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Constructional Project 5MHz 40A Current Probe A current probe is an incredibly useful tool for development, testing and debugging but is usually quite expensive. This DIY version performs well compared to many commercial offerings but at a fraction of the price! Project by Andrew Levido Levido U sing an oscilloscope to monitor the current in a circuit can be challenging. Oscilloscopes are made to measure voltages, so if you can find or add a suitable shunt resistor in the current path, you can measure the current indirectly. However, this is usually only practical at relatively low current levels and only if you can safely connect your ‘scope probe ground to one side of the shunt (which is often not the case). Say there is no suitable shunt resistor, the current runs to several amps or the circuit is not conducive to the safe connection of a grounded probe. You will probably have to use an isolated current probe in those cases. If your circuit operates at mains potential, an isolated probe is mandatory. You can certainly buy such current probes. The problem is getting good performance at a reasonable price. You can opt to spend thousands of dollars on a high-end 50-100MHz probe from one of the big names in test equipment, or you can spend $100 or less on Ali­ Express for a no-name probe with a bandwidth of just a few hundred hertz. I wanted an inexpensive, high-­ performance current probe, so I built my own. The resulting probe, described here, can measure current up to ±40A, with a bandwidth from DC to 5MHz. Its output is fully isolated from the measurement terminals, so you can safely measure the current of mainspowered devices. The output, available on a BNC connector, is scaled to 100mV per amp, so it is in the range ±4V. The device is powered by an internal rechargeable lithium-ion cell. Charging is via a power-only USB-C socket. Current Probe Features & Specifications » » » » » » » » » » 18 Current measurement: bi-directional Output scaling: 0.1V/A (±40A translates to ±4V) Input/output isolation: 420V RMS (600V peak) ‘reinforced’ Maximum current: 40A peak (35A continuous) Bandwidth: DC-5MHz Power supply: onboard Li-ion cell Battery life: approximately 30 hours Charging: USB Type-C socket (5V DC) Charging time: approximately 3 hours from flat Charging current: 300mA (optionally 100mA) Scope 1 shows a typical ‘scope capture made using the probe. This is the mains inrush current of a variable-­ frequency motor drive unit, which has a large capacitor bank charged via a bridge rectifier from the mains. A softstart circuit limits the inrush current at power-on. The vertical scale of the scope capture is 2V per division, corresponding to 20A per division. You can see that the peak charging current is about 34A in the first half-­ cycle, with a reduction in the current each cycle after that as the capacitors charge. Another example capture is shown in Scope 2. Here, a short is applied across a bench power supply set to a 6A current limit. The peak current spikes to almost 50A (showing some headroom in the current probe’s design) but rapidly drops as the power supply current regulation circuit begins to operate. Within a millisecond, the current is brought under control and limited to 6A. Design The heart of the current probe is the ACS37030 chip from Allegro Microsystems. Like many similar devices, this uses a Hall effect sensor to measure a current indirectly by measuring the magnitude of the magnetic field it produces. See the separate panel for some background on how the Hall Practical Electronics | November | 2025 5MHz 40A Current Probe effect works and how it is used in this application. Hall sensors are useful in these applications since they work with DC; however, their frequency response is typically limited to a few hundred kilohertz. The ACS37030 family of sensors is particularly interesting because it pairs a Hall effect sensor for DC and low-frequency signals with inductive sense coils for high-­frequency signals. They are available with full-scale current ratings of ±20A, ±40A or ±65A and come in a 6-pin SOIC (small-outline integrated circuit) package with 3500V RMS isolation. All for less than $8 in low quantities. The block diagram of the ACS37030 is shown in Fig.1. You can see the Hall current sensor at lower left (“Dynamic Offset Cancellation”), with the inductive sensor just above it. The output from the transducers is conditioned by two separate signal chains, which come together at a summing junction. The resulting signal is buffered and offset to produce the output signal. An advanced digital subsystem uses calibration data stored in non-volatile memory to manage the gain of the two signal paths, providing an accurate output over the whole frequency and operating temperature range. Since the sensor uses a single 3.3V supply, the output signal swings around a zero-current level of 1.65V, provided by an internal bandgap reference. For the 40A device used here, this output voltage is 1.65V±33.3mV/A for a maximum output swing of just under 0.3V to 3.0V. Notably, the 1.65V reference is available on one of the pins. Some chips lack such a facility, and it is very difficult to zero them. Even if you have an external trimmable reference voltage that you adjust to get 0V output at 0A current, any differential drift between the two reference voltages will cause the output accuracy to deteriorate significantly. To display the output voltage conveniently on an oscilloscope, we must remove the offset and amplify the signal to get a ±100mV/A signal based around zero volts. The most straightforward way to achieve this is to use an instrumentation amplifier. An instrumentation amplifier is a high-­precision differential amplifier based on op amps that usually uses a single resistor to set its overall gain. Practical Electronics | November | 2025 Fig.1: the ACS37030 current sensor features a Hall sensor for DC and lowfrequency measurements, plus an inductive sensor for higher frequencies. These are combined by some clever circuity to provide a flat response from DC to 5MHz. Scope 1: the inrush current for a variable-frequency motor drive as measured by the current probe. The scale is 20A per division. Scope 2: This scope grab, made using the current probe, shows the current supplied by a short-circuited bench power supply, at 10A per division. The current peaks at almost 50A before being rapidly brought under control and limited to 6A. 19 Constructional Project be difficult (read expensive) to emulate with discrete components. Fig.2: the INA849 instrumentation amplifier uses the classic three-opamp topology with six lasertrimmed matched resistors. One external resistor, Rg, sets the overall gain. The instrumentation amplifier used in this project is the INA849. It is a classic three-op-amp configuration, as shown in Fig.2. The input stage consists of two non-inverting amplifiers with internal 3kW feedback resistors. A single external resistor, Rg, sets the gain of this stage according to the formula G = 1 + 6kW ÷ Rg. The second stage is a differential amplifier. As the name suggests, it amplifies the difference between two voltages but strongly attenuates any common-mode signal. In the case of the INA849, the differential gain is Circuit details unity. Therefore, the output voltage is given by the formula Vout = Vref + (Vin+ – Vin-) × (1 + 6kW ÷ Rg). The REF terminal is often connected directly to ground, as per the figure, but you can use it to add an offset to the output if required. We could build our own instrumentation amplifier from discrete op amps, but it’s convenient to use an integrated package like this because the commonmode rejection depends on the close matching of the resistors. Packages like this use internal laser-trimmed resistors matched to a precision that would Now we can turn to the complete circuit diagram (Fig.3) to see how it all works. The ACS37030 (IC1) is powered by a 3.3V rail supplied by low-dropout (LDO) 3.3V linear regulator REG2. The sensor output voltage and the 1.65V reference are applied to the instrumentation amplifier’s non-inverting & inverting inputs, respectively. Achieving a differential gain of three requires a nominal Rg value of 3kW. While 3kW resistors are available (it’s a common E24 value for 1% resistors), I used a combination of fixed resistors plus a trimpot to allow an adjustment range of about ±3% around this figure. This allows the user to trim out any gain error in the sensor, which could be as much as ±2%. The trimmer has the added advantage of obviating the need for high-­ precision resistors here. The ACS37030 data suggests that Fig.3: the current probe circuit reveals that the signal path is very simple, consisting of just the instrumentation amplifier with its associated gain and offset trimming. The balance of the circuit is the power supply and battery charger. 20 Practical Electronics | November | 2025 5MHz 40A Current Probe along with the ±2% gain error, there could also be a potential offset error of up to ±10mV. That would translate to ±30mV at the output after amplification. This is why I am driving the instrumentation amplifier’s REF pin with an adjustable offset trim voltage of ±50mV derived from the divider that includes trimpot VR2. The offset trim voltage is buffered by op amp IC4a since the input impedance of the REF pin is relatively low. The instrumentation amplifier's output goes to the output BNC connector (CON3) via a 100W resistor to protect the amplifier IC from short circuits at the output. The rest of the circuit is the power supply. We require a ±6V split supply for the amplifiers. This was chosen because the common-mode input voltage of the instrumentation amplifier can’t be any closer than 2.5V from either supply rail. Since our maximum input voltage extends to 3V, we need supply rails of at least ±5.5V. I chose ±6V to provide a bit of headroom. These rails are derived from a single Li-ion cell via REG6, which contains a boost converter to create the positive rail and an inverting converter to provide the negative rail. The switching Mosfets are internal to the package, but the inductors and rectifier diodes are external: L1/D2 for the positive rail and L2/D3 for the negative. The output is regulated by providing voltage feedback via two 100kW/20kW resistive voltage dividers, which reduce the ±6V outputs to ±1V, matching REG6’s internal feedback target voltages. The R1283K regulator can operate with an input voltage of 2.5-5.5V, which is ideal for a single Li-ion cell. When power switch S1 is on, the cell is connected to the DC-to-DC converter. If it is off, the cell is instead connected to IC5, a MAX1555 dual-­input Li-ion battery charger. This linear device charges the cell at 100mA if powered via the USB input or 300mA if powered from the DC input. The USB input is useful for charging from legacy USB hubs, which may not be able to supply more than 100mA. However, this current probe is designed for a USB type-C power source that can supply at least 500mA, so the higher charge current is used to minimise the changing time. The PCB has provision for either configuration. LED1 will be on while Practical Electronics | November | 2025 the battery is charging and will switch off once full charge is reached. The USB-C connector is a power-­only type with just a subset of the normal 24 pins. These include the power pins and the control channel (CC) pins, which are terminated with 5.1kW resistors to ground. This tells the source that it should supply 5V. The power input is protected from overvoltage by a TVS diode (TVS1) and a series PTC resettable fuse, PTC1. Construction All components are mounted on a double-sided PCB coded 9049-01 that measures 56.5 × 76.5mm. The component overlay diagram (Fig.4) shows where everything goes. To keep it compact, this project uses almost entirely surface-mounted parts. I managed to mostly avoid any difficult-­to-handle parts; all passives are M2012/0805 size (2.0 × 1.2mm) or larger and all but one of the semiconductors are in easy-to-solder SOIC-8, SOT-223 or SOT-23 packages. Unfortunately, the DC-DC converter (REG6) is not available in anything other than a DFN (dual flat no-lead) package, so that is where I suggest you start the assembly process. It really helps if you have a hot air reflow station. These stations are useful for soldering chips like this one and also make desoldering many SMDs straightforward. They are not terribly expensive if purchased online. The easiest way to get REG6 soldered down is to use a soldering iron to lightly tin the pads, including the thermal pad in the centre. The solder should just cover the pads and not be too lumpy. If you put down too much, you can use solder wick to remove the excess. Next, apply a generous amount of flux paste, position the chip carefully (making sure its pin 1 mark is oriented correctly) and hold it in place with tweezers while you use hot air to gently reflow the solder. Once the solder melts, surface tension should pull the chip neatly into place. Any visible solder balls or bridges can be removed with solder wicking braid and a hot iron. After cleaning the area with isopropyl alcohol or similar, you have completed the hardest part. Mount the rest of the surface mount parts using your preferred method. I apply a dab of solder to one pad first, then position the component with tweezers and reflow that pin with the soldering iron. With just one pin soldered, I can tweak the location if necessary to get the other pin(s) into a place I am happy with. Finally, I solder the remaining pin(s). I find this works for two-pin devices like resistors and capacitors, as well as for the ICs. Fig.4: all components are easy-tosolder surface mount or through-hole types, with the exception of REG6. It requires a little more care but is easily achievable for the hobbyist. 21 Constructional Project Parts List – 5MHz 40A Current Probe 1 ABS handheld instrument case, 92 × 66 × 28mm [Hammond 1593LBK] 1 double-sided PCB coded 9049-01, 56.5 × 76.5mm 1 14500 (AA-size) Li-ion cell with solder lugs (BAT1) [eBay 357385477329] 2 6.8μH 1A SMD inductors, M3225/1210 size (L1, L2) [Murata 1276AS-H-6R8M=P2] 1 0.75A 24V PTC fuse, SMD M3225 size (PTC1) [Littelfuse 1210L075/24PR] 1 100W 4.9 × 3.9mm SMD trimpot (VR1) [SM-42TW101] 1 1kW 4.9 × 3.9mm SMD trimpot (VR2) [SM-42TW102] 1 PCB-mounting sub-miniature DPDT toggle switch (S1) [E-Switch 200MDP1T2B2M6RE] 1 red panel-mounting binding post (CON1) [Cal Test Electronics CT2232-2] 1 black panel-mounting binding post (CON2) [Cal Test Electronics CT2232-0] 1 right-angle PCB-mount 50W BNC socket (CON3) [Molex 0731000105] 1 USB type-C power-only socket with through-hole mounting pins (CON4) [Molex 217175-0001 or equivalent] 2 panel-mount 3mm light pipes, 15mm long (for LED1 & LED2) [Dialight 51513020600F] 2 Koa RCUCTE SMD test points (TP0, TP1; optional) 4 No.4 × 6mm self-tapping screws 1 100mm length of 1.0-1.5mm diameter tinned copper wire Semiconductors 1 ACS37030LLZATR-040B3 5MHz 40A current sensor, SOIC-6 (IC1) 1 LD1117S33 or equivalent 3.3V 800mA LDO regulator, SOT-223 (REG2) 1 INA849DR 28MHz instrumentation amplifier, SOIC-8 (IC3) 1 LM358 dual single-supply op amp, SOIC-8 (IC4) 1 MAX1555EZK-T Li-ion battery charger, SOT-23-5 (IC5) 1 R1283K001B-TR buck/boost switching regulator, UFDFN-14 (REG6) 1 yellow SMD LED, M2012/0805 size (LED1) 1 green SMD LED, M2012/0805 size (LED2) 1 SMBJ5.0CA 5.0V TVS diode, DO-214AA (TVS1) 2 30V 1A schottky diodes, DO-214AC/SMA (D2, D3) [MBRA130LT3G, SS14] Capacitors (all SMD M2012/0805 50V X7R unless noted) 1 100μF 25V tantalum, SME case [Kyocera TAJE107K025RNJ] 7 10μF 16V 3 100nF 1 220pF C0G Resistors (all SMD M2012/0805 ⅛W 1% unless noted) 2 100kW 2 20kW 1 2.7kW 1 510W 1 240W 2 56kW 2 5.1kW 1 1.8kW 1 100W 1 0W The current sensor chip, IC1, straddles an unplated slot cut into the board. This slot is to provide plenty of creepage distance between the current being measured and the rest of the circuit. However, it makes the board quite flexible in this area, so handle it carefully after soldering IC2. If the board is flexed too much, it is possible to overstress the IC’s pins and break them – as I unfortunately discovered! Solder the USB connector’s throughhole tabs first to locate it, then the six smaller surface-mounting pins. Case preparation Before you fit the BNC connector, binding posts or battery (well, cell), it’s a good idea to prepare the case. That will allow you to align those larger connectors properly. The cell should be left off until the testing described below is completed. Drill the enclosure's cover and end plates as shown in Fig.5. The USB slot can be made by drilling two 2.8-3.0mm holes at either end and then filing out the plastic between them. Now drop the PCB into the case, resting on its mounting bosses, insert the BNC connector through the hole in the end plate and drop it into its mounting holes on the PCB. If everything lines up, you can tack-solder the BNC connector in place from the top, then remove the whole assembly from the case and solder it properly, taking care that it doesn’t bend as you do so. If it doesn’t fit perfectly, you will need to enlarge the panel hole slightly and try again. Now attach the input terminals (binding posts) to the appropriate end plate and tighten the nuts. Connect the input terminals to the PCB using a few lengths of tinned copper wire bent over the terminal studs, through the PCB slots, and soldered in place. Remember that this connection could carry up to 40A, so ensure it is solid. Remove the assembly from the case and trim off any excess wire. Make sure that there cannot be any shorts between the terminals! Testing With the switch to the right, the probe is powered; to the left, it can be charged via USB. 22 If you have a current-limited bench supply, it’s a good idea to test the circuit before soldering the cell to the board. Set the onboard switch to the on position, towards the BNC connector. Set the bench supply to 4V with a limit of around 100mA. Then, taking care to Practical Electronics | November | 2025 5MHz 40A Current Probe connect it with the correct polarity, hold its output leads to the two battery pads and monitor the current draw. The power supply should not go into current limiting; the circuit should only draw about 20-30mA. LED2 (green) should light. If all seems well, you can disconnect the power supply, switch off the onboard power switch and solder the battery in place. If something is wrong, check all your soldering carefully and verify that all components are installed correctly. Before soldering the cell to the PCB, make sure the power switch is in the off (charging) position, with the toggle switch away from the BNC connector. Once the cell is installed, treat the board with care. Inadvertently shorting things now could be catastrophic, as Li-ion batteries can source a lot of current. We used a PCB-mounting cell but they are unfortunately quite hard to find, so we suggest instead you source a cell with small solder tabs, like the one in the parts list, then attach it to the pads with short, stiff wires (be careful to get the polarity right!). Check that the cell voltage is 3.04.2V. Switch the unit on, and LED2 should light again. Check for 6V across the 10μF capacitor immediately to the left of L2 and the similar capacitor immediately to the left of D3. That will verify that both supply rails are correct. If the readings are wrong, switch it off immediately; you most likely have a problem with the power supply section. Check REG6 and its surrounding components, especially L1, L2, D2 and D3. Once the power supply is working correctly, you can check the battery charging circuit by switching the unit off and connecting a suitable USB supply. The yellow charge LED (LED1) should light, and the voltage across the battery should begin to slowly rise. When the battery voltage reaches about 4.2V, the charge LED should go out, indicating that the battery is fully charged. Depending on the battery’s initial state of charge, that could take a few hours. Final assembly You can now apply the front panel label, shown in Fig.6 (download from siliconchip.au/Shop/11/490). Fig.5: drilling the case is straightforward. The USB slot is best made by drilling two 2.8-3.0mm holes at the ends and joining them with a file. You can see how the finished case looks at left, and how the PCB slots into the case shown enlarged above. Practical Electronics | November | 2025 23 Constructional Project Once it is in place, carefully cut out the LED holes and insert the light pipes from the outside. They can be secured with a drop of cyanoacrylate (super) glue on the inside of the case. Slip the end plate over the BNC connector, switch and USB socket and screw the whole assembly into the base of the enclosure with 6mm self-­tapping screws. Calibration To calibrate the Probe, you will need a current-limited bench power supply capable of sourcFig.6: the label artwork for the front of the enclosure. Print it on sticky-backed paper, cut out the outline and apply it (or laminate it, or use your preferred label-making method). Use the case as a template to cut the holes for the light pipes. The Hall Effect Edwin Hall first described the Hall effect in 1879, just a decade after Maxwell published his seminal work on the interaction of electric and magnetic fields. The lower left diagram shows how it works. A current (green arrow) flows through the long axis of a conductor that is subject to a magnetic field perpendicular to the direction of current flow (lavender arrow). Hall discovered that under these circumstances, the electrons making up this current – which flow in the opposite direction to the current – would experience a Lorentz force pushing them towards one side of the conductor, as shown by the curved blue arrow. As a reminder, Lorentz’s law states that a charged particle, such as an electron, moving in a magnetic field will experience a force at right angles to both the direction of the field and its velocity. This is the basic principle by which electric motors and generators work. The build-up of negative charge on one side of the conductor (and the corresponding positive charge on the other side, where there will be a dearth of electrons) produces an electric potential across the conductor. This Hall voltage is proportional to both the conductor current and the strength of the magnetic field. The Hall effect also works in semiconductors, although the polarity of the Hall voltage may be different in some semiconductors where ‘holes’, rather than electrons, are responsible for current flow. In practical Hall effect sensors, the current to be measured passes through a conductor surrounded by a magnetically permeable core. The Hall sensor is positioned in a narrow gap in this core, so the magnetic field produced by the current in the conductor passes through the element perpendicular to both the excitation current and the Hall voltage measurement terminals. Since the excitation current is fixed, the Hall voltage is proportional to the magnetic field strength, which is, in turn, proportional to the conductor current. Magnetic Core Fixed Current Conductor Hall Sensor Hall Voltage Sensing 24 ing a few amps and a multimeter. If you have two meters, so much the better. First, set the offset trim. Connect the meter, switched to a low voltage range, between the probe output test point (TP1) and ground (TP0). With the unit switched on and nothing connected to the input terminals, adjust the offset pot (VR2) for a meter reading close to 0V. You should be able to get it to less than ±1mV. To trim the gain, configure the power supply to deliver a few volts and set the current limit to 3A or whatever maximum your power supply will deliver. Switch your meter to read current (remember to swap the probes to the correct jacks), select the appropriate range, and connect it across the power supply. The supply should go into current limiting and regulate the current somewhere near the setpoint. Record the current value displayed on the meter. Now switch the meter back to volts and connect it back to TP0 and TP1 as before. Connect the current probe inputs across the power supply without changing any of the settings. The output voltage should read close to one-tenth of the current reading you noted earlier. For example, if you measured the current to be 3.02A, you should see something like 0.302V on the meter. If the reading is a bit off, adjust the gain pot VR1 to get it as close as possible. If you have two meters, you can measure the input current and output voltage at the same time (the current meter goes in series with the probe across the power supply). That will be a bit more accurate (and easier) than switching the meter around. Using it Due to the high currents that the probe can handle, probes (alligator clip wires etc) should not be used unless both the voltage and current are low (under 50V DC/AC & 5A). For higher voltages/currents, you can cover the exposed wires that are attached to the binding posts with heatshrink. As there is exposed metal on the binding posts, if any voltage above 50V is applied to the Probe, that end of the device must be considered live. Position the Probe so that nobody can come in contact with that end, and also to keep the isolated measurement end away from any high-voltage wiring. The Probe itself has a high isolation, but you must ensure that isn’t degraded by any external shorting hazards. PE Practical Electronics | November | 2025