[Design View / Design Solution]
Measuring Mains Current Doesn't Have To Be Difficult
Try this simple loop technique, which offers an inexpensive and reliable alternative to other current-sensing methods, when monitoring mains current.
Another Allegro sensor, the A1321ELHLT- T, is used in this application. Like the ACS712, the A1321 generates an output voltage proportional to the applied magnetic field. However, the A1321 is considerably cheaper than the ACS712. And since the mains current now flows through the PCB track, not through the sensor package, the power dissipation in the sensor itself is no longer an issue. Housed in a SOT23 package, the small size of the A1321ELHLT-T means that the current loop itself can be quite compact, thereby taking up relatively little PCB real estate.
The quiescent output voltage of the A132x family of sensors is nominally 50% of the supply voltage, and the output sensitivity of the A1321 variant is 5 mV/ Gauss. Therefore, with the sensor powered from a 5-V supply rail, the configuration shown in Figure 2 will produce a small ac signal swinging about a quiescent dc level of 2.5 V. Figure 3a shows the output signal obtained with a sinusoidal current of 4.9 A rms (50 Hz) flowing through the loop.
Clearly, the signal must be amplified and converted to a dc voltage before it can be processed by further circuitry, such as an analog-to-digital converter or comparator. There are many ways in which an ac signal can be converted to a dc level.
One solution combines an amplifier with an averaged absolute value circuit (Fig. 4). The circuit amplifies the ac portion of the signal, strips out the sensor’s quiescent dc level, and then generates a dc voltage proportional to the absolute value of the ac waveform.
The sensor output (pin 2 of IC1) contains significant HF noise (Fig. 3a, again): this is filtered out by R1 and C2 (Fig. 4, again). The corner frequency of this low-pass filter is much higher than the frequency of the mains signal (50 Hz/60 Hz) and, therefore, has little effect on the amplitude of the mains signal fed to the amplifier stage.
The amplifier formed by IC2, R2, R3, and C4 provides high gain for the ac content of the sensor’s output signal and unity gain for the dc content. Consequently, the signal at IC2’s output is an amplified version of the mains signal riding on a dc level of 2.5 V. The ac gain is given by:
ac gain = 1 + R2/(R3 + XC4)
where XC4 is the reactance of C4. With the values of R2, R3, and C4 as shown in Figure 4, the nominal ac gain is approximately 36 at 50 Hz/60 Hz.
The remainder of the circuit functions as an averaged absolute-value converter. The converter comprises two stages, the first being a differential-output absolutevalue converter built around IC3a. The second stage comprising IC3b is a traditional differential amplifier.
The combination of the two stages along with the integrating function provided by capacitors C7, C8, C9, and C11 performs single-ended absolute-value conversion. The result is a single-ended dc output voltage at VO, which is proportional to the peak-to-peak amplitude of the signal appearing at the output of IC2.
The converter is based on a circuit described in Reference 1, but with the important addition of R4 and C6. This low-pass filter entirely removes the ac content of the signal at IC2’s output and leaves only the dc content (nominally 2.5 V), which provides a reference potential at IC3a’s non-inverting input.
This reference potential could have been generated by means of a potential divider connected to the supply rails, but the low-pass filter approach ensures that the reference voltage is always exactly equal to IC1’s dc output level (which can vary from 2.425 to 2.575 V).
OP-AMP SELECTION When choosing components for the circuit, select op amps with low input bias current. Ideally, IC2 should have a wide output swing, and IC3a and IC3b should be rail-to-rail I/O types. Op amps with low input offset voltage are preferable for IC3a and IC3b to minimize dc offsets. Although IC3 is shown as a dual device, two single op amps could be used just as well.
Figure 5 shows an actual implementation of the scheme. The inner diameter of the mains track loop on the bottom of the PCB (Fig. 5a) is roughly equal to the size of the Hall Effect sensor located on the top of the PCB (Fig. 5b). When implemented with SMT components, the whole circuit occupies an area not much larger than a postage stamp.
The magnitude of the ac signal generated by the sensor is very sensitive to the dimensions and geometry of the track loop. Therefore, when the board tracking is finalized and the first prototype sample is ready, it may be necessary to adjust the gain of the circuit to get the optimum variation in VO for a given range of mains current. The easiest way to achieve this is by adjusting the value of R3 while keeping all other values constant. When laying out the PCB, take care to ensure that the width of the mains tracking is adequate for the maximum current value that will be encountered.
Figure 6 shows the actual response generated by the layout of Figure 5. Note how the circuit produces a perfectly linear response over a mains current range of around 0.4 A to nearly 13 A rms. The sensitivity of the overall circuit in terms of output voltage relative to input current is around 350 mV/A. Other sensitivities can be obtained by changing the amplifier’s ac gain.
Keep in mind that the circuit only generates an average measure of the absolute signal value. This is fine for truly sinusoidal waveforms in which the rms value is proportional to the peak-to-peak value. Certain types of load, though, can distort the current sinewave and produce erroneous results.
thank you for your stady about current sensor but I want to use 3503 current sensor insted please send me the sechmatic whith it;s amplifire
Anonymous -October 06, 2009
I want to buy 100 pieces of acs754 please how match send me
wafic -September 26, 2009
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