Online current measurement circuit

When testing electrical equipment, sometimes in order to measure the current more accurately
, it is necessary to disconnect the circuit and connect it in series with an ammeter for measurement. Although the
circuit needs to be restored to its original state later, this not only affects the normal
operation , but may also cause new problems. potential faults. If using an online current
meter, there is no need to destroy the circuit board. This enables online
measurement of current. The online current measurement circuit is shown in Figure 6-52. It uses
an operational amplifier to introduce negative feedback, and its "virtual ground" point has
extremely small input resistance to ground.
    When it is connected in parallel with the copper foil on the printed circuit board,
the current forced to flow through the copper foil instead flows through the feedback branch (meter branch) of the
op amp , thereby achieving a circuit without affecting the normal operation of the electrical equipment. In this case
, measure the current of a certain branch.
    In the figure, the internal resistance of the 50VA ammeter is RM =2kfl, and the full-scale voltage drop of the meter head is UD - 50yA × 2kfZ = 100mV. This voltage is far from the
linear dynamic range of the op amp. If the most sensitive level of the measured current is ImA. Then the equivalent resistance of the feedback branch is RM-lOOrrN/lrriA-
ioon. Assume that the open-loop gain of the op amp is G,-120dB [lOf/times], then the equivalent resistance between the op amp's virtual point and the ground RMf =RM/G,=lOOfl/l06=
0; ImfZ. The copper foil is equivalent to a current source with very small internal resistance at this time, and its internal resistance is the wire resistance of a section of the branch being measured. With the 1mm wide printed circuit
board copper foil as the side, its resistivity is measured to be 2mfl/cm. If the distance between the two probes on the copper foil (length of segment AB) is lcni, then the equivalent resistance of the current source
is Rt - 2rrd: l!. Therefore, the error introduced by wind is 5%. like. The length of the NB segment is 5cm, then the error introduced by Rs is 1%. Obviously, the open-loop gain Gv of the operational amplifier
is one of the key factors affecting the measurement error. Choosing a high-gain operational amplifier will help reduce the measurement error. In addition, the input
offset and internal noise of the op amp will cause errors. Therefore, while selecting a high gain op amp, the low drift, low noise and high input
impedance of the op amp must also be taken into consideration. The test result of this circuit using low-drift op amp F033 is that the needle of the most sensitive meter swings a small division (that is, the error is 2%), which basically
meets the requirements. If you use an auto-zero operational amplifier instead (such as 5G7650, etc.), the effect will be better.
    The transistor 3DK4 in the circuit is used to increase the power driving capability. VD1 and VD2 are input protection tubes. VD3 is a level shifter, used
to adjust the operating points of transistors VT and VD3.