ACHS-7123 Failed without ever passing current through it

Hi All

Anyone have any failures of this device.?
I never got to putting any current through it, just using it to calibrate the software at
the zero current point where it should put out about 1/2 of the supply.
After about 2 weeks of playing with it, it suddenly failed this morning and gives me 0v output.



How’ve you got it wired up? I’ve had a couple of incidents reported with the ACHS current sensors before quite a while back reporting lower voltages than would be expected in the V/A curve. But both times it has been a continuity issue or a short.

Would it be possible to upload a photo of how you’ve got it set up?

It is worth noting cooking these boards is certainly possible, although it generally requires a pretty impressive amount of current or overvoltage. We’ll see what we can do to work out what’s gone wrong with this 7123.

Hi Bryce
The pic shows the 7123 connected to a AD654 which has a very high input impedance. The only difference between the circuit shown was that I tested it on 5vdc and not 10vdc.
As I said, never even got to connect anything to the current terminals.
Screenshot from 2023-12-05 13-59-22


Sorry, I should have clarified, I meant a photo of the physical board itself so we can identify the revision you’ve got and whether the connections being used are suitable. Looking at your schematic I can’t see any issues at a glance (also 5V DC should be fine)

Here’s a link to the ACHS-7123 by the way for anyone else troubleshooting this to jump to the board and specs quickly:

This may seem a stupid question BUT???
You have actually measured 10V from VCC to ground haven’t you?

Do you have any sort of a magnet anywhere in the vicinity of this board. That’s how hall effect devices work. They are influenced by the presence of a magnetic field to produce the output voltage.

I am with Bryce here. You would need a very large current to damage the current input side of things. Even if that bit was damaged the device should output VCC/2 with zero current unless the delicate bits are fried somehow.

This might be a possibility as indicated below

where the supply voltage is quoted at 8V max.
Cheers Bob

Extracted from Core description text
Pololu therefore recommend careful reading of the ACHS-7121/7122/7123 datasheet (1MB pdf) or ACHS-7124/7125 datasheet (1MB pdf) before using this product.

Also this description quotes a maximum logic supply of 5.5V

So I think we could assume that 10V VCC may have fried the chip electronics.

Also worthy of note. The 30V referred to relates to the current sensing part of things which is isolated from the logic.


Hi All

The device failed during 5vdc testing, and I confirmed that voltage to ground.
The 10 vdc was a mistake I was about to make with new ones coming.
It was driven by the AD654 spec. Glad you picked that up.

There were no magnets or anything near the board…
All during testing it’s output was around the 2.525 mark.
And definitely there was no current passed through it at any time, as I was testing on my desk and didn’t have a current source.

I belive in the rule, that if an electronic component lasts for more then a month, it will last. This one failed after a few weeks.

I’t currently in the workshop, so will get the model etc to you.

Hi All

Photo attached.
Screenshot from 2023-12-06 14-26-28

Hi J.W.
I agree it’s a pretty weird failure mode. Is it possible something has just come loose in the project since initial testing?
We appreciate the photo - showing the broader context of how it is connected and what it is connected to would be very helpful

where are you performing your measurements? On the board? or with the connected instrument?
to isolate any connection issues, if the device is being powered correctly you ought to be able to unplug whatever is connected to OUT pin and read 5V across the GND and Vcc pins with a multimeter
then we could expect you would measure about 2.5V between GND and OUT

forgive me if you’ve already tried this part - it’s just quite important to know exactly where the measurements are being taken specifically, to eliminate connection issues or otherwise.

Are you connecting directly to those pins which I assume are header types or have you got this plugged into a breadboard. If the latter I would not discount the breadboard or one of its connections being faulty. Plug it into another part of the breadboard. These pins are a bit larger than most of the el cheapo breadboards can handle and pretty well destroy the connection for anything smaller.
Cheers Bob

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Hi All

I had the unit plugged into a breadboard, but because the header pins were bent at 90 degrees, I was able to measure right at the board to confirm Gnd, +5v and usually 2.5v. That changed after a couple of weeks testing software.

The idea is that the output of the 7123 goes through 44.5k to the AD654 voltage to frequency converter. The frequency output goes directly into GP18 of a Pico.

Very simple really.
I will receive the new units in the next few days at which point I will have completed a test PCB.

Hi All

Installed a new unit in the same circuit and all is well.
Some lessons learned though.
Still without connecting any current through the ACHS-7123, I found the greatest
stability on output was to use a separate 5vdc supply. If using the Pico Vsys, any usb activity will change the output.
Installed the 1nF Filter capacitor and a 1uF Power supply bypass.
With the output connected to the AD654 V to F converter, I am getting an output frequency stable within 1 Hz. Note that i am using a 1 minute interrupt to read the total count.

Hi JW.

I thought this was discovered a long time ago. The general opinion became that a separate supply was best for any type of sensor. Although it took a long time for that to be accepted.

I feel sure if that had been shown earlier (like at the very beginning) someone including myself would have picked this up.

Personally if I have a solid 5V supply for powering say XMAS LEDs (in one case of mine) I will power an Arduino with this via the 5V input but never the other way around . Don’t have much bother using this principle.

Good all is sorted now.

That is a worthwhile plus. Put this power application down as one of them.
Cheers Bob
You can use the same 5V supply to power RPi (or Arduino) and a sensor, just do it separately and don’t power the sensor VIA the RPi (Arduino).

Actually my initial testing was on the bench, but as it is going into a ute with Lithium batteries, it will be powered from a 13 to 5vdc stepdown.
But even with a seperate 5vdc wall wart there was still a lot of noise, which the bypass cap minimised. Will see later what amount of noise the switched mode stepdown will create.

It is a fact that the output of the current sensor will be half VCC modified by the current through the sensor. It would be fair to say then that the output will vary by half the variation at VCC added to whatever the output voltage is at the time. As the variation of the output is usually only mV (2.25V for 30A) this can be serious.

The buck converter will be basically a switching regulator so some switching artefacts could be present. If the wires carrying this 5V are any length they will have some influencing inductance which will be worse if thin wires are used. This will affect the switching glitches and the result is probably what you refer to as “noise”.

This can be minimised by keeping this wiring as short as possible and increasing the size if practicable. I suggest placing the 13V to 5V converter as close as possible to the sensor and other electronics and keep them all together physically. If this is not possible an Electrolytic cap of a few hundred µF and an High frequency bypass of about 10nF in parallel with it be connected across the 5V supply as close to the electronics package as possible. If the electronics is housed in a metal enclosure this HF capacitor bypass would be ideal if it were a “feed through” capacitor and the enclosure grounded. “Feed Through” caps are very good at this sort of thing.

The reason for this HF cap is that electrolytic have quite a lot of inductance in themselves and don’t do much bypassing at higher frequencies.

Don’t ever (unless it cannot be absolutely avoided ) have a filter cap on both ends of the same wire. They will form a tuned circuit with the wire inductance and actuallyy INCREASE any high frequency “noise” over the resonant range. That I can guarantee.
Cheers Bob

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Thanks for that Bob.
Will take note when I convert the Vero board into a PCB.

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