M V Sainath

Peak Detector Circuit A waveform can be easily observed in a oscilloscope, but there are certain waveforms that change rapidly and it is impossible to determine some of the characteristics of the waveform. One such characteristic is the peak value of the signal. The peak detector is a circuit that is used to find the peak value of the signal in such cases. There are some basic uses that are associated with the peak detector circuits. One of the prominent one is the use of the peak detector circuit for measuring the sound level of a particular place, it determines the loudness of a sound at a place and helps the concerned people keep it under control. This circuit a simple circuit and does not use a lot of complex and rare equipment. It works on the basis of detecting the highest value of a signal given to it and storing it. Though there are some circuits like rectifiers, which gives an average value of the signal given to it, but sometimes it is important to find the highest value of a signal, in those cases a peak detector circuit is used. There are ways to measure the peak value of a sinusoidal signal, so generally the peak detector circuits are used estimate the peak value of an oscillating signal that is not a sinusoid. This circuit is also helpful because the tradition voltmeters or multimeters cannot determine the peak value of signal inherently as there is no function that is built into them. List of materials 1. 2. 3. 4. 5. 6. Transformer LM358 Opamp IC Diode – 1N4007 Resistors Capacitor Connecting wires Circuit Diagram The circuit diagram for the peak detector circuit is given below. Connect the components accordingly to avoid any damage to the circuit or individual component. Transformer It is a general-purpose chassis mounting mains transformer. Transformer has 240V primary windings and centre tapped second windings. The transformer acts as step down transformer reducing AC 240V to DC 12 V. A transformer is an electronic device which is used to regulate AC voltages. A transformer can be designed to either step up or step down a voltage hence, the regulating function. The working of a transformer is simple, there are two windings which are closed placed to each other. Due to the alternating current through them, they produce magnetic field around them. The mutual induction interaction between the two coils is the reason why power transfer is possible in a transformer. This varying magnetic flux induces a varying electromagnetic force or voltage in the secondary winding. One of the most commonly used core for transformers is high permeability silicon steel. The steel has permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. A schematic diagram of the transformer is given below. LM358 Opamp IC The LM358 IC is an op-amp IC. It is a low power, dual channel op-amp IC. It has two internally frequency compensated, independent, high gain op-amps. It is made so that it works off of a single power supply and can operate over a wide range of voltages. There are many applications of this IC which include the DC gain block, transducer amplifiers and the conventional op-amp circuits. This IC has eight pins package. The pin out is shown in the figure below. The internal structure of the IC is shown in the figure above. The IC as discussed above has two independent op-amps. The terminals 1 and 7 are the output terminals of the op-amps. The terminals 3 and 5 are the non-inverting terminals whereas the terminals 2 and 6 are the inverting terminals. There are the normally present ground and VCC terminals at 4 and 8 respectively. This IC along with being economical and easily available has some more redeeming features of itself that are more towards the electronic side of it. Some of the features of listed below-. Its main selling point, the two op-amps are internally frequency compensated The range of single power supply is 3-32 V. The range of dual power supply is -16 to -1.5V or from 1.5V to 16V. Voltage gain is 100dB and the bandwidth is 1 MHz. Supply current drain to the IC is very low. It is generally in the range of 500µA. There is a small offset voltage at the input, which is generally about 2mV. Common mode voltage obtained from the IC contains the ground potential. The differential input voltage and the power supply voltage given to the IC is comparable. 1N4007 Diode 1N4007 is a PN junction rectifier diode. These types of diodes allow only the flow of the electrical current in one direction. So, it can be used for the conversion of AC power to DC. 1N4007 has different real life applications, e.g. free-wheeling diodes applications, general purpose rectification of power supplies, inverters, converters etc. The pinout for the given Diode is given below. Pin no. 1 2 1N4007 Diode Pin name Anode cathode Charge +ve -ve The diagram above shows the symbolic and the actual picture of the 1N4007. The understanding of any component of an electrical circuit is vastly improved when the electrical characteristics of that device is known. The electrical characteristics of the diode 1N4007 is tabulated below. 1N4007 electrical characteristics Parameter Values Forward voltage at 1.0 A 1.1 Reverse current at 25°C 5 Total capacitance at 1.0 MHz 15 Maximum full load reverse current at 75° 30 Average rectified forward current 1 Peak repetitive reverse voltage 1000 Units V uA pF uA A V The diode 1N4007 features are as follows: • Low leakage current • Low forward voltage drop • High forward surge capability This diode has a lot of real life applications in embedded systems, a few of the major applications associated with the particular diode are given below: 1. 2. 3. 4. 5. 6. Converters For switching purposes in embedded systems Freewheeling diodes applications Inverters General power rectification of power supplies To avoid reverse current and protecting microcontrollers like Arduino or PIC microcontroller. Working A very basic form of this circuit involves only a connection of a diode and a capacitor in series. A transformer is used to step the voltage of the supply to around 6V. The signal that is traversing through the diode, in the positive half cycle, it will be forward biased and allow the current to pass through it. During this time, the capacitor that is connected to the diode starts charging and charges till the diode remains forward biased, or the capacitor reaches the maximum limit of charge it can accumulate. Now that the charging of the capacitor is done, when the signal becomes negative, the diode will no longer conduct and the capacitor holds the peak value of the previous cycle. This is the basic function of a peak detector circuit. This is the working of the most basic type of peak detector, only using a diode and a capacitor. But there are modified versions of the same peak detector circuit which involves the use of opamps. The circuit that is discussed in this article deals with the opamp based peak detector circuit. A small modification to the basic peak detector circuit explained in the previous paragraph is done using opamps to remove the voltage drop because of the diode. The idea of a peak detector using an opamp is to form a buffer circuit that will hold the peak value that is given by the signal. This buffer circuit will made by connecting the diode in a feedback loop with the opamp. This means that, the input that is given to the positive terminal of the opamp, will be the signal received at the output terminal. While the oscillating input to the peak detector circuit is in its positive half cycle, the opamp output is high, that makes the diode become forward biased. This is now directly connected to the capacitor which will get charged to the voltage that is it applied to. That means, the capacitor will keep on charging itself to the maximum value of the input signal. The circuit in this phase is said to be in working in a voltage follower buffer circuit. Similarly, when the signal after half a time period becomes negative, the opamp will give a low output which will switch the diode for a forward biased state to a negative biased state. Now similar to the function of the basic peak detector, the capacitor now holds the peak value that it registered in the previous cycle until the diode remains in reverse bias state. This value that capacitor is holding is measured and it determines the peak value of the signal. The capacitor eventually discharges in a load resistor that is connected in the circuit for that very purpose.