Measure contactless dc current 0-100mA

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
487
Is there any sensor available to measure dc current from 0-100mA contactless?
Contactless is the problem, looked for some sensor half effect but they are for very high currents only
 

AlbertHall

Joined Jun 4, 2014
9,272
ACS712-005 has a full scale of 5A that might do the job. It will output 18.5mV at 100mA so you would probably need to amplify that to get good readings.
 

Reloadron

Joined Jan 15, 2015
5,110
Is there any sensor available to measure dc current from 0-100mA contactless?
Contactless is the problem, looked for some sensor half effect but they are for very high currents only
Yes, the non intrusive is the difficult part. There are some hall effect units available but as you mentioned high current. A 0 to 50 ADC won't help you much with a 0 to 4 VDC output. There is a line offered by CR Magnetics their CR5210 CR5211 and CR5220 offer a split core with a low range of 2.0 ADC and outputs of +/- 5 volts, +/- 10 volts or 4 to 20 mA.

You can always give them a call depending on your location. You may want to note that an input range of 2.0 amps is -1.0 amp to 1.0 amp. I have used their stuff in the past and it's good and reliable. However also note the sensor you want will be about a $160 USD DC Current Transducer. I am not aware of any Chinese made clones or similar transducers for a low price.

Ron
 

MisterBill2

Joined Jan 23, 2018
5,181
There are devices to measure that current, HP used to make a great clamp-on measuring system that read DC and AC and the probe part was a few hundreds of dollars. I came up with a way to do it, using a hall effect sensor and a servo amplifier that completely gets around the sensor non-linearity issues. It is based on the sensor detecting the flux across a gap in a ring encircling the current carrying conductor and the servo amplifier passing current through a winding on the same circular core to cancel the field produced by the conductor. Because the turns ratio is known and the current in the winding can be measured, the conductor current can be accurately calculated. The servo amplifier does not have to be linear, it just needs to have no uncompensated DC offset.
 

MisterBill2

Joined Jan 23, 2018
5,181
I already described a way to get an accurate non-contact reading of the DC current in a wire. Start with the second sentence of post #5 to see the explanation.
How accurate does the measurement need to be?And could the current carrying conductor be run past a permanent magnet to provide a force to move a magnet? I have seen ammeters made like that. In addition, there are "flux gate" systems for measuring the magnetic field generated by a current carrying conductor. They somehow use the rate of decay of an externally created magnetic field to deduce the current in a wire.
 

tautech

Joined Oct 8, 2019
58
Is there any sensor available to measure dc current from 0-100mA contactless?
Contactless is the problem, looked for some sensor half effect but they are for very high currents only
At such low levels of current multiple wraps around the hall sensors core will transform the current to levels you can readily measure.
 

MisterBill2

Joined Jan 23, 2018
5,181
The problem with that device is that it needs to be connected in series with the load. Certainly it is isolated but it is not a "non-contact" arrangement.
BUT that device in the DigiKey link will certainly work with my flux-cancelling servo-amplifier arrangement, with the load circuit wire passing through the gap below the coil. The question becomes one of pricing.
 

Marley

Joined Apr 4, 2016
341
You can get "current transformers" for DC. They use a hall effect sensor with electronics to deliver a voltage output proportional to current.
This one - link - (for example) has a range up to 850mA. Simply wind your current carrying wire 8 times through the hole to get close to 100mA full scale. This will give you a fully isolated current sensor.
 

MisterBill2

Joined Jan 23, 2018
5,181
Yes issue is contactless.
Will check crmagnetics devices.

@MisterBill2, can you explain bitof more of your setup?
The concept is using a second winding on the core that is used to concentrate the magnetic field from the current carrying wire into the hall effect device. That second winding is driven by an amplifier with the input being controlled by the hall device output. The current from the amplifier produces a magnetic force which is intended to cancel out the flux from the conductor being monitored. The intent is to completely cancel the field and be able to easily measure the current needed to do that cancellation. And since the turns ratio is also known, it is simple to figure the current in the wire being monitored. It is a true "servo-system" but with no moving parts. The big limitation is the frequency response of the amplifier and the coils.

I do not recall if there is any variation in the measured current being mentioned.

I hope that this explanation makes some sense.
 

DickCappels

Joined Aug 21, 2008
6,185
Its a fluxgate magnetometer which is a lot like what @MisterBill2 described in post #5. It has no moving parts and is in general very reliable.

A high resolution CRT monitor I worked on when people used particle accelerators to display TV pictures and computer outputs used such a circuit to tell when the monitor was rotated with respect to the earth's field. The monitor factory where it was made was next to a railroad line and all the monitors in the factory would degauss themselves every time a train went by. Great idea but with drawbacks.
 

MisterBill2

Joined Jan 23, 2018
5,181
Looks like this solves the purpose : https://www.lem.com/en/ctsr-03p
1. its contactless
2. can measure -300mA to +300mA with 1.9% accuracy.

can connect 16 bit adc and oversample it for better measurement.

do anyone else ever used their product, its reliablity?
That device could probably be quite accurate with the addition of calibrating it with a known current source prior to use, and powering it from a well regulated power supply. So if the physical aspect of the device is satisfactory it may be the solution.
 

Reloadron

Joined Jan 15, 2015
5,110
anyone else ever used their product, its reliablity?
Have I used their product? No but have used similar. It has a few nice features like external reference but also has a 2.5 volt internal reference. Availability seems to be an issue as I am seeing lead times of 14 weeks plus. The cost is about $35 USD. Lower considerably than the CR Magnetics version I looked at.

Ron
 
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