isolated in most cases to avoid it to come in contact with any circuitry when not in use. |
Above: Low-frequency response of the CT-2.
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Being a passive, bidirectional probe, the CT-1 and CT-2 may also be used to inject a signal, useful for analyzing feedback systems. Beware, however, of the phase shift; The -3 dB cutoff is about 760 Hz for the CT-2. For some purposes it may be required to introduce a compensation network to flatten the phase response.
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Picture to be added
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Chauvin Arnoux E27 AC/DC Current Clamp
The E27 is a current clamp/probe with a nice form factor with not too large "long-nose style" jaws that allow you to probe wires inside a variety of devices to test. The current capacity up to 100 A is sufficiently large and the frequency span from DC to 100 kHz is sufficiently wide to cover events in power supplies, chargers and electronic loads, or to monitor the drive current for shakers, for instance. For monitoring MOSFET switching currents in switched mode supplies you need an ever wider bandwidth, and a smaller form factor, but then the price tag of the probe will increase for sure. The resemblance of the predecessor E3N with Fluke 80i-110s, Tektronix a622, Keysight 1146, and AEMC SL261 is quite striking. The latter is not surprising as AEMC Instruments is a part of the Chauvin Arnoux Group. Finally, a tip: A second-hand Chauvin Arnoux E3N and similar competing models found on auction sites are often priced higher than a brand new Chauvin Arnoux E27 from an autorized dealer! Check out prices before getting tempted to get a used one! |
Fluke Y8100 DC/AC Current Probe
The Y8100 is a Hall-effect probe with 20 A or 200 A full scale, over the specified DC - 1 kHz range, thus primarily targeting power installation applications. Having a 2 V full-scale output, the Y8100 is connected to a multimeter, digitizer or an oscilloscope. The trimmers are located under the adhesive covers, which is quite impractical. The idea is to tear off the covers to access the trimmers, and then replace the covers when the adjustments are done. Fluke offered replacement covers with a calibration kit, but the most practical solution is to pierce holes to gain permanent access the trimmers: Most likely, you want to adjust the trimmers regularly. The Y8100 is powered by 4 AA batteries, so if your measurement session drags out, you need to modify the Y8100 to be powered for an external DC supply, or go for another probe. The graphs show the response between 1 Hz and 10 kHz for two different adjustments of the AC trimmer. Note that the y-axis is only relative, being a product of a current sensing resistor and a number of windings going through the core. When trying to flatten the response to 1 kHz, the response tends to peak, so the adjustment will be a compromise to get the desired response. If you follow the calibration instructions in the manual, you are likely to get a very peaky response. |