Calibrating a 100A DC shunt

[enut11]

OK, for DC the PS must have some very impressive rectifiers. Can you tell me more about your power supply.

[beanflying]

You might get some tips out of the Valhalla Calibrator manual. While it is only 10A max it only uses 1-2V in the test path. Circuit diagrams are at the last few pages. Its bigger brother might have a similar manual out there but I have never looked.

[enut11]

The ones I saw were 12v. How do you get the voltage down to a few volts? Is this an internal PS mod?

[joeqsmith]

The ones I saw were 12v. How do you get the voltage down to a few volts? Is this an internal PS mod?

No mods.  They are programmable.   If you wanted 12v for example, the Cotek AEK-3000-LV-12  can supply up to roughly 200A.  Have a look: 

https://www.amazon.com/AEK-3000-12-Switching-Supply-Programmable-Single/dp/B00YO4ZFSA

Like most supplies you can run CC or CV.  All programmed over the serial bus.  Software interface for this supply is trivial.  This would be a much better setup than what I have with my old stacked supplies. 

[enut11]

@joeqsmith, love the exercise bars on the BIG one.

Anyway, very high current supplies are way out of my hobby budget but it is interesting to see what is out there.
Thanks
enut11

[enut11]

Still working on my setup for the 20A tests. At 6v my programmable 300w load was still overheating as only a quarter of the elements were on. Now testing to see if 3v compliance is enough.

[enut11]

Been working on a stable low voltage high current supply and I think I have it.
https://www.eevblog.com/forum/projects/gigabyte-psu-as-a-high-current-source/

Next step is to improve the cooling for the 300W load resistor. With a 5v PC supply the resistor will have to handle about 100W which is a lot less than when I used a 12v supply.

[enut11]

OK, for DC the PS must have some very impressive rectifiers. Can you tell me more about your power supply.

It's just a couple of OTS supplies in parallel with a LEM sensor.  A small embedded computer controls the current/voltage and talks to the PC which is running a simple LabView program to automate it.

If you want to buy something, I would start with posting your requirements.  If you are just wanting to know which brands to look at, I suggest Cotek.  They offer some with a PC interface. 

I worked for a company where we had a few large Sorenson  power supplies.  Large meaning four eyehooks on the corners to hook the chains to in order to lift it.  I would run these from a PC as well.  I looked on eBay to see if I could find one.  No luck but maybe something like this:

https://www.ebay.com/itm/303119807724

Just depends what you are after.

@joeqsmith, how does the LEM current sensor work. Is it stand alone into a voltmeter or does it need interface electronics?

[Berni]

The LEM sensors mimic a current transformer, except that it works down to DC.

So what you get out of it is a smaller current that is related to the measured current by an exact factor like say 1:500 (depends on the model). So the only thing is does is extend the measurement range of your existing current measurement equipment like a bench DMM.

Tho since it is an active device it does need power. These things run from split rail +/- 15V and needs a fair bit of current, so it might make sense to build a power supply box for them.

[joeqsmith]

OK, for DC the PS must have some very impressive rectifiers. Can you tell me more about your power supply.

It's just a couple of OTS supplies in parallel with a LEM sensor.  A small embedded computer controls the current/voltage and talks to the PC which is running a simple LabView program to automate it.

If you want to buy something, I would start with posting your requirements.  If you are just wanting to know which brands to look at, I suggest Cotek.  They offer some with a PC interface. 

I worked for a company where we had a few large Sorenson  power supplies.  Large meaning four eyehooks on the corners to hook the chains to in order to lift it.  I would run these from a PC as well.  I looked on eBay to see if I could find one.  No luck but maybe something like this:

https://www.ebay.com/itm/303119807724

Just depends what you are after.

@joeqsmith, how does the LEM current sensor work. Is it stand alone into a voltmeter or does it need interface electronics?

I have a friend who designed sensors for them.   I used a similar nulling technique to design a totally different type of sensor.  Basically a hall type sensor is used to measure the flux in a gap.  Current is controlled to null the field so the hall always works in the same region.  Improves temperature stability.   The nulling current is then proportional to the signal we are measuring.   The Have a look at LEM's closed loop description
 
https://www.lem.com/en/hall-effect-current-sensors

[enut11]

I see the LEM sensors come in a variety of packages and up to several hundred dollars. As with everything, there is a cost/benefit choice. What do you get for the extra money, linearity, accuracy, range? Which one would suit a 0-50A setup if aiming for a 0.1% measurement?

[joeqsmith]

I see the LEM sensors come in a variety of packages and up to several hundred dollars. As with everything, there is a cost/benefit choice. What do you get for the extra money, linearity, accuracy, range? Which one would suit a 0-50A setup if aiming for a 0.1% measurement?

Assuming you didn't mean 1.0% or even 10%, good luck.  I think that one I use is +/-1% but that's just the sensor, not the system.   

Someone did post that 0.05% shunt early on.  You could try finding something that wasn't traceable and guess.  I don't see the point.   Personally if I had a need,  I would send off the parts for proper cal with a report.  Far cheaper and traceable.   I would have about as much confidence in something I constructed with OTS parts as I would with the guys buying these cheap standards to align their DMMs.   I would need to send off my custom calibrator to have calibrated.  Even then.... 

[Kleinstein]

A main difference between the good one and the cheap ones is the linearity and accuracy. To a small part also the range, when going really high, like > 200A.  The cheap ones are mostly hall effect based and thus mainly a thing for relatively high currents. They are also often direct reading and not using compensation. This limits the linearity and stability but also means the get away with low power.

The Ultrastab ones use the fluxgate principle and are much more accurate and linear, but usually also more expensice and they need significant more power. If one is lucky one may get a used one for a relatively good price, but the new ones are expensive. As kind of current-transformer the ratio is very close to an exact integer set by the turns ratio. So very little drift and accuracy problem.

At least in Germany there are 2 types from VAC offered very cheap ( < $2) that also use the flux gate principle, but for a smaller range (e.g. 16 A RMS, 50 A peak) and limited accuracy of 0.7%. The more normal price for these is likely in the $10-$20 range. I have some and looking at the internals I was a bit disappointed. Still impressive what the reach with the limited power and simple contruction. As a closed system they can be a bit more sensitive than most DC clamp meters.

[joeqsmith]

For $1500 USD maybe the FNPZR0100A,  0.01 ohm, 0.05%, 500W, 1ppm/degC.   Should be able to keep the temperature fairly stable with only 25W.   At a half volt,  signal may be high enough to directly read it.   Maybe then send it off for calibration and use as a standard?   Of course, that doesn't cover the cost of the power supply.... 

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/760/FNP_Series.pdf

[Berni]

You can easily find the LEM Untrastab flux gate transformers on ebay for under 50 bucks:
https://www.ebay.com/itm/235153418291

The main benefit of them being that the conversion ratio is exactly und exactly the turns ratio. So even if you buy one of these used in unknown condition you don't have to worry about it being out of spec. It is either going to be broken and give no sensible output (or give output that is so wrong it is easy to tell) or it will work fine and be accurate. You won't suddenly have one of the turns of wire fall off the core and change your ratio from 1:1750 to 1:1751

This is also why they are so stable during operation since warming up a transformer also won't change the turns ratio (tho the flux null detector can, but it is specifically designed to minimize it). Since the entire thing operates using currents and not voltages, this means there is no thermocouple voltages to worry about. If you ever tried to measure things in the low microvolts you will see just how difficult it is to keep things stable when your own body heat creates stray voltages all over the place.

[joeqsmith]

You can easily find the LEM Untrastab flux gate transformers on ebay for under 50 bucks:
...

I've never used one but thought I would have a quick look.  OP was asking for 50A.  Maybe something like the it-60-s would be a good fit.  First thing I notice, it's discontinued according to Digikey.  Looking at the datasheet, there is nothing about the accuracy. 

https://www.lem.com/sites/default/files/products_datasheets/it_60-s_ultrastab.pdf

Their website makes a few claims but there's no meat on the bones.   
https://www.lem.com/en/product-list/it-60s-ultrastab

[Berni]

Sure there is.

Linearity error is 3 ppm max and temp co. is 2.5 ppm/K max. Initial offset is 250 ppm max and it might drift by 2.5 ppm/month.

Sure they tend to measure a lot more than 50A, but they are so stable that they are perfectly usable even down into miliamps.

What other accuracy stats you need to know?

[PartialDischarge]

  Looking at the datasheet, there is nothing about the accuracy. 

I believe the original manufacturer is Danfisyk not LEM and their specs are strange too.
The offset is given in ppm and only linearity not accuracy is given.

https://xdevs.com/doc/LEM/doc/867-700I_Installation_Package.pdf

[alm]

I guess absolute accuracy depends on the value of the burden resistor (and stability / tempco, obviously) and needs to be calibrated for the specific system. See the other threads on DCCTs and TiN's article on xdevs.com that were probably linked before in this thread or one of the other recent high current threads.

[JohnG]

For practical purposes, these behave as current sensing transformers that work down to DC. They have a fixed number of turns and a closed-loop feedback from the flux-gate sensor. They drive a current through the turns to cancel the flux from the conductor being sensed, and this current is run through an external burden resistor. The sensed current is reduced by the  turns ratio, which is an exact integer. The ppm and linearity readings refer to the current conversion ratio of the sensor.

IIRC, the flux gate sensor is quite good at determining the zero flux point with low offset, and the turns ratio is pretty close to an exact integer. This leaves the burden resistor as the key component determining the accuracy. However, the burden resistor is measuring a current reduced by the turns ratio, and is no longer in the main current path. Thus, you can use a much larger resistor value than you would for a shunt, making it much easier to get a high accuracy resistor, and giving a much larger sense signal than would be practical with a standard shunt due to the need to avoid dissipating too much power in the shunt.

Normally, these sensors are quite expensive in the $1k to $3k range. I purchased two brand new ones on ebay for $100 each, the ITN 600-S: https://www.lem.com/sites/default/files/products_datasheets/itn_600-s_ultrastab.pdf

These are 600 A, with a turns ratio of 1500. If you have the luxury of being able to wrap multiple turns through the sensor, I recommend getting the higher current ones. The initial offset is less, perhaps due to reduced sensitivity to the Earth's magnetic field, and if you need more sensitivity to lower currents, you can wrap multiple turns through the sensor. For example, with ten turns, the max effective range becomes 60 A.

I use these in a high accuracy efficiency measurement system for power converters. One of the great things about this system is that I can run a single current through an extra winding that I wrap through both current sensors and use a single current run simultaneously through both sensors to account for differences in the two sensors, thereby eliminating a major error source (efficiency is a ratio-metric measurement). The working windings used can then be optimized to take advantage of the full range of the sensor. For example, in a 2.5 kW, 48-12 V converter measurement, I can put 8 turns on the input sensor and 2 on the output sensor and use most of the sensor range.

You can also find some older ones sold under Danphysik. They are good as well, but the newer ones have some protection against user error, like sending a large current through the sensor when it is not powered.

Another company that makes these is Danisense. Here are some prices for new ones: https://gmw.com/product/ds/#productPricing.

Hope you find this useful,
John

[enut11]

Thanks @JohnG, @alm, @Berni, @joeqsmith and @Kleinstein
The magnetic current sensors are a fascinating technology that I can explore at a later date. Meanwhile, I will persist with improvements to my present setup to see what can be achieved.

The load resistors consist of 4 windings, a pair of 0.3R + 0.67R. Various series/parallel combinations are possible by changing links. I have improved the cooling and there are now 4 fans, 2 through the tubes and 2 overhead.

For the power supply, I am using a modified PC PSU. The nominal 5v output settles on ~4.6v at 20A but is very stable under load.
https://www.eevblog.com/forum/projects/gigabyte-psu-as-a-high-current-source/msg5164392/#new (see Reply #8).

Running the load resistors at ~4v/20A (instead of 12v previously) means they are much more stable for the few minutes of the test.

Picture of the new test setup below incorporates suggestions from Forum members, shunts on edge for better cooling and improved sensor wire connections to the logging DMMs.

As before, I will be comparing the voltage from a DUT to a calibrated unit. For these tests the Cal unit will be a 100A 0.25% Murata (small black). The large black unit was also calibrated (2002). The white unit is the shunt I purchased from Bulgaria and the one I need to be characterised as best I can.

I have a further 2 instruments, one monitoring the stability of the power supply and a clampmeter monitoring the test current.

[JohnG]

Make sure you don't melt the fans stick in the end of what looks like copper tubes.

John

[enut11]

Hi @JohnG. The fans blow fresh air into the tubes and work well.

Setting up for the first test with the new setup. Decided to duplicate results from Reply #1. DUT is 100A from Bulgaria and Ref is the 100A (MP, 2002 Cal).
enut11

[alm]

I don't remember if this was discussed already, but you could build up to 100 A. Say you had a DMM that you trust to 10A. Take x identical shunts, put them in series with DMM, and measure voltage drop at 10A. Then put them all in parallel, if necessary compensate differences between shunts with a series resistor, and send X*10A through it. Measure voltage across each shunt add and up for total current. Use this to calibrate one or more X*10A shunts or DCCT. Repeat until you're at 100A.

[enut11]

@alm, it has some merit but outside the capabilities of my equipment at this time.
enut11