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Topic: how do i interface a split-core current transformer? (Read 21430 times) previous topic - next topic

travis_farmer

ok, THIS is what I am working with for a current sensor. or will be when they arrive from china.
I have never used one of these, so I have no idea how to interface it to an Arduino analog in.

The project is a DCC++ base station, and I am measuring the current going to the track. I just need help finding a circuit to interface it, and what formula in needed on the Arduino to get the amps being drawn.

Thanks in advance for whomever solves my mind from drawing a blank. :D

~Travis

jackrae

Here you are :-
http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface
Good luck

jackrae


dwightthinker

I'm not sure this is what you want.
Most trains run on DC, not AC.
Split core transformers only work with AC unless a
hall effect sensor is used rather than a transformer winding.
Even then using cores and Hall effect devices can
be sensitive to position relative to the earth magnetic
field.
There are inline DC current sensors, with isolation, but I
 don't recall what there name was.
Dwight

jackrae

Travis
As Dwight states, you cannot use a conventional CT for DC sensing.  I failed to fully read what you were endeavouring to do.  A hall sensor with a 5 volt output is really what you require.  Whilst these can be influenced by residual magnetic fields they are probably better than using a low value shunt resistor and associated amplifier.  Something like one of these will interface directly

jackrae

So, if I understand correctly, you have an AC power line with digital "noise" on it.  And I presume it's the AC power you wish to meter.  If that is the case then your CT will probably work but you will have to put a filter on the arduino input to eliminate the "noise" that will appear on the arduino input.

Have you looked at thislink which details a ct connected to an arduino

jackrae

One of the problems you might come up against is the high frequency of the "noise" being too high for the magnetic material of the transformer core.  If that is the case then it may also be too high for hall sensors as well.   Have you any idea what the signal frequency is ?  You might have to end up going for a shunt resistor, which is much less sensitive to frequency, then condition the low voltage (millivolts) from it so's it's readable by the arduino.

jackrae

At only 7Hz, the ct won't operate but both a hall sensor and a shunt resistor will
If you go for one of these hall sensors then you get both isolation and virtually no line loss.  This unit is limitted to 5A but there are plenty of higher current versions available

If however you want to use a shunt resistor, its value depends very much on current drawn by the track equipment.
I would imagine that you would want to limit shunt volts to no more than say 5% of system volts since this is voltage "lost" to the track equipment.
By way of example, if system drive voltage is say 12 volts and the track equipment draws say 5 amps then 5% of 12 volts equals 12 x 0.05 = 60mV,  and at 5 amps the required resistor would be 60mV/5 or 12mOhms. (That is milliohms)  Power disipation will be V x I = 0.060 x 5 = 0.3Watts
As you see, the signal voltage is quite low, certainly too low to give a meaningful input to an arduino, so the next requirement is an amplifier.

jackrae

Travis,

I think it's time to take a big step backwards and ask what exactly it is that you're trying to do.
When I thought about things after my last post where you said the signal frequency was 7Hz, little alarm bells started to ring so I did some googling on DCC signal protocols.  It seems the system frequecy is as high as 250kHz.  Might I suggest you have a read of this document before going any further.  It is quite illuminating.

MarkT

Sorry, I forgot to mention. DCC++ is a digital signal imposed onto AC track power.

Thank you for pointing that out though.

~Travis
What carrier frequency?  You'll need a ferrite core that handles the frequency in question,
standard CT's are good for low frequency, but the grade of ferrite might not have high
frequency response since it doesn't have to.  There are many grades of ferrite.

Are there any docs for DCC+?

[ ah - missed the last post.  Hmm, not very well documented alas.  They do give circuits. Note that
CTs are used for large currents, not 30mA as seems to be the signal here.  Can't really figure out
the modulation frequency from that document, I'm suspecting its quite low though ]
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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