Current shunt recommendation

[Dacian Todea]

I get quite a few question related to current shunts so I decided to make this post.

SBMS0 supports any shunt between 0.0400mOhm and 9.9999mOhm that means all shunts in the Riedon RS range https://riedon.com/media/pdf/RS.pdf  except for the smallest 5A one but including 1200A.

As I order parts for SBMS0 production I also ordered a few of this Reidon shunts one from each category the RSA smallest ones up to 150A so I got the 100A 100mV one then one from the midd size RSB and got the 500A 100mV one and finally the huge RSC 1000A 100mA.

Interestingly the 500A 100mV was the only one coming with a certificate of calibration showing the exact current shunt value of 0.200045mOhm so that will round out to exact 0.2000mOhm in the SBMS settings.

I got my current shunts from Digikey that should be available in almost any country   https://www.digikey.ca/  But Riedon also has his own website and I think you may be able to buy directly from them https://riedon.com/how-to-buy/

Murata also has a similar line of current shunts and I have the small 10A 100mV you can see in one of the photos below.

I attempted to measure the current shunt resistance but I do not have precision enough power supply and multimeter to get to the level of precision required to check their calibration (not even close).  I have used a constant current power supply set at 3.0065A measured with Wavetek model 52 bench multimeter the last digit was not stable on that power supply so it can be in the 3.0060 to 3.0070 range the 3.0065 was sort of the average and where it was most of the time.

In any case the table below contains the results

On the left side you have the spec and on the right side the voltage reading in mV at the fixed 3.0065A current then calculated shunt resistance based on that result and then calculated error.

Even with my limited measurement accuracy and resolution all the Riedon and Murata shunts where in spec below the 0.25% error while the two shunts from China had an order of magnitude larger tolerance.

The 100A 75mV shunt was looking like calibrated as it had a cut but it is even more out of spec than the 200A 75mV one that had no calibration done and it makes sense that had no calibration as the value of resistance was already higher than spec so they can not calibrate by removing material as that will make things worse.

In any case even the low cost China shunt are usable if you are on a budget since you can easily calibrate the error out using the SBMS0 settings so you just compare the reading of the SBMS0 with a known good multimeter and adjust the resistance value until your multimeter measurement and SBMS0 measurement match. But there are other advantages to the Riedon and Murata shunts as they already have a insulated base and they are more compact. And a shunt will always have value even in 10 or 20 years shunts will likely not change as they are just a power resistor and so you can reuse tose for decades.     In fact the funny thing is that the 1000A 100mV shunt was manufactured in 2013 :) based on date on the box so not a very popular current shunt size.  Also while you can use the 1000 or even 1200A current shunt the SBMS0 will limit the max reading to 750A

Also max continues for all shunts is 66% of the rating as you can see in spec so if you have say a 600A continues load you will need the 1000A shunt.

 

To see the full size photo just right click on the photo and select open in new tab.

Not sure how clear it is but the bottom screws heads are deep and will not touch the base you install this on and there is a clear resin on top of them.

SBMS0 for size reference.

On the right side is the 10A 100mV murata very similar to the RSA series and below the 200A and 100A 75mV inexpensive shunts.

The bottom of the 100A 100mV Riedon and the 10A 100mV Murata looks like they are not manufactured by the same company fairly significant differences.

Material seems to also be different but form factor is very similar. 

[Randy Armour]

Dacian,

Thanks for posting this information. It is helpful to know Riedon shunts are verified as a good choice.  I purchased some from www.shunts.com.   A banner on the site indicates it is the official e-commerce site for Riedon shunts.  Pricing is almost identical to Digikey, althought I can't spreak to shipping charges outside the US. 

My order arrived in 2 days.  Well packed and included a small catalogue of their products.  

www.shunts.com is easy to navigate and limited to Riedon shunts and some nice class T fuse blocks (fuse included).  A good reference, if nothing more.

Randy    

[Dacian Todea]

Randy,

That is a good link. I did not remembered that one but I think you or someone else provided that link to me a few days ago.

Digikey has fairly fast shipping next day in US but for me in the middle of nowhere in Canada it takes a bit more thos they always ship in the same day I order or next day if is after hours.  

[Paul Barrette]

I got myself a pair of RSB-500-50 from Digikey at 39.00 CAD each. Good price for the unit ... good price from Digikey. Shipping was very quick... I live in southern Quebec. Very impressed with Digikey as an electronics provider.

https://www.digikey.ca/en/products/detail/RSB-500-50/696-1597-ND/4967074?utm_medium=email&utm_source=oce&utm_campaign=3298_OCE20RT&utm_content=productdetail_CA&utm_cid=1346132&so=64625461&mkt_tok=eyJpIjoiT1RnNVpqRTFPR1l3WWpaaSIsInQiOiJuZ1wvZFptVkw5Tjh1cUYrc1pWOCtGU0ZHNzZVeDNrNUx0S09vc2dPNTVWblhCVlNGdmJ1S3NyM1wvMGROTXUxUlBNU1BKdzJ0bmJxMjZIWFhJU0tjcDBKXC9PbjN1RlhxM1UzYkhcL1owXC9PUXBOQVZHMzNlOXdvaEpcL2xLSnorNXZENiJ9

Very impressed also by the construction of the unit. Reidon seems to build quality units

[Don Fukushima]

It seems Reidon and Murata current shunts are 50 or 100mV, not available at 75mV. and since you bought 100mV, is that the preferred spec?

[Dacian Todea]

Don,

Prefered depend on your application.

SBMS0 has a input range of 90mV that mean for example with a 100A 100mV shunt SBMS can measure up to 90A but since it uses the entire range measurement resolution will be very good.

An 100A 50mV shunt will have half the resolution in the same measurement range but also less heat dissipated on the shunt.

So say at 50A you will have 50A you will have 25mV drop on the 50mV shunt and 50mV drop on the 100mV shunt so 2.5W as heat on the 100mV shunt and half that 1.25W on the 50mV shunt.

So if you care most about accuracy you will go with the 100mV shunt and if you care more about heat (maybe cramped space and very high currents) then you may want to go with the 50mV shunt.

Of course a 200A 100mV shunt will have the same resistance as a 100A 50mV shunt but the 200A 100mV will be larger to deal with the extra heat that needs to be generated.

The 75mV where somewhere in the middle and is a very common value for those low cost shunts from China.

[Don Fukushima]

FYI - Your reply required more thought - Looking around the internet it seems 50mV rating is more common so that is what was selected, even though the "50mV shunt will have half the resolution in the same measurement range but also less heat dissipated on the shunt". 

Reducing heat is a consideration as the shunts will be in an enclosed electric breaker box. There was no indication of resolution for either value so adopted assumption that half resolution is sufficient for my use; 

[Don Fukushima]

Forgot to mention, Murata shunts were purchased from mouser.com as Digi-Key was short inventory.

[Dacian Todea]

Don,

A 200A 100mV current shunt will have the same resistance (0.5000mOhm) as a 100A 50mV shunt so they will have the same resolution.

The 200A 100mV will be larger so that it can handle the increase amount of heat that needs to be dissipated at the higher current.

The SBMS0 ADC resolution is 16bit that means plus minus 15bit that will be 32768

So for example with the 200A 100mV shunt the SBMS0 can read only up to 90mV thus 180A max and for that there are 32768 steps.

180A / 32768 = 5.5mA will be the smallest step so resolution in this example will be 5.5mA that is the smallest change in current that can be read.

[Plamen]

 Hi Dacian,

To be able to use 120A continues, the shunt must be  200A. The resistance for a 200A/75mV will be 0,3750 mOhm. I didn't understand how it applies in the above calculation if i want to calculate the smallest resolution for that shunt to compare with 200A/100mV. How You get those 90mV that SBMS0 can read?

  - The ADC resolution of 32768 is constant for SBMS0, is it the same for SBMS120 too?

[Dacian Todea]

Plamen,

With a 200A 75mV current shunt the SBMS0 is able to measure up to 240A   (90mV/75mV = 1.2 ;  200A x 1.2 =240A)

240A / 32768 = 7.3mA is the smallest change in current that can be measure with that current shunt.

[Plamen]

Thanks for Your patience and time spent here Dacian!

[Demosthien]

On Tuesday, 14 July 2020 09:33:16 UTC+1, Dacian Todea wrote:

With a 200A 75mV current shunt the SBMS0 is able to measure up to 240A   (90mV/75mV = 1.2 ;  200A x 1.2 =240A)

240A / 32768 = 7.3mA is the smallest change in current that can be measure with that current shunt.

So, in relation to the 500A-100mV shunt from your test batch.

90mV / 100mV = 0.9

500A x 0.9 = 450A

450A / 32768 = 0.0137A 

 That shunt+SBMS0 can measure a maximum of 450A to an accuracy of 0.0137A (13.7mA)?

Is that good or not so good?

[Dacian Todea]

All those shunts can be used to a max continues load of 66% of their rating meaning in this case 330A so if your continues load is below 330A then that is the best option shunt it is a lot below that then you can use a lower rated current shunt.

But yes if that is your requirement with peak up to around 450A (can be for a few seconds even a minute or two) then average for long term is below 330A that is a good choice.

[Wadzanai kanongovere]

Dacian and all

I plan to use 4 panels(180 W each,connected in parallel with 2 DSSR20) in a 12V system,what shunt size must I use for the PV side.

I plan to have a maximum continuous load of 250W,so I calculated a maximum current of 20Amps,so I calculated shunt size of 30Amps,but I have no idea how to calculate the PV shunt size also correct me if my battery shunt size is incorrect.

Regards all.

[Dacian Todea]

The PV panels can provide up to around 40A continues maybe 48A absolute peak so a 100A 100mV shunt will be good there and you can use the same value current shunt for load if you like symmetry but if your max load is 20A then a 30 or 50A current shunt can be used.  

[Casey & Gina]

Those Riedon shunts look nice! I was considering replacing my cheap Chinese shunts with ones from Blue Sea Systems (https://www.amazon.com/dp/B000MMC8K6) but the Riedon ones look to be very similar build quality for a significantly lower price.

I am currently using 250A shunts, but my 3000VA inverter recommends a 300A fuse, so once I install that I guess the shunt could fail before the fuse, though realistically I think the fuse could be smaller without issue, as even if pushed to peak capacity of 4800W it would only draw 200A... Regardless, I am considering adding another inverter and more DC loads in the future which would definitely require a bigger shunt. I was planning to switch the load shunt to 500A, but it would be better to jump to 750 or 1000A to be sure that the shunt isn’t too small, though it is extremely unlikely I’d ever actually hit even 66% of 500A.

In the SBMS0 manual you suggest that larger shunts will have less resolution, and most of the time, my load amperage is quite low. How much of a difference will this make when only drawing a couple amps? Does it make any difference to use a 50 vs. 75 vs. 100mV shunt? I would be interested in learning how to calibrate the shunts with a multimeter!

Random semi-related question: I noticed today that if I draw more than 100A from the battery bank, the color of the amp measurements on the SBMS0 for load and battery change to a different color. What does this indicate?

[Dacian Todea]

The 66% rating of the shunt is for continues use at that level it can of course work up to full rating for a minute or so (is just about heat) and even above rating for seconds.

You should not need more than a 250A shunt for a 3000VA inverter.  The smaller rating shunt will provide better resolution at small currents so you will not want to massively oversize the current shunt that you need.

See the discussion in this thread as it explains exactly the resolution for different current shunt's and how to calculate.

The color change at over 100A was an old future for the other SBMS models like SBMS120 and over 100A was high for those so decided to show with a different color. Not anything to worry about. 

  

[Casey & Gina]

The need for a bigger than 250A shunt is not for the single Multiplus 3000, but for two in parallel plus other DC loads.

I am thinking of just switching the Multiplus 3000 to a Quattro 5000 instead, which should still be plenty for my desire to run multiple AC appliances at once and has a second AC input which could be useful at some point - more importantly 1 Quattro 5000 would consume considerably less space than 2 Multiplus 3000s.

For the Quattro 5000, Victron recommends a 400A fuse, though the peak wattage is documented as 10,000W, and 10,000W / 24V = 417A, which is a little higher. I would be just fine with a 400A fuse I think. Let’s say I don’t expect more than 100A DC load - I still need a 500A shunt at least, right? More strange is that they recommend only a 500A fuse for a Quattro 8000, although its peak wattage would actually need 667A.

Let’s say DC load is ~50A (tank heaters in winter, refrigerator, lights, various electronics) and while having a cooktop burner on high (~75A) the A/C or heat cycles on (~50A) - at this point I don’t need more than a 250A shunt, and with a Quattro 5K wouldn’t be in overload like with the Multiplus 3K. But if DC load increases by 50A, or another AC appliance starts drawing another 50-75A, a 250A shunt would fail.

I will say that it is unlikely that I would exceed 250A most of the time, but I want the system to be robust enough to handle it. The Quattro can draw 167A without going into overload, and I am designing the system to allow up to 80A 24V load along with a 70A 12V converter. So 317A and occasional spikes higher via AC should ideally work without problem. A 500A shunt would alleviate concern - I think I would never hit 500A and 317A is 63% of 500A so it seems a good fit.

Whether I use a 250A or 500A shunt, can a fuse be used between the battery and shunt, to protect the shunt? I am thinking that it may be best to have something like this:

Battery
- 500A fuse - 500A shunt
- 400A fuse - Quattro 5K
- 150A fuse - DC loads (individually smaller fused)
- 250A fuse - 250A shunt - DSSR20s

[Dacian Todea]

Yes 1000VA is peak but for a very short period so a 400A fuse should be OK and yes a 500A shunt will probably be a good idea with a Quattro 5000 or two Multiplus 3000 and some DC loads. Peak power is for less than a second and fuses do not heat up that fast.

The shunt can work at spec current or even above for a few seconds but continues load like over 10 minutes should be below 66% of the rating so no more than 165A continues.

No the fuse can not be between battery+ and shunt the fuse needs to be after the shunt and if you want you can have them both in an electrical box so they are protected. You can have the PV shunt and the PV fuse in the same box as all this are at the same potential.

[Robert Muster]

I’m running 3 x Victron Phoenix 3000VA inverters in parallel using a 900ah LiFePO4 and an SMBS120. The system has been running my house (230V AC) coming up to 3 years at the beginning of Dec this year. I’ve got a main fuse after the shunt of 500amps and I’ve individually fused each inverter with 200amp fuses (possibly not necessary). There are no additional loads through the shunt and fuses as I am using the SBMS120 for other DC loads. The AC loads are rarely more than 30amps AC, well within the limits of the fuses and the 3 inverters.

I probably could have used 2 x 3000VA inverters but I erred on the side of having more capacity than I needed.

Bob
South Australia

[Casey & Gina]

Ok, I guess that fuses after the shunts should offer the same protection - the fuse should fail before the shunt or wiring to the battery. Can a 600A battery disconnect switch be used between the battery and shunts? I currently have 2 disconnect switches - one after each shunt, but it’s probably not valuable to have two separate switches.

I’m torn between using a 500A shunt and fuse, versus something smaller like a 300A which would impose a lower limit on everything, but would probably be all I actually need. I In practical terms, what does the difference in resolution look like between a 300A and 500A shunt? Does the mV rating make any difference?

[Dacian Todea]

Casey & Gina,

No you can not have anything that can disconnect the battery from the shunt including a disconnect switch. That should also be after the shunts and yes you either need two of them or one with dual separate contacts as you will not want load and chargers to be connected together without a battery since they can get damaged.

Yes the mV rating makes a difference and a 500A 100mV shunt will have the same resolution as a 250A 50mV shunt. But no matter what the resolution will likely be more than good enough as SBMS uses a very high resolution (16bit) ADC. 

[Julien]

If I well remember an old discussion, we cannot put a breaker or equivalent to disconnect battery to shunt but we can put a breaker on the negative battery side right?

[Casey & Gina]

A switch on the negative seems like a good solution. Is there any danger in having the positive connections live if the negative is disconnected?

[Dacian Todea]

Julien,

You will not want to fully disconnect the battery from the circuit no matter if is on the negative or positive side.  You will need to have a switch for Loads and a separate isolated switch for charge sources (the two separate switches can be connected to a single physical switch handle) but you will not want the charger's to feed the loads without the battery in the circuit as that can damage the loads or and chargers.

Casey & Gina,

It depends. In a vehicle the body of the car is connected to negative so if your LiFePO4 battery is also connected to body of the car the positive wire can still produce a current if shorted to the body of the car.

Also see above you need separate switch circuit for loads and charge sources and there is no advantage on having that on the negative side as opposed of positive side as shown in the SBMS0 user manual in the main diagram where there is a circuit breaker for loads and one for charge sources and so those can be used to disconnect the battery from the circuit. 

[Casey & Gina]

Thank you as always for the valuable advice, Dacian! I will just keep 2 disconnect switches as I have them set up already, increase my load shunt to 500A (ordered from DigiKey today), and get some fuses in place to protect things...

Have you noticed the cheap shunts “drifting” over time? I had emailed you once before when the SBMS was showing a -1A load when everything was off & disconnected. That was solved by unplugging the BMS ribbon cable and reconnecting, and everything was fine for a good while. But now I am seeing a 0.2-0.3A draw on the BMS even with everything disconnected. I could reset the BMS again, but wondered if this is something possibly caused by the shunt.

[Dacian Todea]

Shunt's can not drift over time not in the sense that the zero current point will move. Are you sure that everything is disconnected ? A photo of the first monitoring screen and one of the diagnostic screen when current through the shunt is zero may be helpful.

[Casey & Gina]

I am fairly sure - I had turned off the disconnect switch I have between the load shunt and everything else. But the input switch was not turned off so the DSSR20s and DEXT16 were still live. I intended to check better when I got back today, but the SBMS0 screen was frozen and the buttons did not respond.

This is a bit concerning - loads were still active, and although I had plenty of battery charge, I worry that this could happen and let loads drain the battery too low, or alternatively let the batteries overcharge (I am assuming that charging remained off when the BMS froze this time as the batteries would have been full since early this morning). I had it left on the first Monitoring page, as I usually do, but showing watts rather than amps as usual since yesterday.

If there’s anything I can do to provide diagnostic information, please let me know. I will keep an eye on it and let you know if it happens again.

Since restarting the BMS, the load is again showing 0 when it should. I powered on the BMS with both disconnect switches turned off, and when I turn them back on, the negligible load of my idle converter and a few LEDs don’t draw enough power to register - 0 is shown where I was seeing a fraction of an amp in the same conditions last night. Something seems to have caused it to read no load as a different value for a little while before the BMS froze.

[Dacian Todea]

Casey & Gina,

Is hard to say what you may have interpreted as a frozen screen. Like pushing the OK button in the monitoring screen will pull down a small menu and while in that menu the data in background will stop updating. I say this because of you being confused about the Watt instead of Amp readings as you can change that using that menu by pushing the OK button and selecting Current or Power. 

The main microcontroller can not freeze but is possible the screen communication to microcontroller got corrupted either due to a bad contact, humidity (water condensed on PCB) or a high magnetic field that induced voltages. In that case the LCD will retain the last information on screen (as screen has his own display memory) but the microcontroller will function normally in the background.

If all loads and all charge sources are connected correctly to SBMS0 then during power ON there will be no current through the shunts as SBMS will keep them OFF during power ON sequence.

Please test that all loads are turned OFF when you go to SBMS EXT IOx menu and set EXT IO3 as type 0 and same for all charge sources the charging should stop when EXT IO4 is set as type 0

[Casey & Gina]

Hi Dacian,

I am not confused about watt vs. amp readings, and intentionally changed it to show watts the night before it froze up and left it there. That was the only difference from usual, as I most often leave the Monitoring screen on the display.

The screen still showed the Monitoring page, with the battery at 99%. It was not updating like usual and pressing the buttons did nothing whatsoever. Humidity could not have been the cause - I am in a very dry desert area. Dust on the other hand could be a factor...

I forgot though - I do not currently have remote on/off for the loads connected to the BMS, as I still need to get an optoisolator connected so that the BMS can control my inverter, 12V converter, and BatteryProtect for 24V loads all together (I have an optoisolator, and will work on this soon). The battery was full though, so there would be no need to turn off loads - I just incorrectly worried that the screen being frozen meant the microcontroller wasn’t working either.

Thank you for the reassurance! I will let you know if the zero point appears to change again.

[Casey & Gina]

The "phantom load" showed up again this morning.  I have turned both battery disconnect switches off to be sure, prior to taking this picture.  There is definitely no current moving through the shunt, and it was reading 0 yesterday after I reset the SBMS.  Here are pictures of the requested screens.  The load amount is constantly fluctuating - after taking these pictures it is hovering around 0.2, whereas before it was hovering around 0.02.

[Dacian Todea]

Casey & Gina,

Where you unable to capture the phantom load in a photo ? In the photos you sent the readings are normal.

Especially if you can capture that while in the diagnostic screen will be very useful to diagnose.

[Casey & Gina]

Dacian,

My photo of the Monitoring screen shows a load of 0.027A and Batt at -0.037A.  Those should be zero, right?  The diagnostic screen is there too.  

I am not there now but can get more photos later.

[Dacian Todea]

Casey & Gina,

No the 0.027A and -0.037A is correct and represents the calculated self consumption of the SBMS0

[Refinished]

On Tuesday, July 14, 2020 at 5:48:11 AM UTC, Dacian Todea wrote:

So for example with the 200A 100mV shunt the SBMS0 can read only up to 90mV thus 180A max and for that there are 32768 steps.

180A / 32768 = 5.5mA will be the smallest step so resolution in this example will be 5.5mA that is the smallest change in current that can be read.

Dacian - Using the example given above, if I were to exceed the range of the SMBS0 on the high side on rare occasion and for short durations, what data will the SBMS0 use and log?  If I ran 205A thru the example shunt for 10 seconds, will the SBMS0 simply think that it's 180.22A and I introduce a tiny SOC inaccuracy or will something else happen?  Thank you.

[Marinepower]

I purchased a shunt recently that I thought I would share with the forum.  I was having a hard time finding a shunt that took 5/16 or 3/8 battery cable lugs that was less than 200amps.  My use case is 80 amps max charge/discharge.  All the shunts rated less than 200amps seem to take only 1/4 inch lugs, which is awkward for large battery cables.

I found this alternate series from Riedon.   The 150 amp version (RSN - 150 - 100B)  seems like the better right choice in the my application.   https://riedon.com/media/pdf/RSN.pdf.    Cad $ 55 from Digitkey.

Disclaimer, I have not received it yet so cant comment on the quality or fit for the SBMS0.  But the specs seem to match.

MP

[Dacian Todea]

There is not a problem using a 200A current shunt for max 80A charge discharge rate. My own system has a 200A 75mV current shunt and my max discharge rate is below 80A

There will not be much of a difference between a 150A and a 200A shunt in terms of measurement resolution.

[Marinepower]

Thxs Dacian,

my typical loads are 0.7 amps to 3.5 amps, with a once a day,  short duration 80 amp load and charging current typically about 70amps.  I was unsure how sensitive the SBMS was was in detecting the shunt voltage drop, so was trying to keep as low as possible shunt value to capture the small loads. 

I'm sure i'm just over-thinking this.

MP

[Dacian Todea]

That is also my typical Load with around 4 to 5A during the day and around 1A at night and the measurement resolution is more than sufficient with the 200A 75mV shunt I have. 

[Troy Gordon]

Thanks for posting this.  My project is using a decommissioned hybrid bus battery.  Thought about just taking the packs when purchased, but have since found several useful components in there such as a shunt!!  After reading this thread realize it will work no problem, just may not be as accurate as possible for the small load placed upon it.

[Dacian Todea]

Yes any shunt will work all you need to know is the shunt resistance to set up in the SBMS menu.  Usually the current rating and associated voltage drop is written on the shunt and from those you can calculate the shunt resistance. 

[Casey & Gina]

Is it even required, technically, to use shunts at all?  As I understand it, high and low cutoffs are based on voltage - the shunts just provide useful monitoring.  But in theory, if one didn't care about that, would it be possible to use an SBMS setup without any shunts?

[Dacian Todea]

Battery will still be protected even without shunts but there will be no cell balancing as that only happens when SBMS0 sees a minimum of 300mA of battery charge current and also the SOC is not calculated thus you do not know the state of charge of your battery. I will SOC is super important info and so you should have at least the battery shunt to get this and cell balancing.

You can get a cheap current shunt for just 10 or $15 and if you also use the PV shunt you get extra info like PV energy production and Load consumption else with just one shunt you only have the battery current so just the SOC calculation.

[Electrons4me]

@Dacian, Thanks for pointing me to this shunt post, very informative!

Cheers

E4M

[Allan Yates]

Found this article when searching for thoughts on AtoD precision for shunts. For those looking for shunts, especially in Canada, I have been sourcing them from Canadian Shunt Industries (http://cshunt.com). No connection other than a satisfied. I don't have any details on the precision of their shunts. I use their 200A/100mV instead of the supplied 100A/50mV shunts supplied with Blue Sea meters, so to get a high full duty cycle rating.

Allan.

On Monday, June 22, 2020 at 4:08:45 PM UTC-4 electr...@gmail.com wrote:

I get quite a few question related to current shunts so I decided to make this post.

SBMS0 supports any shunt between 0.0400mOhm and 9.9999mOhm that means all shunts in the Riedon RS range https://riedon.com/media/pdf/RS.pdf  except for the smallest 5A one but including 1200A.

As I order parts for SBMS0 production I also ordered a few of this Reidon shunts one from each category the RSA smallest ones up to 150A so I got the 100A 100mV one then one from the midd size RSB and got the 500A 100mV one and finally the huge RSC 1000A 100mA.

Interestingly the 500A 100mV was the only one coming with a certificate of calibration showing the exact current shunt value of 0.200045mOhm so that will round out to exact 0.2000mOhm in the SBMS settings.

I got my current shunts from Digikey that should be available in almost any country   https://www.digikey.ca/  But Riedon also has his own website and I think you may be able to buy directly from them https://riedon.com/how-to-buy/

Murata also has a similar line of current shunts and I have the small 10A 100mV you can see in one of the photos below.

I attempted to measure the current shunt resistance but I do not have precision enough power supply and multimeter to get to the level of precision required to check their calibration (not even close).  I have used a constant current power supply set at 3.0065A measured with Wavetek model 52 bench multimeter the last digit was not stable on that power supply so it can be in the 3.0060 to 3.0070 range the 3.0065 was sort of the average and where it was most of the time.

In any case the table below contains the results

On the left side you have the spec and on the right side the voltage reading in mV at the fixed 3.0065A current then calculated shunt resistance based on that result and then calculated error.

Even with my limited measurement accuracy and resolution all the Riedon and Murata shunts where in spec below the 0.25% error while the two shunts from China had an order of magnitude larger tolerance.

The 100A 75mV shunt was looking like calibrated as it had a cut but it is even more out of spec than the 200A 75mV one that had no calibration done and it makes sense that had no calibration as the value of resistance was already higher than spec so they can not calibrate by removing material as that will make things worse.

In any case even the low cost China shunt are usable if you are on a budget since you can easily calibrate the error out using the SBMS0 settings so you just compare the reading of the SBMS0 with a known good multimeter and adjust the resistance value until your multimeter measurement and SBMS0 measurement match. But there are other advantages to the Riedon and Murata shunts as they already have a insulated base and they are more compact. And a shunt will always have value even in 10 or 20 years shunts will likely not change as they are just a power resistor and so you can reuse tose for decades.     In fact the funny thing is that the 1000A 100mV shunt was manufactured in 2013 :) based on date on the box so not a very popular current shunt size.  Also while you can use the 1000 or even 1200A current shunt the SBMS0 will limit the max reading to 750A

Also max continues for all shunts is 66% of the rating as you can see in spec so if you have say a 600A continues load you will need the 1000A shunt.

 

To see the full size photo just right click on the photo and select open in new tab.

Not sure how clear it is but the bottom screws heads are deep and will not touch the base you install this on and there is a clear resin on top of them.

SBMS0 for size reference.

On the right side is the 10A 100mV murata very similar to the RSA series and below the 200A and 100A 75mV inexpensive shunts.

The bottom of the 100A 100mV Riedon and the 10A 100mV Murata looks like they are not manufactured by the same company fairly significant differences.

Material seems to also be different but form factor is very similar.