The value of the mA current sense resistor for BM869S? - Page 1
Forum section: electronics / testgear

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 02:04:03 pm
content: Could you please tell me the value of the current sense resistor in BM869S?How many ohms and what is the rated power? Thank you.

author: 2N3055
author_rank: Super Contributor
timestamp: July 20, 2020, 02:41:09 pm
content: From spec:DC Current:Burden voltage500.00µA 0.15mv/1µA5000.0µA 0.15mV/1µA50.000mA 3.3mV/mA500.00mA 3.3mV/mA5.0000A 45mV/A10.000A 45mV/ASo burden voltage here is voltage drop on resistor.. Use Ohms law and calculate resistance. Nominal power at every range will be resistance in range times nominal current squared...

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2020, 02:46:22 pm
content: Actually the burden voltage will result from the total resistance of the current path including fuse and any other protection devices. The value of the current sense resistor will be less than this.

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 03:18:50 pm
content: The burden voltage helps us to find the value with approximation, but I wish to know the exact values of the resistor installed. I am especally interested about the nominal power.

author: HKJ
author_rank: Super Contributor
timestamp: July 20, 2020, 03:57:43 pm
content: Quote from: aurel9x1 on July 20, 2020, 03:18:50 pmNearly all meters have 1ohm or 0.99ohm resistor for the mA range, looking at the voltage drop I will say this is also the case for this meter.

author: Fungus
author_rank: Super Contributor
timestamp: July 20, 2020, 04:14:49 pm
content: Quote from: HKJ on July 20, 2020, 03:57:43 pmI'd say it looks more like 3.3 Ohms.Quote from: 2N3055 on July 20, 2020, 02:41:09 pm

author: Monkeh
author_rank: Super Contributor
timestamp: July 20, 2020, 04:24:29 pm
content: Quote from: Fungus on July 20, 2020, 04:14:49 pmThe fuse will make up a large portion of that.

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 05:45:23 pm
content: Does anybody have the specs of the current resistor used in any of the other meters? Usually is used an 0.98ohms resistor, but what about the nominal power?I know it can't be smaller than 1W, but I am interested of the exact power rating chosen by the manufacurer which probably is greater. Also, the resistor should have a very low TCR, very low inductance, very good long term stability and should be able to withstand high voltage for a couple of miliseconds in case of short-circuit, so it must be some special resistor, not "just an 1ohm resistor"...If anybody knows the exact specs for BM869S or for any other dmm would be great.

author: 2N3055
author_rank: Super Contributor
timestamp: July 20, 2020, 05:49:11 pm
content: Measured is 0.03 Ohm and 1.79 Ohm and 101.82 Ohm with fuses..Fuses will be lower resistance when cold..To speculate, that would be 1 and 100 Ohm for mA i uA ranges.. For A range I don't know, could be 0.01 Ohm or maybe 0.02 , depending on fuse..Power will be some factor more than max dissipation to minimize heat up..Why is it important? What do you do? Are you repairing one, or what?

author: 2N3055
author_rank: Super Contributor
timestamp: July 20, 2020, 05:49:56 pm
content: Quote from: aurel9x1 on July 20, 2020, 05:45:23 pmAha, search for current shunt resistors, or current sensing resistors..

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 06:09:54 pm
content: I am searching and 'researching' the subject for a couple of days, but unfortunately I wasn't able to find the exact specs.The closer I could get was the value found in an old schematic of a fluke dmm. Fluke is using 4.99ohm (at least in the old models, I don't know about the new models). I think a bigger value offers better resolution and better precission but trades it for a higher burden voltage and higher power dissipation, so the resistor should have a better TCR and higher nominal power.

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 07:22:11 pm
content: Quote from: 2N3055 on July 20, 2020, 05:49:11 pmI am not repairing one, I need to understand how the imput circuit works in order to use one in some specific circumstances.

author: Fungus
author_rank: Super Contributor
timestamp: July 20, 2020, 07:28:19 pm
content: Quote from: aurel9x1 on July 20, 2020, 07:22:11 pmThat's easy: Take the lid off, put it in mA mode, pass a known current through it and measure the voltage drop between the 'internal' end of the fuse and the COM terminal. Use Ohms law to get the resistance.

author: HKJ
author_rank: Super Contributor
timestamp: July 20, 2020, 07:32:54 pm
content: Quote from: Fungus on July 20, 2020, 04:14:49 pmI based my conclusion on the values I measured in my review:https://lygte-info.dk/review/DMMBrymen%20BM869s%20UK.htmlUsually the mA fuse adds roughly 1ohm on top of the resistor value.

author: HKJ
author_rank: Super Contributor
timestamp: July 20, 2020, 07:34:20 pm
content: Quote from: aurel9x1 on July 20, 2020, 07:22:11 pmYou can see how a typical input is designed here:https://lygte-info.dk/info/DMMDesign%20UK.html#Current

author: tooki
author_rank: Super Contributor
timestamp: July 20, 2020, 08:44:58 pm
content: Quote from: aurel9x1 on July 20, 2020, 07:22:11 pmIf you have one, why not open it up and find the resistor? If it’s marked you’re all set, and if not you can measure it with a good bench meter, or measure a voltage drop across it like Fungus says.

author: aurel9x1
author_rank: Contributor
timestamp: July 20, 2020, 09:04:05 pm
content: I need to know the values of the resistor in order to actually use the meter. I am especially interested in the nominal power the resistor is rated. I can't measure that.For example, I want to use the meter to measure a current through a live circuit that is taking 500mA ...more or less. Can I use the meter to continuously measure 500mA and log it to a computer?What about 600mA? What happens if the circuit starts to take more and the current slowly rises to 1,3A on the mA scale or if by mistake I'll try to measure 1,3A? According to the datasheet, the fuse won't blow, so will the imput circuit be able to dissipate continuously 1.3W without decalibrating the current resistor? For how long? (depends of the power rating of the resistor).If the resistor won't be able to accept this current for long, it means the protection diodes should do their job, but the 4 diodes won't conduct before the imput is reaching 1,4V (aka 0,7V per diode, aka 1,4A through resistor - assuming it's an 1ohm), so no protection at 1,3A.What will be the effect on the precission of the measurements if the current goes back to 400mA? Will be any error?Many assumptions can be made, but I need the real values, that's why I asked for them.I think the manufacturer supposed the average Joe will measure the current for a minute or two and than take the meter out of the circuit, so didn't mentioned anything about the long term effects.//for example Keysight mentions in the U1280 manual that 440mA to 600mA can be measured for max 20hours and the meter should be cooled down, also the delta function should be used in order to compensate for the heating error. How they got to 20 hours is beyond me. Nevertheless, it is obviously that 600mA is already the limit for the mA scale and more will stress the resistor in any meter no matter how good it is. The max current depends of the ability of the resistor to dissipate the heat and is important especially when overloading with about 50% (no fuse blows).//The imput protection works if the average Joe by mistake connects the meter to the mains (fuse instantly blows), but what happens if he puts just 1.1A? Will the resistor be able to dissipate that heat (easy to say 'yes', but if it would be so easy we would have the 600mA limit)?

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2020, 09:45:04 pm
content: It seems like you should probably address these questions to Brymen customer support rather than asking the internet for guesses?Be aware that it is very easy to blow the low range current fuse with even a small over-current, as I found to my cost. If you try to measure 1000 mA or more, don't count on the fuse surviving.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2020, 10:32:22 pm
content: Maybe easiest to use what ever shunt you feel need. Nice thing about this approach, you can decide if you need the fuse or not and you can optiomize the shunts value for your specific requirement.

author: J-R
author_rank: Super Contributor
timestamp: July 21, 2020, 02:10:46 am
content: Link to the fuse datasheet showing the current/time relationship being mentioned:https://www.mouser.com/datasheet/2/87/Bus_Ele_DS_1047_DMM-B-336843.pdfGiven the BM869s has 0.6A printed directly on the front, I think it would be wise to expect 0.6A to be the absolute maximum continuous rating in absence of a manufacturer's direct statement.However, the BM869s will go in OL at approximately 531mA and you will not get a reading above that level, so 500mA is probably the sensible target maximum.The resistance of the mA fuse is approximately 0.7 Ohms, which is a significant portion of the meter's total resistance in mA mode of approximately 1.75 Ohms (leads excluded). Regardless, in any case, to get a meaningful value for placing the DMM into your circuit, you would want to use an appropriately accurate meter with Kelvin leads and simulate the connection method as best as possible in order to also gather the connection resistance values.Side note, the BM869s does quite well with single digit mA readings in 10A mode, would that not work for your application?

author: aurel9x1
author_rank: Contributor
timestamp: July 21, 2020, 02:46:18 am
content: I think the best solution would be to use the A scale. Anywhay, I have a couple of questions.1. If the resistance on the mA scale is about 1.75ohms, why the burden voltage is 3.3mv/mA?2. Agree, 600mA should be the limit, but if I put 1.2A through it, will the device be protected or not?3. If the internal resistor is 1ohm, and there are four diodes for protection, and two diodes should open when overloaded, it means that the protection will be activate only at 1.4V, right? What happens at 1.3V aka 1.3A? Can the resistor handle that?For example an old fluke has 5ohms input resistor and is using 6 diodes in series for protection, that means max 4.2V so the protection is activated at 840mA.In my mind things are not clear. The burden voltage does not match the input resistor value and also the protection does not match, so I think I miss something.

author: aurel9x1
author_rank: Contributor
timestamp: July 21, 2020, 03:04:33 am
content: The fluke at 840mA will produce over 4W, but having an 1ohm resistor at 1.3A will only generate 1.3W of heat. So it can accept a higher current and generate less heat, but if this is the case, why the 0.6A limit? And why all the other things that not match?

author: J-R
author_rank: Super Contributor
timestamp: July 21, 2020, 04:46:35 am
content: The BM869s is absolutely right around 1.75-1.85 Ohms throughout the mA range (depending on connection type and leads). I pushed 0.9A for about 10 seconds and after returning the current to to 500mA and even 1mA the meter sat at around 2.8 Ohms. There was no downward trending, such as from cooling. I then pushed up to 1A but the fuse blew. After replacing the fuse with an identical one, I let it sit at 750mA for quite some time and it remained very close to 1.8 Ohms with no drift to speak of.Pushing the higher current through the fuse most likely damaged it permanently, increasing the resistance value slightly.The BM235 and the 121GW use a 440mA fuse that has a higher resistance value of about 1.8 Ohms, resulting in a total DMM resistance of approximately 2.5 Ohms for the BM235 and 3.0 Ohms for the 121GW.So:1. Seems like either a typo or it will depend on the installed fuse.2. With the Eaton/Bussman fuses referenced, it appears yes, the fuse will blow before you can realistically reach even 1A.3. Brymen most likely did their homework and the fuses should be the weak link in all (normal!) cases.

author: aurel9x1
author_rank: Contributor
timestamp: July 21, 2020, 01:27:27 pm
content: From the datasheet, I understand that the fuse does not blow before 1.25A and even at 1.3A it takes about 100hours. Maybe I miss some effect or misinterpret the data. The graph is a little blury, so, I've just took a screenshot of the graph and added my notes using red color. Please take a look at my notes and let me know what I've missed.My annotated picture attached. Thank you.

author: 2N3055
author_rank: Super Contributor
timestamp: July 21, 2020, 02:22:57 pm
content: You seem to be very particular how measurement needs to be done. Which is actually a good thing.In which case best recommendation is what Joe recomended.Set a measurement fixture with a shunt of your choice, with fusing and everything, and than measure voltage on shunt and log that. Conversion to current is trivial.There are many good resistors to be used for that. This way you will even be able to achieve better measurement specs than built in current measurements, if properly calibrated..

author: aurel9x1
author_rank: Contributor
timestamp: July 21, 2020, 02:49:07 pm
content: I suspect one of the two:It might be a bigger than 1ohm shunt. Some Flukes use 5ohms, and I think a higher value offers better precision and resolution. Burden voltage increases, but the precision and resolution are very important, especially for this class, so this could be the case.orthere must be some suplementary components in series added, maybe for protection or to improve the measurements in high frequency, maybe a coil or something. That would explain why in CC the measured imput resistance is 1.7ohms, but things might be different in CA and this could explain the statement in the manual.I think there is no typo in the manual, I suspect they designed it that way, added some components to improve the schematic and that would explain why it works (which is obvious from the real life usage).