Addressing Challenges in Current Measurement
Our readers have provided valuable feedback, highlighting the challenges associated with measuring currents in the positive lead of a power supply. In response to these concerns, the circuit presented here proves to be a helpful solution in numerous scenarios. While the design itself isn’t groundbreaking, its practical utility makes it a valuable tool for addressing the complexities often encountered in current measurements.
Principle of Current Measurement
To measure current accurately, the essential shunt or current measuring resistor, R1, is incorporated in series with the load. The voltage drop across this resistor is directly proportional to the current passing through the load. Employing the operational amplifier’s (opamp) inherent tendency to minimize potential differences between its inverting and non-inverting inputs, a compensating current flows from the emitter of T1 to the inverting input of IC1, with a magnitude of UR1/R2. Simultaneously, this current passes through R3, resulting in a voltage of UR1·R3/R2, which at the provided values equals 2 V per ampere. This voltage can be displayed using a moving coil instrument or an appropriate indicator.
Opamp Requirements and Circuit Considerations
A crucial aspect to note is that the inverting input of the opamp is effectively linked to the power supply. This necessitates an opamp with an input common-mode range that encompasses at least the positive supply rail. Additionally, the output of the opamp must be capable of swinging close to the power supply voltage. Failure to achieve this may prevent T1 from turning off sufficiently. Hence, a high-quality ‘rail-to-rail’ opamp is indispensable. The LT1677, chosen for this application, is specifically designed for this purpose and boasts essential characteristics, making it an ideal choice for the circuit’s successful operation.
- rail-to-rail input and output;
- extremely low noise (3.2 nV/√Hz at 1 kHz);
- gain/bandwidth 7.2 MHz;
- offset 60 µV;
- power supply range 3 to 30 V
LT1677: Ideal for Small Signal Processing
The distinctive traits of the LT1677 render it exceptionally suitable for handling small signals with precision and accuracy.
Ammeter Functionality and Symbol
Ammeters possess the ability to measure a broad spectrum of current values by diverting only a fraction of the total current through the meter mechanism at high values. The majority of the current is carried through a shunt in parallel with the meter. In circuit diagrams, an ammeter is represented by a circle containing the capital letter ‘A’ inside it.