Help configuring ACS724 to monitor/limit current of Iso Variable AC Power Supply - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: March 12, 2021, 02:53:42 am
content: I am DIYing an Isolated Variable AC Power Supply using these components...- DIY'd Isolation Transformer - Made from two APC 1500 UPS transformers, placed back to back - Approx 360 Watts Max- Staco 3PN1010 Autotransformer - 10A Max- Incandescent bulbs, in series, for current control AKA Dim Bulb Tester- ACS724 Hall Effect Current Sensor- Harbor Freight Freeby MultimeterI would like to be able to read the current and possibly control/limit it with a constant current circuit (and nix the light bulbs.)The ACS724 Hall Effect Current Sensor might do for monitoring... It will read AC, but I am not finding any information as to how to configure it in an AC circuit.It is on a breakout board. Some of these boards come with an op amp circuit. Mine doesn't have the op amp onboard, but it could easily be added.I would prefer to read the ACS724 directly with the HF Multimeter, as opposed to employing a MCU, because I don't code.I am not finding an AC Constant Current circuit with its explanation to work from. I have a copy of the BK Precision 1655 schematic, but I have little AC experience and I do not know how to read it well enough to break out the sections that I need. Building an analog approach is within my skill set, but not designing it, because it is AC. I can understand that a MCU might be useful to do the limiting, but, again, I don't code.Summing it up... I need an analog AC Constant Current circuit, an explanation of how the ACS724 works in an AC configuration and instructions for how to employ it thusly.If someone wants to contribute the C code for a PIC, I can surely build up the hardware for a digital approach. I have collaborated on PIC projects. I handled the hardware build.I am sorry to be rambling a bit. I seem to be tired. Thank you for your help.1655_compo.pdf(101.06 kB - downloaded 741 times.)ACS724 Hall Eff Curr Sens.pdf(1181.72 kB - downloaded 660 times.)

author: t1d
author_rank: Super Contributor
timestamp: March 12, 2021, 11:09:08 am
content: Bump...

author: t1d
author_rank: Super Contributor
timestamp: March 14, 2021, 03:52:16 pm
content: Looks like the clue I needed was to investigate Triac/Diac circuits. I have not finished studying this type of solution, but, if it is a PWM approach, how would varying the operational Hertz effect the DUT test results?Edit: It appears that this PWM solution only adjusts the duty cycle and that it does not effect the frequency. Do I have this correctly?

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2021, 05:12:15 pm
content: I'm not sure exactly what you are attempting--is this a specific-purpose setup or do you want a general-purpose iso-variac? Is the goal isolation for measurement or noise reasons or is it safety?Using a symmetric back-to-back step-down to step up arrangement will result in significant losses and poor load regulation. Whether you can compensate for that with the Variac is questionable. A typical isolation transformer has few extra primary windings so the the open-circuit voltage at the output is a bit higher than the input.As far as measuring the current goes, you should study the datasheet for the ACS724 as it will require a translation of the output to read what you want. The explanation is in there--it looks like you would typically have an AC output signal of less than 1VRMS with a DC offset of 2.5V. It could be done with a multimeter, but the HF cheapo is one of the few DMMs in the world that isn't going to work. It doesn't have a low ACV range and it doesn't read AC current directly. There are probably easier ways to read the AC current.A constant-current AC circuit would be very complicated and a triac would be a terrible way to try and accomplish it. The output would be very noisy and rough. The BK1655 doesn't have any current control or limiting other than the circuit breaker and perhaps the impedance of the transformers, which greatly reduce available fault current. If you really want a current limiting device for testing, you probably should just stick with the light bulbs.

author: ledtester
author_rank: Super Contributor
timestamp: March 14, 2021, 05:17:40 pm
content: Which ACS724 chip do you have? Some can only measure DC current and others are bidirectional. For instance, have a look at this page:https://www.pololu.com/product/4045and where it says "Alternatives available with variations in these parameter(s):".If you have a bi-directional ACS724 then using a multimeter to read out the AC current is going to be problematic. From the Pololu page:QuoteSo the output waveform will be a sine wave with a DC offset of 2.5V. Set your multimeter to AC voltage and see what it says for various AC currents through the ACS724. Your multimeter may not like the 2.5V DC offset -- maybe a DC blocking capacitor will help here.If all goes well your multimeter will read an AC voltage which is proportional to the AC current.I think you'd be better off just getting an AC ammeter like:https://www.aliexpress.com/item/4000368728947.htmlYou can also get LED displays -- search for "AC current meter LED", for instance.QuoteIf you use a triac you will be altering the output AC wave shape and this can have consequences for the devices you are powering. For resistive loads like light bulbs this isn't a problem, but it could be a problem for an inductive load -- like another transformer.Being resistive the dim bulb tester won't distort the sinusoidal form of the AC wave and is a very practical and safe way to limit AC current - especially when working with high voltages.

author: t1d
author_rank: Super Contributor
timestamp: March 14, 2021, 07:15:33 pm
content: Thank you, bdunham7 and ledtester. You both gave me very good information that is right on target. I will continue to study on it.