DIY

ACS730 Oscilloscope Current Probe, 500 kHz

ACS730 oscilloscope current probe, 500 kHz  ACS730 500 kHz oscilloscope current probe I came across such a homemade product. According to the rules of the site, it does not attract a full-fledged article, but I think it would be right to post it all the same, at least in topics.
If you've ever worked with power electronics or high-current devices before, you might be wondering what the current waveform of these devices looks like.
Well, since this is not such an unusual topic, there are already a number of commercial solutions. The devices are called current probes and allow engineers to see and measure current signals in an oscilloscope up to tens or hundreds of amperes.
Without these devices, it would be very difficult to work, but the question is the price. Not everyone can afford to buy it.
There are a huge number of current probes on the market, each of which is designed with a different approach. Most of them allow to measure current in a non-invasive way, relying on special magnetic transducers, which is the main reason for these devices so expensive.
Such a device is very difficult to reproduce for the average user, since there are no resources available on the market for this. The current probe described in this article uses two terminals and a wide frequency range current transformer IC, which works just like a multimeter.
This allows for an accessible design that is easy to make.
Step One: Features
The device is developed on the basis of the ACS730 microcircuit from Allegro MicroSystems (possibly an analog for aliexpress), which has a bandwidth of -3 dB at 1 MHz and a measurement range of + -50 A. In fact, it is a galvanically isolated Hall-effect current converter in a SOIC8 package. This allows you to measure 2400 Vrms ac line voltage up to 420 Vpk rms or dc.
Next comes a signal conditioning stage based on a pair of AD823ARZ precision high-speed op-amps selected to meet bandwidth specifications current sensor. They scale the signal to 100 mV/A.
The probe is powered by a 1000mAh LiPo battery, charged at 200mAh via Mini USB with MCP73831 IC. The positive and negative voltages required by the sensor come from the TPS65133 power converter. The power consumption rises to about 50 mA, allowing continuous operation for 15-20 hours.
In a nutshell, the probe performance can be summarized as follows:
– Scale: 100 mV/A
– + -25 A nominal measuring range (40 Apeak)
– Nominal output voltage: + -2.5 V (+ -4 V max.)
– Bandwidth: DC – 500 kHz -1 dB (1 MHz -3 dB)
– 15-20 hours of work.
ACS730 oscilloscope current probe, 500 kHz Step two: theory
As indicated in the previous step, the main component that makes the probe work is the ACS730 (U2), which outputs 2.5V at zero current. Then, if a current is applied to its terminals, the output power changes at a rate of 40 mV/A in the + -50 A version. This gives a voltage of 0.5 V to 4.5 V depending on the polarity of the current.
Like you, You may have noticed by now, there is always a 2.5V DC bias in the measurement signal. So the next section is the AD823ARZ op amp, which buffers the 2.5V obtained through the voltage divider from the 5V power rail. Using a dedicated voltage reference will result in much better noise performance, but at the expense of an increase in the total cost of the instrument.
The second stage of op-amps then de-bias by subtracting the buffered 2.5 Vdc level from the output (IC2A). In the last step, a gain of 2.5 is applied to obtain a 100 mV/A scale (IC2B).
Probe should not be used with currents higher than 40 A. Due to limitations in the PCB substrate, 25 A seems to be a more reasonable maximum current, although higher transient currents can still be measured.
The rest of the diagram is mostly self-explanatory. The TPS65133 (U3) chip supplies 5V positive and negative current to the op amps and current sensor. It is a power converter that switches in the megahertz range to maintain good regulation and noise control.
A 3.7V 1000mAh LiPo battery powers the instrument. The MCP73831 (U1) maintains the specified battery at 200mAh when the MiniUSB port (J1) is connected. When charging is complete, the LED turns on.
ACS730 oscilloscope current probe, 500 kHz Step third: performance and limitations
The ACS730 offers a-3dB bandwidth of 1 MHz and a rise time of 0.6 microseconds, which is very good, but for the current probe, the attenuated waveform is not very reliable and distorts measurements. Thus, the wizard suggests using the probe to test signals in a 1 dB bandwidth at 500 kHz.
For an op amp stage, this is not a problem. As you can see in the images, the sine wave at the 1 MHz input does not undergo any fading or distortion at all. This goes far beyond the technical characteristics of the current converter.
ACS730 oscilloscope current probe, 500 kHz Oscilloscope current probe ACS730, 500 kHz ACS730 oscilloscope current probe, 500 kHz < a href = "https://usamodelkina.ru/uploads/posts/2021-04/1618207662_1-7.jpg" rel = "prettyPhoto"> ACS730 oscilloscope current probe, 500 kHz Step four: experimental results
The master conducted a series of accuracy tests in which the probe showed excellent results. At the moment, he doesn't have an AC power supply available to show the actual bandwidth of the probe, but he tested the overall DC performance with a regular bench top power supply.
When the power supply is set to 1A DC, the current clamp reads 0. 96 A, and the multimeter shows 0.0995 V at the output of the sensor, which is exactly the way it should be. A current of 5 A leads to even better results: with an actual clamp-on current of 4.86 A, the meter reads 0.4855 V. This is excellent accuracy for a € 30 device.
Oscilloscope tests show quite a lot of noise coming from the probe. Although it is significant at low level currents (less than 1 A), it is less relevant for higher amplitude currents. This may be due to improper signal testing with a long ground oscilloscope probe.
ACS730 oscilloscope current probe, 500 kHz < a href = "https://usamodelkina.ru/uploads/posts/2021-04/1618207692_1-9.jpg" rel = "prettyPhoto"> ACS730 oscilloscope current probe, 500 kHz ACS730 oscilloscope current probe, 500 kHz < a href = "https://usamodelkina.ru/uploads/posts/2021-04/1618207642_1-11.jpg" rel = "prettyPhoto"> ACS730 oscilloscope current probe, 500 kHz ACS730 oscilloscope current probe, 500 kHz < a href = "https://usamodelkina.ru/uploads/posts/2021-04/1618207684_1-13.jpg" rel = "prettyPhoto"> ACS730 oscilloscope current probe, 500 kHz ACS730 oscilloscope current probe, 500 kHz < a href = "https://usamodelkina.ru/uploads/posts/2021-04/1618207709_1-15.jpg" rel = "prettyPhoto"> ACS730 oscilloscope current probe, 500 kHz ACS730 oscilloscope current probe, 500 kHz Step fifth: design files and documentation
The archive below contains all the information for manufacturing this design.
CAD Files (Fusion 360 and EAGLE 9.X):
– F360 Assembly Project (Assembly .f3z)
– F360 electronics project (ACS730_probe.f3z)
– F360 schematic (F360_ACS730_sch.fsch)
– F360 board schematic (F360_ACS730_PCB.fbrd)
– EAGLE schematic (EAGLE_ACS730_sch.sch) < br> – EAGLE PCB layout (EAGLE_ACS730_PCB.brd)
– STL model of the presented box (3DModels.zip)
Print files:
– Schematic (ACS730_Probe_Schematic.pdf) < br> Product files:
– Bill of Materials (BOM_ACS730_25A.txt, BOM_ACS730_25A.csv)
– Gerber files (ACS730_25A.zip)
This project was completely done with Fusion 360 The electrical project files were exported as Fusion 360 and EAGLE 9.X compatible files. Link to Google Drive to access project files:
https: //drive.google.com/file/d/14_Hp3iuTNa1wZ3X5k …

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