How To Make Non-Zero Initial Current Sense Voltage Zero? - Page 1
Forum section: electronics / beginners

author: Trurl
author_rank: Regular Contributor
timestamp: April 12, 2024, 10:21:09 am
content: I've got an HP DPS-1400 (HSTNS-PD43) server PSU. The first small pin on the underside of the outputs is a CS(Current Sense) pin. I'd seen online that its output is 25mV/A. When I test it at low amps (1~4 amps) the output indeed seems to increase by approximately 25mV/A.The issue though is that at 0 amps the CS output is NOT zero volt, instead it is 26mV. Hence I need to "subtract" 26mV from this CS output voltage.What would be the simplest way to do this (without using fancy gizmos like an Arduino, just regular "jelly bean" components)?I'd like to connect this CS output to an ICL7107 based volt meter with the voltage reference adjusted to 2.5V(instead of the typical 1V) so that I can monitor the current.

author: xvr
author_rank: Frequent Contributor
timestamp: April 12, 2024, 11:12:10 am
content: Operational Amplifier?https://www.electronics-tutorials.ws/opamp/opamp_5.htmlIn your case you need RR IO opamp.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2024, 01:19:46 pm
content: What supply voltages do you have available to power a circuit?Does the ICL7107 based volt meter have a zero adjust?If the ICL7107 based volt meter has a differential input, then its reading can be offset by 26 millivolts by shifting the negative input up 26 millivolts.

author: Trurl
author_rank: Regular Contributor
timestamp: April 12, 2024, 02:03:15 pm
content: Interesting that you mention the method of offsetting the negative input by 26mV... I recall in the past I'd done just that for offsetting a non-zero initial CS voltage from an Allegro hall type current sensor (it was a model that swung both positive and negative current readings but the CS output was all positive voltages so I had to "subtract" the lower half - 2.5V). In that case, I used a wire off a 5V regulator for the ICL7107 based volt meter, setup a halving voltage divider to supply the offset voltage and connected it to the negative side of the the voltage inputs. I was surprised that it worked* even though the negative input was connected to the ground of the supply voltage (initially I wasn't sure if the offset voltage would actually be noticed by the ICL7107's differential input in such arrangement).Since the volt meter has a TL431 (2.5V reference), I'm thinking I could attach a wire to it and add a 1:100 voltage divider to supply 25mV to the negative input of the differential input of the ICL7107. This, assuming it would work, would be simpler than using a separate op amp etc.* Update: Way back when I had done the offset method, I initially encountered weird fluctuations(like drifting) of readings not only on one meter but all others that shared the same 5V supply source/ground. The 5V supply for the offset voltage and the 5V supply for the voltmeter(ICL7107) were 2 different sources, and so that may have caused a ground loop issue. I recall I had to use small isolated 5V DC-DC converters for the 5V supply voltages on the meters that implemented the subtractive offset voltage (on the differential input's negative terminals). I think the fluctuations were due to the 5V ground being affected by the volt meter PCB which links the differential input's negative terminal with the main ground of the 5V supply voltage input. At any rate, the isolated DC-DC converter used for 5V supply of the voltmeter resolved the fluctuation issues.This time, even though I'll be using only one source of power (the main server PSU itself, as opposed to separate adapters) with a 5V regulator in between, I think I'll use the same isolated DC-DC converter on the meter to eliminate any possibility of the meter affecting the server PSU other than acting as a minimal load.

author: kevin.gibbs
author_rank: Regular Contributor
timestamp: April 25, 2024, 12:34:15 pm
content: I would just remember to subtract 1 Amp from the measurements. If you want to keep that meter, you can make a subtractor on an operational amplifier.

author: Trurl
author_rank: Regular Contributor
timestamp: May 30, 2024, 01:29:12 pm
content: Ha! You's soSLY~! If the gizmo don't behave, I stick a lil thought cloud "post-it" say "(-1Amp)"! LOL. Cheers~!

author: Terry Bites
author_rank: Super Contributor
timestamp: May 30, 2024, 03:01:34 pm
content: We've just been here recently, no?Disconnect INLO from COM and REFLO.Add the offset at INLO

author: Trurl
author_rank: Regular Contributor
timestamp: May 30, 2024, 03:54:58 pm
content: Thanks for the detailed pointer!I've realized that the Intersil et al's datasheets areREALLY"spot on" as ya'll Brits say... it's these cheap-ass voltmeter PCBs that were made just assuming that,"oh, ... no one willEVERneed to use the differential input feature, who the hell knows about it anyway... so hell... link them 'LO' pins all to ground, and who cares if a 7-segment LED would have to be desoldered to get to the linking traces to cut them! Let'em go tuh hale!"Well, I guess I'm one of those odd balls, havin' some fun. LOL!Oh... BTW, I know you're "recovering" and all, but I finally posted my manually probed and traced schematic of the HSTNS-PD43 server PSU's auxiliary PCB in the 'Projects, Designs, and Technical Stuff ' section:https://www.eevblog.com/forum/projects/hp-hstns-pd43-psu-hack-feedback-circuit-(partial)/If you are at all interested in deciphering the type of compensation circuits in PSUs, and like to break down how it all works, you'll have a ball.Cheers!