Current Sense Output Disrupting Ammeter Powered With Most SMPS But Not All(???) - Page 1
Forum section: electronics / beginners

author: Trurl
author_rank: Regular Contributor
timestamp: June 12, 2024, 03:39:48 pm
content: PLEASE HELP! LOAD HAVE MERCY!Recently, I posted with excitement my "solution" to a finicky ammeter based on an ICL7107 IC that is setup to take a CS(Current Sense) output from a server PSU as the ammeter's input(IN HI).**Refer to the thread "ICL7107 Volt Meter Hacked To An Ammeter w/ 2.5V Ref & In Lo Offset (Solved)!" (sorry, linking isn't working likely because I'd edited the thread's title a few times, please search title in "Beginners" section).That "solution" involved the ammeter being powered by a secondary PSU(instead of the server PSU that the ammeter is monitoring). The secondary PSU is a 12V 3A rated SMPS. This seemed to resolve the issue of the ammeter going "stoopid"(readings fluctuating), which seems to be caused by the CS output connection to the ammeter, and perhaps current through the ammeter back to the CS output's source server PSU(?).As the secondary 12V PSU above is a bit bulky, I tried powering the ammeter with other low power SMPS "wall warts" (7.5V, 9V) + a 5V regulator(LM7805) in series to the ammeter, and found the same wild fluctuations in readings(jumping about, polarity flip-floppin' etc.).Thinking perhaps that these fluctuations might be caused by potential difference between the CS source and the ammeter's supply source, I tried connecting another 12V SMPS(different than the "secondary" PSU mentioned above which the ammeter seems to work fine with) and this other 12V SMPS also causes the ammeter to fluctuate "wildly". Therefore, I don't think the issue is due to potential difference between CS source and ammeter supply source.I'm pretty certain that the real issue(perhaps current flowing from the CS output through the ammeter and back to the CS output's source supply) is still not truly resolved even in the secondary 12V PSU that seems to work with the ammeter. If I cannot pin point the cause of the issue, I'm considering using a clamp type or inline hall sensor for current monitoring to definitely isolate the ammeter from the CS source.What do you think is the true cause of the issue of the ammeter fluctuating with most supply sources but not(at least noticeably) with the one "secondary" source I mentioned above?PLEASE HELP!

author: coromonadalix
author_rank: Super Contributor
timestamp: June 12, 2024, 04:00:00 pm
content: schematics of your installationi use ammeters with integrated shunt but need external supply, i do use small isolated supply for the ammeter TO BE SURE i dont provoke any problems on sense lines or voltage returns etc ...as you may guessbut some of them, are in the ground line and others are on the positive line (witch i use), the ones on negative ground may case voltage drop as we know E = Rx I .... and create voltages differences if you parallel some psuand you have some you supply themselves by the source psu ... witch i do no use for the same reasons ..i use real dmm chipsets in the voltage and ammeter panels (icl 7106 7107 or 7135) not the cheap china based mcus you find every where ....they are all on the positive linethe ground is the ground with no voltage differences or gimmicks who may interfere

author: Trurl
author_rank: Regular Contributor
timestamp: June 12, 2024, 06:11:45 pm
content: The connection is so simple, I hope that the photo below and the ammeter's core schematic will be enough for a grasp of my setup.Ammeter SetupWith Server PSU(upper left, HP HSTNS-PD43), Ammeter Supply SMPS(12V) & LM7805(breadboard), ICL7107 Ammeter(lower center), Resistive Load(6Ohms, upper right), Clamp Meter(top center)The white wire from the PD43(server PSU) is connected to a lead that is soldered to the first underside pin which is the CS(current sense output with roughly 25mV/A). The CS output is connected to the ICL7107 ammeter's IN HI, while the IN LO(black dangling wire) is linked with supply ground on the PCB.The black SMPS(12V) below the server PSU is the "good" SMPS that the ammeter works with(the reading is stable,though one or two counts higher than the clamp meter).But other SMPS supplies with the ammeter cause "wild" fluctuations[regardless of output voltage: 5.2V (connected directly), 7.3V, 7.5V, 9V, 12V(different than the "good" one) in EXACTLY the SAME setup with their output voltage stable, yet causes the ammeter to fluctuate wildly(unstable, drifts upwards like a counter, flip-flops polarity etc. etc.)]. WHY?The "good" supply does have that ferrite core at the output which the other supplies do not. Could THAT cause such a difference?Ammeter / ICL7107 Core Connections SchematicNote:The actual ammeter above uses a TL431 output of 2.5V directly into the REF HI (instead of the more common 1V via a divider as shown), and the -5V supply is generated via a C1815 NPN Transistor(not the ICL7660 in the schematic). Therefore, an output of 2A from the server PSU(PD43) will result in about 50mV CS output(25mV/A) and the "ammeter" processes this and displays 2.0(as the reference voltage is set to 2.5V, not 1V).Any comment/diagnosis would be VERY appreciated.Cheers~

author: Trurl
author_rank: Regular Contributor
timestamp: June 13, 2024, 08:22:30 pm
content: I think I may have found the cause of the "anomaly".When I tested the differences between the ground potentials(voltages) of the server PSU(PD43) and the supplies for the ammeter, I noticed that there is no difference with the "good" supply, whereas there are fluctuating ground potential differences(~ 60 to 130mV) in the others.As I was plugging and unplugging the supplies, I realized that the "good" supply uses a 3 conductor AC cord(grounded) whereas the others use a 2 conductor non-grounded AC cord or are 2-pronger "wall warts".Hence the "good" supply and the PD43 share a common AC ground, where as the other supplies that cause ammeter fluctuation are "floating" in relation to the PD43.I'll have to find another 3 conductor powered SMPS and see if it also turns out to be "good" and report back.

author: Trurl
author_rank: Regular Contributor
timestamp: June 13, 2024, 10:04:54 pm
content: YES!!!The fluctuations in the ammeter is caused by the ungrounded("floating") "wall wart" supplies.Grounded supplies* work fine like the "good" 12V grounded supply.*such as my monitor's 20V supply.As I'm hoping to use a more compact supply for the ammeter, this leads to the question:is there a safe and simple way to convert a non-grounded("floating") 2-prong "wall wart" supply into a grounded 2-prong + "ground enabled" supply?Thanks in advance

author: Trurl
author_rank: Regular Contributor
timestamp: June 14, 2024, 08:51:19 pm
content: Thanks to Zero999who skeptically suggested to try "grounding" the negative of the supply's DC output in another thread*, I've found that doing so completely eliminates the fluctuations!*https://www.eevblog.com/forum/beginners/safe-simple-way-to-convert-floating-wall-wart-to-grounded/msg5541505/#msg5541505The fluctuations were caused by ground potential difference between the AC grounded server PSU(PD43) and the "floating" supplies(various smps bricks & wall warts alike).Now I can get on with fun of building out the PD43 voltage boosted supply(14V 100A) with modular volt/current meters in a protective briefcase.