Re: Magnetic coupling and current transducer

I don't think you want a current transformer if you are looking at DC.
No?

regards,
Bob
N9NEO

Ignoramus15879 wrote:
On 16 Jun 2006 07:16:52 -0700, Yzordderrex <yzordderrex@xxxxxxxxxxx> wrote:
Lem sensors are in most cases good for 3X the rated current.
Well...Sorta... That is usually a peak requirement before the amplifier
saturates. I do not think they will handle 3X continuous. If you got
a few of the 50amp parts then you can put them in parallel and sum the
output voltages.

You can make a shunt yourself some kind of material that isn't so
dependant on temperature. Like copper is near 0.4%/c. I think maybe
they make em out of brass, but please don't quote me there. If you
need isolation use an Agilent serial optocoupler solution - I think
It's Avago now - They're getting to be as bad as Mot with the name
changes.

Another potential solution for you would be to use an Allegro
Microsystems part. In some cases I will mount a hall effect sensor
next to some current carrying tracks on a pc board and no need for a
flux concentrator like a gapped toroid or tape wound. I met with
allegro last week and they have some very interesting parts they call
LEM killers.

I can poke around my toybox or intercept a used one on the way to the
scrap pile. Your cost would be shipping. Unfortunatly a very good
friend of mine just recently left Lem, but I'm sure I can hook you up
if time isn't a concern. I am interested in your welder cause I play
around with welding too. I am strictly buzz box now, but I have a
giant diode bridge and some inductors kicking around. I design large
inverters for a 9-5 so maybe one day I will build a slick welder like
yours.

Thanks Bob.

I could not find any high enough rated current transformers at
acceptable prices, hence my interest in current shunts. Summing
outputs is kind of a pain, since they are referenced to ground (middle
point between + and - supply leads, I suppose).

If you have a CT that is capable enough -- if that is what you
referred to when you mentioned your scrap pile -- I would be most
interested.

Thanks!

igor

Sorry if I repeated the thread at all. I read it with only one eye
open.

regards,
Bob
N9NEO



Ignoramus19197 wrote:
On Thu, 15 Jun 2006 16:03:18 GMT, Rich Grise <richgrise@xxxxxxxxxxx> wrote:
On Thu, 15 Jun 2006 07:59:35 -0700, Jim Thompson wrote:
On Thu, 15 Jun 2006 11:03:33 -0400, John Popelish <jpopelish@xxxxxxxx>
Ignoramus19197 wrote:
On Thu, 15 Jun 2006 10:16:11 -0400, John Popelish <jpopelish@xxxxxxxx> wrote:

Here is a little algebra problem for you:
How much change in resistance of the 300 amp shunt (and its end
connections) must there be to alter the current through the 50 amp
path by 5 amps (10% of full scale) when there is 300 amps total
passing through the parallel pair of shunts? I haven't done this,
yet, but my gut tells me that it is a very small resistance change.
You should be thinking in numbers.

Suppose that we have two resistors, R1 and R2, and current I.

Then the individual currents will be

I1 = I*R2/(R1+R2)
I2 = I*R1/(R1+R2)

Right? If R1 or R2 changes by a small percentage, then I1 and I2 would
change by a smaller percentage.

Right. But these two resistors have such low resistance, that I am
concerned that the bolted connections will not be small percentages of
their resistance, over time and thermal cycles.

A 50 mV 50 A shunt has a resistance of 1 mohm.
a 50 mV 300 A shunt has a resistance of 170 uohm.

Are you confident that your bolted connections will be (and remain) a
small percentage of 170 uohm?

That's why one uses Kelvin connections.


He's talking about the connections for the main current, which is split
and goes to two shunts in parallel. Where those conductors connect to
the shunts themselves could affect the division ratio.

|
| 300A
|
o----o----o
| |
~40A [S1] [S2] ~260A
| |
o----o----o
|
etc.

Each of the circles is subject to corrosion, thermal effects, vibration,
and all that happy stuff. :-)

Yeppers.

i


.

Relevant Pages

  • Re: Magnetic coupling and current transducer
    ... You can make a shunt yourself some kind of material that isn't so ... How much change in resistance of the 300 amp shunt (and its end ... connections) must there be to alter the current through the 50 amp ... but my gut tells me that it is a very small resistance change. ...
    (sci.electronics.design)
  • Re: Magnetic coupling and current transducer
    ... You can make a shunt yourself some kind of material that isn't so ... How much change in resistance of the 300 amp shunt (and its end ... connections) must there be to alter the current through the 50 amp ... but my gut tells me that it is a very small resistance change. ...
    (sci.electronics.design)
  • Re: Magnetic coupling and current transducer
    ... they use Hall effect ... You can make a shunt yourself some kind of material that isn't so ... connections) must there be to alter the current through the 50 amp ... but my gut tells me that it is a very small resistance change. ...
    (sci.electronics.design)
  • Re: Magnetic coupling and current transducer
    ... I meant a 300amp LEM ... You can make a shunt yourself some kind of material that isn't so ... connections) must there be to alter the current through the 50 amp ... but my gut tells me that it is a very small resistance change. ...
    (sci.electronics.design)
  • Re: Current measurement
    ... troubleshooting I would say the roll your own shunt and a cheap meter ... A copper shunt looks like a good idea until you look at copper's ... used to make inexpensive RTDs (resistance temperature devices). ... I keep a 100 amp and a 500 amp shunt in my tool box all the time. ...
    (rec.outdoors.rv-travel)