Current transformers and burden resistors

[RocketScienceSmurf]

I have a need for limiting a large three phase AC load (electric boiler with 7 heating elements of 6kW each) and I was thinking about using three current transformers and an Arduino. Since the transformers give AC on the secondary I need to make some kind of signal conditioning before I connect to the Arduino's A/D inputs.

I have made some tests with a small CT, a schottky bridge rectifier and a burden resistor on the DC side of the rectifier (a tip I found here: https://electronics.stackexchange.com/questions/281048/current-transformer-and-bridge-rectifier-why-the-diode-drop ) and it seem to work very well for what I am trying to achieve. In my application I need to send a signal (logic level) if the measured current goes above a certain level and precision and linearity are not crucial.

But what about the burden resistor? I have found some information that seem to indicate that you can't just calculate a value that gives you a reasonable voltage drop at a certain current, but why is that? In my case  I would like to measure up to about 80A. That is roughly 113A peak. If I have a transformer with a ratio of 800:1 that would translate to 141mA and if disregard the drop over the schottky rectifier and I require about 1.1V at the A/D input pin that would mean I had to get a 8 Ohm resistor. What will happen if I use a burden resistor that is way different from the recommended value specified in the data sheet of the CT?

[Mads Barnkob]

Since you are using a arduino, you should take a look at the integrated current sensing ICs from Allegro: http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs.aspx

The recommended burden resistor range could have to do with avoiding too much phase shift from large resistor values?

[RocketScienceSmurf]

Thanks. I have looked at the allegro IC's before when I wanted some kind of current protection on my induction heater, but in this case I would prefer a current transformer. The wires I need to measure are the incoming to a large farm and I can't properly install these current sensing IC's on the huge wires. I think it will have to be CT's. Would the phase shift be smaller with a low value/high secondary current? I noticed a few high current CT's were specified with a 5A secondary. Would that perhaps work better? I only need a resolution of about 0,5A per A/D step

[petespaco]

Maybe you could simply skip the "burden" resistor and clamp the DC output to a bit below 5 volts.
Also, since you won't be drawing much current, maybe use a single rectifier diode and a simple filter capacitor.
  I recently used a cheap CT in a circuit to sense whether a woodworking power tool was on or off using this idea and it worked.
I think I attached a hand drawn schematic of  what I am suggesting.

Here is the CT that I used:
10-Pack New CR Magnetics 3110-3000 Split-Core Current Transformers CTs
Not sure if the 10 pack is still available.  All ten only cost $30USD  in May of 2017.

Pete Stanaitis
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[RocketScienceSmurf]

But wouldn't that result in 4.7V (the zener voltage) pretty much all the time regardless of the current flowing? Today we have 35A fuses, but my plan was to maybe raise that to 50A and then use the the additonal 15A available plus the 16A we already have today when all is available so I need to be able to tell roughly how much current we can use to our boiler at any given time.

[petespaco]

But wouldn't that result in 4.7V (the zener voltage) pretty much all the time regardless of the current flowing?

No.  unless I am missing something, any voltages below about 4.7 would pretty much just pass through to the Arduino.
The zener is there only to clamp to CT's rectified output below the 5 volts max, that the Arduino' analog input  can tolerate.

Read this:
https://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/
and let me know if it makes sense.

I tested the circuit that I sent in my last post with 120 volt AC loads varying from about 150 watts to about 1000 watts.
I guess my attachment didn't work.

Oh, I guess I didn't see that you have to click "(insert Attachment 1)".
Hope it works this time.


Caution: NEVER leave the output of a CT open.  High voltages can occur.

Pete Stanaitis
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[Uspring]

A CT is a loosely coupled transformer, the primary being the wire fed through it and the secondary the CT winding. A replacement circuit for the secondary side is a voltage source in series with an inductance. The voltage source is in phase 90 degrees ahead of the primary current. Consider 2 extreme cases:

1) Burden resistor is 0. Then the current in the secondary is shifted 90 degrees behind the voltage source due to the series inductance, so secondary current is exactly in phase with primary current. This mode of operation is used, when you need exact phase information e.g. in a feedback loop for zero current switching. A non zero burden resistor will introduce some phase shift between primary and secondary current but that can be negligible if the resistor is not too large.

2) Burden resistor is infinite. Then there will be a 90 degrees phase shift between primary current and output voltage. The voltage can be quite large, several kV, since the CT is a step up transformer.

If you're just interested in current limiting, you can choose any burden resistor you want to achieve the desired output voltage. For precise phase information, you should use small burden resistors or zeners clamping to small voltages.

[petespaco]

I don't see any reason to concern yourself with phase shifting if you want to produce a DC signal for an Aruduino's analog input.

Note:  The CR3110 is good for 75 amps and their CR3111 is good for 100 amps.


I just repeated and recorded some testing of rectified outputs with a few electrical devices:


I saw less than 50 millivolts ripple at the output.

I hope this helps,
Pete Stanaitis
---------------

[RocketScienceSmurf]

Thank you both for the information. If I understand correctly I can choose a suitable burden resistor to achieve the voltage I require since my applications does not require precise phase shifts and what not. When I receive my three phase current transformer I will try and make some measurements with both kind of current-to-voltage conversion (single diode with zener and schottky bridge rectifier) and compare the two.