LEM flux-gate current transducer power supply consideration - Page 1
Forum section: electronics / projects

author: dl1640
author_rank: Regular Contributor
timestamp: October 15, 2017, 05:39:55 am
content: i own a LEM flux-gate current transducer, is a very accurate one on the market.this one requires bi-polar power supply +&-15V, to get most accurate measurement result it is suggested that the +15V and -15V shall start at the same time to reduce the risk of magnetization. maybe also need to drop off the supplies at the same time?here comes 2 questions first:1. i need to pick up a switching mode power supply which is small size and can be embedded into a test box, but is a general one will start +15V and -15V at the same time? i have no idea of that and maybe an external circuit is needed to ensure that +15V and -15V is sychronous?2. if it will be a power fail and the current transducer is unfortunately magnetised how do i recover it by de-magnetise? the tranducer itself seems no option to de-magnetise automatically.thanks,

author: jbb
author_rank: Super Contributor
timestamp: October 15, 2017, 07:03:39 am
content: 1. Most off the shelf power supplies don't clearly specify the order in which the outputs come up.If you get a standard mains to DC supply, that will likely be a flyback type, and both outputs will likely come up together. If you're uncertain, check it with an oscilloscope.You may want to use, say, a +- 18V switching supply, followed by +-15V linear regulators to reduce noise. Given you probably forked out a lot for the transducer, consider the power supply an investment to get the most out of the transducer.2. Resetting any magnetisation is called degaussing. I suspect LEM will have some application notes on it.EDIT: a lot of precision equipment (e.g. power meters) include a NULL function which can digitally zero out small offsets for you. This could be very effective if you've only got a wee offset. Large offsets will need degaussing.

author: Echo88
author_rank: Frequent Contributor
timestamp: October 15, 2017, 02:39:13 pm
content: If they need to start up together one could also use a double pull single throw relay, which would be energized by the supply with a small time delay (via a RC-combo which powers the relay-coil) and switch both supply rails simultaneously after a little while.

author: SeanB
author_rank: Super Contributor
timestamp: October 15, 2017, 03:17:02 pm
content: Power the relay from the incoming 15V rails as well, using a 12V relay and an 18V 1W ( or something higher if the relay coil is low resistance) zener diode, so that you need at least 26V present on the power rails before the relay energises, assuming typical 8V pull in on the relay coil.. That way no RC network, and guaranteed not to operate if one rail is missing. Place the usual reverse biased diode across the relay coil as well, and make sure there is enough capacitance on the output of the power supply, along with a series resistor ( around 4R7 to 22R) in the 15V rails to the LEM transducer, and some 10-100uF 35V capacitors for local decoupling as well after resistors. R and C is there to limit rate of rise of the rail, and to reduce arcing on the relay contacts. Resistance value depends on current draw, just there to keep drop under roughly a quarter volt at full power draw of the sensor.

author: dl1640
author_rank: Regular Contributor
timestamp: October 16, 2017, 01:24:13 am
content: just checked my benchtop linear power supply which is the only one could supply +-15V i have now.looks they start up at the same time but with a little bit delay between each other, say about 50ms.to use a mechenical relay is one of my option and i worry about its switch on/off glitches.i need to get one SMPS to verify it first

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2017, 06:31:12 am
content: What LEM sensor you have? The magnetization is only a problem with pulsed DC overloads iirc, not with wrong supply voltage.Or having it near permanent magnets or iron core transformers.

author: dl1640
author_rank: Regular Contributor
timestamp: October 16, 2017, 07:04:09 am
content: model is IT200-S, it can measure both dc and ac current.i was told by another engineer that please keep in mind the supply voltage need come up at the same time,otherwise the ct might be magnetised and an offset can be observed on the secondary side while there is nothing connected on the primary side (primary open). also it is suggested that the primary conductor should be fixed to a position so that is does not move around in the ct hole to get repeatable measurement result. this is to archieve the best measurement uncertainty.however i do not simulate that situation (wrong power supply) in real life, i use a linear supply and so far so good.but to place the power supply and the ct into a single box, i need a good SMPS...or circuit.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2017, 08:02:13 am
content: I wouldn't worry about a few milliseconds of delay. A few hundred, perhaps.Datasheet also doesn't mention specific supply slopes.But, could you use a pair of mosfets to switch the unit on based a supply voltage monitor?

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: October 16, 2017, 09:57:29 am
content: "i was told by another engineer that please keep in mind the supply voltage need come up at the same time"well... then why not just ask that particular engineer how he manages that in his designs?

author: dl1640
author_rank: Regular Contributor
timestamp: October 16, 2017, 12:13:14 pm
content: his company has similar application but he is not the one design the system, that exact schematic is confidential i think.so far the secondary output current is way below 1uA (full scale 200mA) while no primary input current, good news

author: dl1640
author_rank: Regular Contributor
timestamp: October 16, 2017, 12:17:28 pm
content: just i know the ct would be magnetised if primary current applied before power supply switch on.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2017, 01:35:00 pm
content: Quote from: dl1640 on October 16, 2017, 12:17:28 pmDC current or static magnetic fields are a problem.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 16, 2017, 09:10:14 pm
content: Quote from: Jeroen3 on October 16, 2017, 01:35:00 pmNot according to the data sheet.http://www.lem.com/docs/products/hal_50_600-s.pdf

author: dl1640
author_rank: Regular Contributor
timestamp: October 17, 2017, 12:27:31 am
content: both hall type and flux-gate can measure dc current, i would not worry about overload because target is below 100A peak and full range of ct is 200A peak, i only measure dc of sinewave.

author: dl1640
author_rank: Regular Contributor
timestamp: December 10, 2017, 02:09:20 am
content: Thanks all your opinions, especially SeanB's.I may have time to look into this project soon,But before my hands get dirty I am thinking to simulate such circuit on PC,I am not familiar with such simulation software to start this small project,I'm happy that if anyone would suggest a proper software, to see how the circuit would work.