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Gigh side current measurement

Ramakrishna Challa
Prodigy 160 points
Other Parts Discussed in Thread: OPA350

Dear all,

 I have 0.005Ohm current sense resistor.

What's the best differential amplifier to measure the current (max 1A).

Specifications

1) High side current measurement

2) The bus voltage is 48V while the Op-amp would be supplied by 5V.

3) need to measurement the current in both the directions

OPA350 is suitable for this application?

 V=IR Drop across current resistor = 0.005*1 = 5mV, this is very small voltage

Any referense schematics would be of great help.

Thanks and regards

Ramakrishna

  • 0
    TI__Guru**** 150976 points

    Hello Ramakrishna,

    I do not have a single chip solution, but your persistence has motivated me to present a complex solution. As you may have already been told, a 5mV max signal on a 48V line is hard to amplify when DC must be preserved.

    My solution is to make a low side amplifier with gain of 200 using the 48V line as a "ground". This requires an op-amp that has a rail to rail input because the common mode is at +VCC potential. The bidirectional current requires adding an offset to the amplifier on the non-inverting input. I also added a -5V supply that is relative to the 48V line. Because the input signal is so low, it will be necessary to offset VIO using a potentiometer (unless a very low outset op-amp is chosen).

    The output should vary from 48V to 46V as the current varies +/-1A.  Therefore a second diff amp is needed to make the output relative to circuit ground. The second op-amp has a gain of one so its output varies from 0V to 2V. Just about any "single supply" 5V op-amp should work here.

    This circuit is untested.  (edit: the second diff amp will NOT work as drawn)

    Regards,
    Ron Michallick

     

  • 0 in reply to Ron Michallick
    TI__Guru**** 150976 points

    Hello Ramakrishna,

    My circuit violates the common mode range of the second diff amp.
    If a multi chip solution is acceptable, I will fix my design.
    Sorry about that.

    Ron

     

  • 0 in reply to Ron Michallick
    Prodigy 160 points

    Hello Ron,

     Application: I have the fuse of 500mA. I need to replace this bi-direction fuse with small sensing circuit.

    I have attached the my proposed circuit. Please go through it and provide your comments.

     I don`t have TINA software for doing simulation. Can you provide the schematic in pdf or image format that will be usefull forme

    Thanks & Regards

    Ramakrishna

     

     6330.fuse.pdf

  • 0 in reply to Ramakrishna Challa
    TI__Guru**** 150976 points

    Ramakrishna,

    I have questions about your circuit.
    Does analog switch, U3, allow high voltage on the S & D pins?  How low impedance is this switch?

    When The current is too high, should the fuse latch and when will it be reset?

    Regards,
    Ron.

     

     

  • 0 in reply to Ron Michallick
    Prodigy 160 points

     

    Ron,

    I think it will allow,

    Specification of the analog switch:

    Low on resistance (0.25 O typical).

    400 mA current-carrying capability (Continuous Current, Pin S or Pin D)

    35 ns Switcing times

    Thanks,

    Regards,

    ram