Building a Precision 1A Current source, again, in 2020! - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: August 08, 2020, 10:05:06 am
content: Hi all,I've really enjoyed Dave's series on thishttps://www.eevblog.com/forum/blog/eevblog-567-precision-1a-current-source/and I am now looking to build one myself.Thoughts on what I could use as a replacement for the LTC6655? Here are some bits I've already got. 2nd column is the Farnell Code, last column is the manufacturer code:ON SEMICONDUCTOR Power MOSFET, N Channel, 200 V, 18 A, TO-220AB, 110W 2453452 IRL640AVISHAY FOIL RESISTORS Through Hole, 10 ohm, VPR221Z Series, ± 0.01%, TO-220 2820515 Y169010R0000T9LOne potential alternative on the resistor is this one, but it is super expensive and is only 0.1%, not even 0.01%!https://export.farnell.com/vishay-foil-resistors/y21231r00000b9r/res-1r-0-1-8w-to-220-metal-foil/dp/2820485P.S Before you make any suggestions, be kind enough to check if it is available at Farnell (Export) as I live in Asia and currently source everything via a local agent.https://export.farnell.com/

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2020, 11:03:41 am
content: For a precision source I would consider not relying on a accurate resistor, but use a calibration step.Because of the heat problem I would more like consider a lower reference voltage level and thus a smaller resistor, more like 0.1 to 1 Ohms. This would be some kind of current measurement shunt of some kind.The main part to care about is the resistor.Unless one would spend a small fortune or is really lucky the resistor would be more critical than the reference. There are plenty of 1-2.5 V bandgap references to choose from.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 08, 2020, 06:46:16 pm
content: Quote from: Kleinstein on August 08, 2020, 11:03:41 amThanks for the tip!I found this voltage reference as an example though:TI LM4140CCM-1.2 (https://export.farnell.com/texas-instruments/lm4140ccm-1-2/voltage-ref-series-1-25v-soic/dp/1286891)Datasheet:https://www.ti.com/lit/ds/symlink/lm4140.pdf

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 08, 2020, 07:32:48 pm
content: This last voltage reference seems ok. You can easily find 0.05% initial accuracy references too (can't remember now exact part numbers), they are not very expensive.And yes, shunt has to be somewere between 0.1...0.5 Ohm. I mean no more than 0.5 W dissipated power, otherwise you have to add an active cooling to shunt too. (Hot shunt is bad shunt). The cooler - the better.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 17, 2021, 05:52:08 pm
content: Hi Kleinstein,I'm returning to work on this and in the mean time I've built a CC DC load - so I'm going to be using a similar approach here for the CC loop with an op-amp.With regards to the shut resistor I will be using a 0.1 ohm shunt. I have this in stock:https://export.farnell.com/bourns/pwr221t-30-r100f/resistor-thick-film-0-1ohm-1-to/dp/2328296You mentioned using a "calibration step". I could place the voltage going into the Opamp under MCU control by using a 20-bit DAC (or something cheaper), but the DAC's digital value can be "calibrated", and we can adjust this to match the MOSFET I(d) current to what we want.On the bench, the same can be realised with just a linear-pot, which is basically what my DIY CC DC load does.Please confirm, thanks!Cheers, M.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 17, 2021, 06:34:10 pm
content: A 20 bit DAC would be total overkill just for a trim step. With reasonable resistors / ref. the trim range only need to be some 1-5%. So 10 Bit resolution would result in some 10-50 ppm trim steps, which is likely OK. Not many meters are more accurate at 1 A and the shunt would likely drift more.The linked 0.1 Ohms resistor does not look like highest quality. I would prefer more like wire wound, bare wire or foil type. The choice really depends on the quality needed /wanted and currently also what is available.

author: magic
author_rank: Super Contributor
timestamp: August 18, 2021, 02:19:25 pm
content: I don't know how good it is in terms of long term stability, but LM10 could offer a one chip solution to: reference, opamp and adjustment. The rest is one external potentiometer, one external 0.2Ω shunt and one external pass transistor.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 25, 2021, 01:31:52 pm
content: Hi Kleinstein, thanks for the advice, I found this - thoughts?SMD Current Sense Resistor, 0.1 ohm, WSL Series, 1206 [3216 Metric], 250 mW, ± 0.5%, Metal Striphttps://export.farnell.com/vishay/wsl1206r1000dea/res-0r1-0-25w-1206-metal-strip/dp/3596991Cost is £1 per strip, it's the cheapest at that price point. 0.1% is crazy expensive.Thanks M

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2021, 03:50:57 pm
content: The choice of parts really depends an what is wanted. This could be a given acceptable price point (more for the whole circuit, less for single parts) and than looking for a good solution to fit the bill. The other way is to set a required performance and than look for a cost effective (and maybe in stock) solution to match the requitements. For the performance there are different points like absolute accuracy, stability over time and temperature, noise and others.The linked resistor is only 1206 size and this relatively small and thus low power. 1 A and 0.1 Ohms would allready be 100 mW and this normally for a shunt would call for something like a > 1 W power rating to keep the self heating low. It may still work with less for a constant current, but slow warm up may still be inconvenient.

author: mikerj
author_rank: Super Contributor
timestamp: August 25, 2021, 03:54:48 pm
content: It's a bit of a chonker for the application but 20ppm is very good for this price:https://export.farnell.com/rohm/gmr100htcfar100/current-sense-res-0r1-1-3-w-2512/dp/2808335RLNote that the layout of the sense connections to the resistor (i.e. Kelvin connection) will be critical to get good tempco performance.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 09, 2021, 10:24:09 am
content: I want to double check my numbers - if I'm measuring 10A across a 0R1 resistor, that's P = I^2*R right? Hence, 10W power.20A would be 40W power dissipated in the current shunt. Wouldn't it make sense to drop to a 0.01resistor in this case?Considering the 0.01:-https://export.farnell.com/ohmite/pcs2512dr0100et/res-0r01-2w-2512-metal-alloy/dp/3285929-https://export.farnell.com/ohmite/fc4l90r010fer/current-sense-chip-1-0-010-ohm/dp/3217832Both are under 1% and 50ppm and allow between 2 & 4W, which is more than enough headroom (10A^2 * 0.01 = 1W and 20A^2 * 0.01 = 4W so both cases can be supported).

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2021, 10:41:33 am
content: There is a difference between the rating the resistor can survive and the power that can be used, when you expect the resistor to be accurate.So a given resistor may be OK to survive 1 W, but if accurcy is wanted better use only 100 mW.It is definitely good to lower the shunt for 10 A, but 10 mOhm may still be too much, especially with such a tiny SMD part, that need a hughe copper area to only survive the given power.For a 2 W actual power dissipation you don't want something you need tweezers and a magnifying glass.I would more like look for a THT part to be mounted on a heat sink, rated for 10 W or more. So more like something like this:https://www.reichelt.de/widerstand-manganin-5-0-mohm-sil-10-w-1--isa-pbv-r005-f1-p237449.html?CCOUNTRY=445&LANGUAGE=de&trstct=pos_5&nbc=1&&r=1

author: bsodmike
author_rank: Regular Contributor
timestamp: September 09, 2021, 10:48:27 am
content: Thanks - yes I was thinking a THT part would be better, but they are costly for sure.Specs wise this is great though 0.5% and ~5ppm.https://export.farnell.com/ohmite/cs10dr010e/res-0r01-10w-radial-metal-alloy/dp/3604567?st=resistor+sil+10wI'll check for something cheaper - sorry, I need to source through Farnell.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 09, 2021, 10:52:40 am
content: Oh BTW I've been discussing 2x separate scenarios here.1) as per the original thread, my intension then was re. a 1A precision source.2) my more recent reference to pushing 10-20A through is for a separate CC DC load application.With regards to (2), here's something I've been prototyping:https://youtu.be/WmP2RaRIo74Thought I'd mention this so as not to confuse any other readers of this thread as well.TL;DR for the precision 1A source, the SMD part more than fine.

author: magic
author_rank: Super Contributor
timestamp: September 09, 2021, 12:33:50 pm
content: I have used the 3W variant of this OAR resistor for a 10A dummy load. Cheap, indestructible and accurate enough for the job.Backordered at Farnellhttps://export.farnell.com/tt-electronics-welwyn/oar5-r010fi/resistor-1-0r010/dp/1200374?st=oar5