Improved Hall Effect Current Sensor

[commanda]

I built a current sensor using a UGN3503 and a toroid, like Gizmo did here:

http://www.thebackshed.com/Windmill/charger1.asp

and found the interaction between zero and span made it impossible to adjust.

So I dug out a circuit I have used elsewhere, which mostly eliminates this problem.

Step one is to adjust the offset null, so the voltage from TPA to TPB is 0.0mV. This will mean there is 0.0mV across R1, so no current will flow. This same current will (not) flow through the gain pot, P3. So, adjusting the gain pot will not affect the voltage output at U2B. Hence, the output voltage at pin 8, as set by the zero pot, will not change when the gain is adjusted.

Remember, an op-amp will adjust its output voltage to keep the differential input voltage zero. So the voltage at pin 5 will be the same as the voltage at pin 6.

The circuit.


09590240.pdf

I used a rail to rail quad opamp, because I'm using it in unipolar mode. That is, at zero amps, the output voltage is zero.

Used in the more traditional bipolar mode, where the output is at half supply with zero amps, and goes positive or negative from there depending on whether it's charge or discharge current, is much less critical of opamp used, and a common LM324 should suffice in this case.

Amanda

[commanda]

Followup:

The toroids I used were salvaged from pc power supplies, and generally used as output chokes. I cut a slot in them using a mains powered Dremel type tool with a small cutoff wheel. There appears to be quite a bit of residual magnetism in the core, which introduces offset and stability problems in the final application. May I suggest that anyone following this route should deGauss the toroids before use.

I'm also thinking the final sensor may need to be in a Faraday cage to protect from stray magnetic fields, to ultimately get the stability I'm looking for (one part in 1023 or 0.1%).

Amanda