Current Draw at Power Supply vs Current Measured across a series shunt - Page 1
Forum section: electronics / beginners

author: steamedhams
author_rank: Regular Contributor
timestamp: May 07, 2021, 10:01:30 am
content: I am measuring power consumption of a pcb. The current draw of the power supply reads 0.5 amps.When I measure across a series shunt on the board (1 mOhm), the voltage across is 2.7 mV which means 2.7 amps are flowing across that resistor.What is the deal here?? There are a couple of boost/bucks converters on the board, regulators, inductors...etc Is that jacking up the current??Thanks in advance

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 07, 2021, 10:40:14 am
content: Either your power supply or your shunt resistor or your mV measuring device is lying to you. Check them out.

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2021, 10:58:05 am
content: A couple of random thoughts in order of likelyhood...1. Make sure you are measuring directly across the resistorterminalsleads, ie. not sharing solder joints or pads with the current connections, at the 1mOhm level, this can make a huge difference. You need 4 wire aka Kelvin connection. Make separate connections for current and measurement.2. Your current draw could be very spiky in nature and your measuring device isn't handling it well.3. Thermocouple effects - check the reading on the meter with the power off and make sure you're getting a zero reading.My bets are on 1 though.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 07, 2021, 11:09:09 am
content: My bet is on 2.

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2021, 11:19:43 am
content: Yes, it's one or other. Unless the meter has a serious zero offset, thermocouple effect would probably top out at the 10's of uV level.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2021, 11:41:29 am
content: I would say the voltage measured at the shunt resistor is the wrong one. With such a small resistor adding extra resistance to the setup can easily happens. Plus the fact that measure a couple of mV is difficult to get accuracy. I personally do not have a meter that can measure 2.7mV with the certainly of +/- 1mV.

author: tggzzz
author_rank: Super Contributor
timestamp: May 07, 2021, 01:15:11 pm
content: What is the tolerance of the 0.001ohm resistor?What is the tolerance of it and its connections when soldered into the circuit?If you try to measure 0.001ohms, you will find that it is not an easy measurement to make with any degree of precision and repeatability.

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2021, 01:30:36 pm
content: 1mOhm is certainly at the bleeding edge for that sort of current. I can understand that you want to minimise voltage burden but it's probably a step too far. My rule of thumb is to keep the burden at around 10mV in the current range you're interested in, although I do use a 1uV resolution meter. The burden of a standard 100mOhm DMM shunt is too high for most psu applications. Using a separate shunt, as you're doing, is definitely the preferable method.I would be looking at 10mOhm for 0.5- 2A sort of measurement range. These are easier to find in 4 terminal configuration with reasonable (1%) accuracy.Edit: I should qualify all of the above with 'where the burden voltage is a consideration'. Obviously for something like a 1.8 or 3.3V rail, 100mV drop would be excessive. Much less so with regulators being powered from a bench PSU.

author: exe
author_rank: Unknown
timestamp: May 07, 2021, 01:34:32 pm
content: I found that at least two of my dmms cannot reliably measure millivolts (an8002 and an8008). Like, reading is changing when I wiggle the dial. The error I had was +0.5-1.5mV to the "actual" voltage. So, I'd check capabilities of your gear.And, as others mentioned, if it's not a dc voltage, then you can't rely on dmm without some hardware filtering. I found that my dmms have problems with DC offset when measuring noisy signals (can't blame them for that). So, an oscilloscope is your friend, unless the signal is below the noise floor

author: coppice
author_rank: Super Contributor
timestamp: May 07, 2021, 01:41:39 pm
content: At 1 milli-ohm you need to take some serious care with layout if you are using a 2 terminal resistors. 4 terminal resistors help enormously, but are expensive.https://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.htmlhas some good information about suitable layouts for 2 terminal resistors. Note that you really have to get all the copper out of the measured path, as copper has a temp coeff of 0.4% per degree, so even if you compensate for the resistance of the copper being added to your 1 milli-ohm resistor, you still get significant error as the temperature changes.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2021, 05:25:24 pm
content: For measuring power consumption the burden voltage isn't a problem. Measure the voltage across the PCB and increase the supply voltage to have to correct voltage at the PCB. In this case even the burden voltage is at high as 1V wouldn't be a problem.

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2021, 11:10:06 pm
content: Quote from: Gyro on May 07, 2021, 10:58:05 amThat is what I think also. I consider a 4-wire current shunt resistor to be mandatory in this application.It might be possible to do without one by calibrating the current sense output with a known current source but the temperature coefficient will be poor.

author: coppice
author_rank: Super Contributor
timestamp: May 08, 2021, 12:02:04 am
content: Quote from: David Hess on May 07, 2021, 11:10:06 pm4 terminal resistors are certainly an easy solution. However, you can get great results with cheap surface mount resistors if you take the information inhttps://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.htmlseriously. I've worked with 2 terminal SMD resistor shunts down to 500 micro-ohms, and achieved good results using a similar approach to the suggestions in that Analog Devices paper.

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2021, 03:14:42 am
content: Quote from: coppice on May 08, 2021, 12:02:04 amThe problem I was referring to is the +600 PPM/C temperature coefficient of copper which quickly dominates the errors.I notice that Analog Devices didn't show the method I have used in the past with the sense connections coming of of the inside of the resistor pads, but it is not suitable with smaller parts.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: May 08, 2021, 03:31:08 am
content: What is the voltage of the power supply to the board with the switcher?

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2021, 09:02:18 am
content: It would be really nice to hear back from the OP about now.It would be helpful to know...1. What concerns do you have about burden voltage - why the choice of a 1mOhm resistor?2. Are you making use of the resistor in circuit, or is it just there for external monitoring purposes?3. What technology are you using? We don't even whether it is through-hole or smd (although I think even zero-ohm resistors are more than 1mOhm?)4. where exactly are you placing your probes?5. What instrument (make/model?) are you measuring with?6. A Photo would bereallyhelpful!I think we've reached the stage of stabbing in the dark with the information that we currently have.

author: exe
author_rank: Unknown
timestamp: May 08, 2021, 09:46:28 am
content: Oh yeah, photos please!PS never heard of stabbing in the dark (I usually say shooting in the dark). Is it a British saying?

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2021, 11:25:18 am
content: Quote from: exe on May 08, 2021, 09:46:28 amI guess it must be - you tend to forget the gory derivation of some of these sayings - it must date back to the Victorian era, probably much earlier!We have another saying: "I'll take a stab at it" as in I will try (not necessarily with expectations of success). A quick web search suggests that it derives from the late 17th century.

author: ledtester
author_rank: Super Contributor
timestamp: May 08, 2021, 12:26:43 pm
content: QuoteThe phrase "shoot(ing) from the hip" carries a similar connotation.

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2021, 01:07:24 pm
content: Oops, sorry. I edited that bit out for fear of stirring up something.

author: coppice
author_rank: Super Contributor
timestamp: May 09, 2021, 06:53:16 pm
content: Quote from: David Hess on May 08, 2021, 03:14:42 amAren't the C and D layouts they show exactly that? Even small amounts of copper can be a disaster for the temperature coefficent, as I said earlier in this thread, but a sloppy approach to the tap off point can easily make a 1 milli-ohm resistor look more like several milli-ohms, which is an even bigger error. I've seen it happen. Try what Analog suggest with 0805 SMD resistors or larger, and the results can be very good. I'd recommend using a 1206 or larger, though.We researched pretty much what Analog show in the link, and came to the same conclusions about what is practical. They published just as we were preparing our writeup, so we didn't bother publishing it.