How to implement a 4.7mOhm 10A sense resistor? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: March 31, 2016, 06:58:31 pm
content: So this is called for when implementing my own mixed mode power supply... How to make it happen? I cannot find them in my local electronics store, and the current involved is too large to be built using a PCB trace. Help...

author: Monkeh
author_rank: Super Contributor
timestamp: March 31, 2016, 07:11:33 pm
content: Buy one from a regular supplier?

author: technix
author_rank: Super Contributor
timestamp: March 31, 2016, 07:12:48 pm
content: Quote from: Monkeh on March 31, 2016, 07:11:33 pmNot available. 10mOhm minimum and those are expensive.

author: georges80
author_rank: Frequent Contributor
timestamp: March 31, 2016, 07:14:47 pm
content: Digikey/Mouser etc. A PCB 'trace/resistor' has poor tempco.Lot's of sources, just search for a real distributor versus the corner store... And yes, they'll cost more than $0.001 each...cheers,george.

author: Monkeh
author_rank: Super Contributor
timestamp: March 31, 2016, 07:17:31 pm
content: http://uk.farnell.com/welwyn/lrmam1206-r005ft5/resistor-metal-alloy-chip-r005/dp/2079443http://uk.farnell.com/panasonic-electronic-components/erjm1wsf5m0u/resistor-2512-0r005-1-1w/dp/1577579http://www.digikey.com/product-detail/en/susumu/KRL1632E-M-R005-G-T5/408-1565-1-ND/3737747http://www.digikey.com/product-detail/en/ohmite/MCS1632R005FER/MCS1632R005FERCT-ND/2003871http://www.digikey.com/product-detail/en/bourns-inc/CRF0805-FX-R005ELF/CRF0805-FX-R005ELFCT-ND/4430996I didn't even bother looking at Mouser. Resistors are not hard to buy.

author: technix
author_rank: Super Contributor
timestamp: March 31, 2016, 07:18:04 pm
content: Quote from: georges80 on March 31, 2016, 07:14:47 pmI don't really need tempco here as there is a linear stage following that with its own current sensing. So as long as it don't drift too much I'm okay...

author: technix
author_rank: Super Contributor
timestamp: March 31, 2016, 07:22:13 pm
content: Quote from: Monkeh on March 31, 2016, 07:17:31 pmI cannot use a 5mOhm one - the calculation got thrown off there. 4.7mOhm is already borderline. I can go down but not up.

author: Monkeh
author_rank: Super Contributor
timestamp: March 31, 2016, 07:22:56 pm
content: Well they list hundreds of 4mohm too. Those are your choices off the shelf.

author: klunkerbus
author_rank: Unknown
timestamp: March 31, 2016, 07:36:12 pm
content: Depending on what kind of precision you're looking for, some turns of the right size wire with specifications you can trust is sometimes used as a current shunt. Details on doing that are easy to find.

author: dadler
author_rank: Unknown
timestamp: March 31, 2016, 07:38:29 pm
content: If you can go down in value far enough to hit the power rating, these will work:http://www.vishay.com/docs/30101/wsr.pdfI picked up about 15 different values of the above sort of sense resistors from this eBay seller:http://stores.ebay.com/hfoelektronik/Resistors-Potentiometers-/_i.html?_fsub=510193119

author: mazurov
author_rank: Frequent Contributor
timestamp: March 31, 2016, 08:09:57 pm
content: Quote from: technix on March 31, 2016, 07:12:48 pmYour resistor will dissipate 0.5W - this is not difficult at all. Mouser has plenty of small value resistors in this power rating - look under 'current sense resistors'. If you want to save money, make it from regular smt resistors by stacking them on top of one another. 0603 package is typically 0.1W, to get 0.5W use 5 in parallel.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 31, 2016, 08:26:34 pm
content: Just make your own four terminal resistor.Length of copper wire capable or handling 10 amps, solder a low current wire on one end, pass 10 amps through your wire and slide the other low current wire along the copper wire till you get a 10 Amp reading on your equipment, then solder it where it is.Done

author: langwadt
author_rank: Super Contributor
timestamp: March 31, 2016, 09:15:04 pm
content: Quote from: Mr Smiley on March 31, 2016, 08:26:34 pmcopper has a terrible temperature coefficient at ~0.4% per C

author: technix
author_rank: Super Contributor
timestamp: March 31, 2016, 09:23:42 pm
content: Quote from: langwadt on March 31, 2016, 09:15:04 pmOnce again, I don't need good tempco here. I have a post-regulator in place.

author: mariush
author_rank: Super Contributor
timestamp: March 31, 2016, 09:57:39 pm
content: Use 3 x 1 ohm resistors in parallel and you have 0.33 ohm .. but really, at 10A there's going to be quite a voltage drop and power wasted on those resistors, even if they're 1ohm each. They'll heat up and the accuracy will go down the drain if you don't buy a proper current shunt.My suggestion would be to consider a hall effect sensor type IC because they typically have a constant 1-1.5 mOhm resistance and they output a voltage proportional with the current, which makes it easy to read this voltage and convert it to an actual current value and show it on led digits or lcd displays, and it's also easy to push the output through an opamp or whatever you use for feedback in a current limiting scheme.See for exampleACS725for an ok-ish 10A max current sensor, orACS723for a 20A one. There are much better ICs than these two, but they cost almost double, these are already kind of expensive at around $4-5 a piece.The lineary and accuracy of these cheaper chips make are not superb, but they're still good enough for... hmmm, let's say 50mA steps. Really, would you need much finer current control/limit at over 1A of current output?For more accuracy, you could also test each chip and pre-compute values to correct the linearity issues, then you could save a table in the microcontroller that displays or reports the current.I'd say if you want really accurate current measurement under 1A, you could also add a 1 ohm resistor in series with this sensor and when current goes above 1A you could just short the resistor with a mosfet or a relay and use the output of the hall sensor IC. When you're below 1A, just enable the resistor and now you have 1 ohm of the resistor + the 1-1.5mOhm of the current sensor IC, so you have much finer current measurement, but your circuit will have to handle these two cases differently ( the hall effect sensors output 200-264mV per amp starting from 2.5v at 0A, while your 1ohm resistor will have a voltage drop of 1v per A)PS. and speaking of your circuit design inyour other threadshouldn't you use fuses rated for higher than 5A if you want to have up to 5A of DC input from each input? Make it 6A time delay or something like that?And what's the fascination with SS54 diodes, i've seen them in several projects ... they're only going to see up to around 2.5-3A in worse case scenario, and they'll handle this much only provided you give them enough pcb pads for their ends... according to your notes in the picture, you have heatsink with 7 to-220 locations, so you could use twoGBU1002or something similar...

author: nctnico
author_rank: Super Contributor
timestamp: March 31, 2016, 10:37:07 pm
content: Quote from: technix on March 31, 2016, 09:23:42 pmThen just use a PCB trace. In one of my recent designs I just need to know the direction of the current so I measure across a bunch of wide PCB traces in parallel on several layers (it should handle 70A). I also have a temperature sensor on the board so compensating for temperature is an option.

author: Fungus
author_rank: Super Contributor
timestamp: March 31, 2016, 10:59:05 pm
content: Quote from: technix on March 31, 2016, 07:22:13 pmTwo 5mOhm in parallel?

author: technix
author_rank: Super Contributor
timestamp: April 01, 2016, 02:27:10 am
content: Quote from: nctnico on March 31, 2016, 10:37:07 pmI need to make sure 10A does not fry it. How wide should it be then?

author: daqq
author_rank: Super Contributor
timestamp: April 01, 2016, 09:02:58 am
content: Try another supplier. Any of these should work:http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000050528&showResults=true&aa=true&pf=110070932,110105981This one looks nice.http://uk.farnell.com/ohmite/ebwa-nr0040fe/res-current-sense-0-004-ohm-2/dp/2420522While you will not find an EXACT 4.7mOhm value, 4mOhm and 5mOhm are common. There's a 4.5mOhm one -http://uk.farnell.com/te-connectivity/tlr3a20dr0045ftdg/resistor-r0045-2-0w-1/dp/2116019.As for making one home, well, tricky. You can parallel several resistors, but at high currents that's iffy. You can get 4.7mOhm by paralleling: 10mOhm, 10mOhm, 82mOhm and 1.8 Ohm for instance.

author: technix
author_rank: Super Contributor
timestamp: April 01, 2016, 09:05:46 am
content: Quote from: daqq on April 01, 2016, 09:02:58 amIt took me a few tries and a good hour of search to find some local supplier that sells 4mOhm 2512 SMD resistors. My other option was using some PCB traces but since I only have 2 layers that can be tricky.

author: coppice
author_rank: Super Contributor
timestamp: April 01, 2016, 09:21:57 am
content: Quote from: technix on March 31, 2016, 06:58:31 pmFrom your tag I assume you are actually in China. Its easy to get 2 terminal SMD shunts down to 2 milliohms from a number of sources in China. Its less easy to find 4 terminal shunts, but you can get those below 1 milliohm. If you use a 2 terminal shunt you need to take some care with board layout to ensure the shunt performs well. Copper has a temp coeff of 0.4%/C, so even a little copper in the path can really mess up results over the temp range.http://www.analog.com/library/analogDialogue/archives/46-06/shunt_resistors.htmlos a good place to look for guidance in this area.I have sourced 500 micro-ohm 2 terminal SMD shunts in China, but I had to get someone to do some searching, and they were not cheap.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2016, 09:53:31 am
content: If the circuit is that critical that 5 mOhms are too much, take care of the lines going to the shunt - these will add to the voltage drop seen by the main current - just the measuring circuit part might not the it if the sensing wires are correct connected.For adjustment, keep the option to have a few resistors in parallel.If it just needs to be slightly more stable than copper trace, one can better use a piece of brass: easier so solder than proper low TC alloys like Manganin, but significantly better than copper.

author: nctnico
author_rank: Super Contributor
timestamp: April 01, 2016, 01:10:52 pm
content: Quote from: technix on April 01, 2016, 02:27:10 amJust use the normal width from a PCB trace calculator. This will also give you the resistance per length so you can calculate the amount of amplification versus the distance between the measurement points you need to get a signal you can use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2016, 01:55:27 pm
content: People, people, please. The application is for a switching regulator.There is NO COPPER SOLUTION where the inductive reactance will also be significantly less than the DC resistance.Not beyond about 10kHz.It must be a "hunk of nichrome" type, or something like that. It will still need an R+C afterwards for filtering the inductance, but it can be a reasonable value.Tim

author: nctnico
author_rank: Super Contributor
timestamp: April 01, 2016, 05:07:20 pm
content: That is not clear at all from the description. Besides that if it is a switching regulator I'd use Rdson current sensing.Edit: IF the current sensing is to measure the current through the MOSFETs at the switching frequency!

author: Bud
author_rank: Super Contributor
timestamp: April 01, 2016, 05:45:34 pm
content: Quote from: klunkerbus on March 31, 2016, 07:36:12 pm+1

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2016, 06:39:52 pm
content: Quote from: nctnico on April 01, 2016, 05:07:20 pmSadly, OPs often assume others already know all the context...In this case, context is:https://www.eevblog.com/forum/projects/mixed-linear-and-switch-mode-topology-in-a-lab-power-supply/Of course, *I'm* assuming this is the context as well, so I can't really be sure.Tim