Hello,
I am building a closed loop current sensor and having a very hard time finding good documentation about how to make one. I'm hoping I can solicit some help from the many savvy members of this forum.
I will post a schematic as soon as I can, but here's something to get this started.
The general theory of operation is that it's an AC/DC sensor that uses feedback to get high accuracy, and can measure either very high currents without saturating your magnetic core (a toroid with a thin slot cut in it), or alternately, very small amounts of current. The sensor combined a 1. Gapped magnetic core, 2. A hall effect sensor placed inside the gap, 2. An amplifier stage, and a secondary loop of wire that opposes the magnetic field in the primary of the circuit, with N turns ranging from 50 to 2000, depending on what you are trying to measure. The idea is that Ip = N x Is, and by putting a shunt resistor on the secondary, an analog voltage is created as the output. The formula for the circuit, then, presuming a high gain amp is used (say >=100) is: Vout / Ip = Rsh / N
I've designed the circuit with the following parameters:
Vout = +/- 4V
Ip = +/- 500 mA
N = 50
Rsh = 400 ohm
The problem I am having is that at 500 mA, the output, with the secondary wire coil connected, and in series with the shunt resistor gives the same output as if no coil is connected (about 60 mV at 500mA). I measured the current in secondary coil/resistor, and it is only 0.25 mA, versus the 500 mA / 50 turns = 10mA that is expected. The opamp I am using is an instrumentation amplifier TI INA826 with a gain set to 100. I was expecting that it could drive the 10mA required to energize the secondary circuit, but the output is clearly not doing what I was expecting it to do.
First, maybe a dumb question... Can this sensor actually detect DC?
Second, is it possible I need some sort of driver circuit to supply current to the secondary?
Any theories, tips, schematics, or reading materials you might provide on the subject would be greatly appreciated!
Dave
I am building a closed loop current sensor and having a very hard time finding good documentation about how to make one. I'm hoping I can solicit some help from the many savvy members of this forum.
I will post a schematic as soon as I can, but here's something to get this started.
The general theory of operation is that it's an AC/DC sensor that uses feedback to get high accuracy, and can measure either very high currents without saturating your magnetic core (a toroid with a thin slot cut in it), or alternately, very small amounts of current. The sensor combined a 1. Gapped magnetic core, 2. A hall effect sensor placed inside the gap, 2. An amplifier stage, and a secondary loop of wire that opposes the magnetic field in the primary of the circuit, with N turns ranging from 50 to 2000, depending on what you are trying to measure. The idea is that Ip = N x Is, and by putting a shunt resistor on the secondary, an analog voltage is created as the output. The formula for the circuit, then, presuming a high gain amp is used (say >=100) is: Vout / Ip = Rsh / N
I've designed the circuit with the following parameters:
Vout = +/- 4V
Ip = +/- 500 mA
N = 50
Rsh = 400 ohm
The problem I am having is that at 500 mA, the output, with the secondary wire coil connected, and in series with the shunt resistor gives the same output as if no coil is connected (about 60 mV at 500mA). I measured the current in secondary coil/resistor, and it is only 0.25 mA, versus the 500 mA / 50 turns = 10mA that is expected. The opamp I am using is an instrumentation amplifier TI INA826 with a gain set to 100. I was expecting that it could drive the 10mA required to energize the secondary circuit, but the output is clearly not doing what I was expecting it to do.
First, maybe a dumb question... Can this sensor actually detect DC?
Second, is it possible I need some sort of driver circuit to supply current to the secondary?
Any theories, tips, schematics, or reading materials you might provide on the subject would be greatly appreciated!
Dave