1
\$\begingroup\$

In resistive current sensor, can it have two different supplies for the voltage divider circuit and the differential op-amp circuit? Since the op-amp only gets the voltage difference.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Is the schematic meant to show that Gnd1 and Gnd2 are connected or not connected? \$\endgroup\$ – The Photon Jan 13 '14 at 6:17
  • \$\begingroup\$ @ThePhoton I presume by the Gnd1 and Gnd2 and isolated power supplies the answer is no, to the question too. \$\endgroup\$ – Diego C Nascimento Jan 13 '14 at 6:31
  • \$\begingroup\$ Isolated grounds \$\endgroup\$ – Xegara Jan 13 '14 at 12:29
1
\$\begingroup\$

If the grounds in your schematic are connected then the circuit will work fine with two different supplies as long as the voltage level reaching the opamp inputs is within the input common-mode range.

If the grounds are not connected then you can use a Hall-Effect-Based Linear Current Sensor such as ACS712. The sensor output is electrically isolated from the circuit being measured so the two supplies can be completely independent.

enter image description here

\$\endgroup\$
1
\$\begingroup\$

I don't think your circuit will work in any event. The inverting input is connected to ground through a resistor, so the voltage at the inverting input is always at ground (for practical purposes). The voltage at the non-inverting input is above ground when there is current flowing, so the voltage at the output will increase. However, there is no feedback from the output to the inverting input because of how you have connected R3 to ground in the circuit. I think the connection from the right end of R3 needs to be connected between R5 and the inverting input.

Also, you are going to need an op amp with good rail-to-rail input and output to make this work for small currents.

\$\endgroup\$
  • \$\begingroup\$ Sorry. I drew the schematic wrongly. Had it edited already \$\endgroup\$ – Xegara Jan 13 '14 at 12:29
  • \$\begingroup\$ Actually there is no junctions dot at the cross point of the ground line with the inverting input line so there is no connection but you are right that the lines shouldn't cross like this, it's a bad drawing example. The way that R3 has been placed is also confusing because it seems to indicate that the output will be the inverting input rather than the opamp output. \$\endgroup\$ – alexan_e Jan 13 '14 at 15:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.