Current sense shunt resistor on high current pcb - Page 1
Forum section: electronics / projects

author: cmorgan
author_rank: Newbie
timestamp: November 12, 2018, 02:01:33 pm
content: Hello!I have a high current pcb, 2oz copper, 7.5mm wide traces each side, designed to comfortably carry 30A @ 240VAC.I'd like to add a shunt resistor to measure current on the pcb. Resistances from 0.0003 to 0.0001 ohms keep total power below 4W but all of these are surface mount likehttps://www.digikey.com/product-detail/en/stackpole-electronics-inc/CSS2725FTL250/CSS2725FTL250CT-ND/1923235Given that I have the 2oz traces on the top and bottom of the pcb how should I handle adding that shunt resistor? Vias to pull the trace to one side of the board, through shunt resistor and then vias on the other side to go back to traces on both sides?Chris

author: 001
author_rank: Super Contributor
timestamp: November 12, 2018, 03:04:21 pm
content: Quote from: cmorgan on November 12, 2018, 02:01:33 pmuse standard 4 wire reading

author: coppice
author_rank: Super Contributor
timestamp: November 12, 2018, 03:17:13 pm
content: How accurately do you need to measure the current? It matters a lot in these designs. If you want a rough estimate (say 5%) lots of things will work. If you want decent accuracy (<1%) you will need to take great care to achieve a good 4 wire connection (whether you use an actual 4 wire shunt device or not), and you'll either need to avoid the shunt heating too much, or pay a lot for a shunt with an extremely good temperature coefficient.

author: cmorgan
author_rank: Newbie
timestamp: November 12, 2018, 03:18:36 pm
content: Quote from: 001 on November 12, 2018, 03:04:21 pmHi 001.Would you explain what you mean by a 4 wire reading?If it helps this is a single phase two wire, so I have either L / N or L1 / L2 through the device. I didn't want to put two shunt resistors, one on the top and one on the bottom as then I'd have to sum up the current through the two paths. I'm looking at using ATM90E26 for the measurements.Chris

author: Marco
author_rank: Super Contributor
timestamp: November 12, 2018, 03:18:53 pm
content: More like 10 vias, or you could use two shunts and then just resistively combine the outputs.PS. actually on second though, can't easily combine the outputs resistively since you don't have a good common ground.

author: cmorgan
author_rank: Newbie
timestamp: November 12, 2018, 03:24:32 pm
content: Hi coppice.5% would be great, this isn't for metering purposes. I was going to try to achieve a kelvin attachment as best as I could as well.I was originally looking at using CTs but the shunt resistor approach seemed lower cost, less space and less manual complexity to assemble.At this point I'm most concerned that the smd shunt causes me to redirect the current across one layer of the PCB and could impact the current carrying capability. The rest of the components on the high current path are through hole and connect to top and bottom layers. I found some THT shunts but their resistance is lower and at peak current I was worried about dissipation. It's 30A @ 240VAC, maybe my calculations are broken. I was using a peak voltage of something like 400V, so 400V * 30A, to calculate power through these shunt resistors but its 240VAC RMS so maybe that calculation is indicating far more power than will actually be dissipated and could let me use one of those THT shunt resistors at 0.001 ohms.Chris

author: coppice
author_rank: Super Contributor
timestamp: November 12, 2018, 03:26:50 pm
content: Thishttps://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.htmlis a good source of information about getting good accuracy from SMD current sensors.

author: coppice
author_rank: Super Contributor
timestamp: November 12, 2018, 04:31:51 pm
content: The important thing to remember when even trying to get 5% accuracy is copper has a temperature coefficient of 0.4%/C. Even small amounts of it in the measured path can seriously screw up accuracy.

author: 001
author_rank: Super Contributor
timestamp: November 12, 2018, 08:20:40 pm
content: Quote from: cmorgan on November 12, 2018, 03:18:36 pmThis is trick to eliminate wire errorhttps://en.wikipedia.org/wiki/Four-terminal_sensing

author: jbb
author_rank: Super Contributor
timestamp: November 12, 2018, 08:36:01 pm
content: I think LEM make a surface mount IC which senses the magnetic field of current flowing through a PCB trace. It won’t be very accurate but maybe good enough for your purposes.

author: Marco
author_rank: Super Contributor
timestamp: November 13, 2018, 12:46:35 am
content: Quote from: cmorgan on November 12, 2018, 03:24:32 pmVery expensive though, for that cost you could put a SMD shunt on both sides and use some method to measure both (DPDT or two current amplifiers).

author: t1d
author_rank: Super Contributor
timestamp: November 13, 2018, 02:07:54 am
content: TE Connectivity/TE.com may have a resistor to consider.https://www.te.com/usa-en/plp/hs/X262v.html?q=&n=42563&type=products&samples=N&instock=N

author: tszaboo
author_rank: Super Contributor
timestamp: November 13, 2018, 10:33:53 am
content: Quote from: cmorgan on November 12, 2018, 03:24:32 pmYou do realize that you are not calculating the power the right way?You never have 240V on a shunt, and the power isnotP=U*I*RYou have a shunt with voltage drop of like 1 volt.And P= I*I*R.So for a 1 mOhm resistor it is 30A*30A*0.001 = 0.9W

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2018, 11:43:10 am
content: Yes, the power calculation is wrong. To dissipate <4W at 30A, you need a resistance of <4.444mOhm.R = P/I2= 4/302= 4/900 = 0.00444430A across a 4.444mOhm resistor would be a voltage drop of:V = I*R = 0.004444*30 = 0.13333V.Aiming for exceptionally low voltage drops, increases errors due to noise and the offset voltage of the amplifier. For example, if the amplifier has an input offset error of 100µV and you're using a 0.004mOhm shunt, the current offset error will be:I = V/R = 0.0001/0.004 = 0.025A = 25mA.

author: coppice
author_rank: Super Contributor
timestamp: November 13, 2018, 11:56:44 am
content: Quote from: Hero999 on November 13, 2018, 11:43:10 amWhy would amplifier offsets matter when measuring AC? You estimate and remove an DC. The real issue you need to worry about is the CMRR of the input to the electronics. In the OP's case this is not an amp. Its a differential ADC input with a really good CMRR. Its designed for shunts around 500 micro-ohms when the maximum current is 30A.

author: nctnico
author_rank: Super Contributor
timestamp: November 13, 2018, 01:32:56 pm
content: Quote from: coppice on November 12, 2018, 04:31:51 pmYou can use that. In a design which didn't need very accurate current sensing I had a temperature sensor (cheap NTC) near the trace and measured the voltage drop across a piece of PCB trace. Together with the temperature you can get current readings and compensate for the temperature.

author: r0d3z1
author_rank: Regular Contributor
timestamp: November 13, 2018, 01:58:31 pm
content: Quote from: coppice on November 13, 2018, 11:56:44 amI confirm this, usually the problem is the CMRR, could you be more precise about the circuit, is it a low side sense ? phase sense ? high side sense ?For a low side current sense you can use this kind of shunt with a difference amplifier. You must be careful with the resistance due to the soldering which is comparable to the shunt value. For this reason, you have to use a 4 wire sensing as already suggested.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2018, 09:01:29 pm
content: Quote from: coppice on November 13, 2018, 11:56:44 amYes, you're right, I missed it was AC, the offset voltage is no problem.

author: perieanuo
author_rank: Frequent Contributor
timestamp: November 15, 2018, 04:16:05 pm
content: Acs723?Envoyé de mon iPad en utilisant Tapatalk