LM3915 based AUDIO POWER METER Circuit Schematic With Explanation

This VU (volume-unit) meter is capable of monitoring anddisplaying power levels present at the speaker terminals ofan stereo audio power amplifier.
The levels are displayed in ten discrete steps. This meter is not designed to give an accurate display of the power levels.It is designed to give an approximate visual indication of the audio power output of each channel. For many situations - disco, parties - this is all which is required: the flashing LED's add atmosphere to the situation.
LM3915. This is a monolithic Dot/Bar Display Driver IC made by National Semiconductor. It takes an analog voltage input on pin 5 then drives 10 LED's providing a logarithmic 3dB/step analog display. When measuring power, a 3dB increase means that the power input has doubled. As the power doubles, an additional LED will be lit until the maximum is reached. The display can be bar or moving dot depending on the connection of pin 9 to the positive supply. The LED drive current is regulated which eliminates the need for current limiting resistors.
The supply voltage can be between 3V to 25V. You can download the data sheet from the National Semiconductor website at: http://www.national.com/
The IC is suited to signals with a wide dynamic range such as audio, power, light intensity. In many applications a bargraph meter is faster, more rugged and has higher visibility than an analog (moving coil) or LCD meter.

PARTS LIST

R1-----10K
R2------1K
R3-----390
R4-----2,7K
P1-----100K TRIMMER POT.
C1-----10mf 25v
D1..D10---LED red
I.C. ----LM3915

VHF field strength meter Circuit Schematic With Explnation

This is a simple and low cost wide band VHF field strength meter. The field strength is measured by converting the radio signal to DC and measuring it.





The RF signal will be picked up by the coil and rectified by the diode D1.Even a very small DC voltage is sufficient to alter the biasing of FET and it will be reflected in the meter as an indication of the presence of a radio signal. The meter can be calibrated by adjusting the preset R2 to make meter M1 read ZERO in the absence of any radio signal. This circuit is not very sensitive, but can sense radio signals from hand held FM transmitters up to a distance of few meters( ideal for theoretical demonstrations).



Notes.

* The circuit can be assembled on a general purpose PCB.
* Use a 9V PP3 battery for powering the circuit.
* Use a 250uA FSD current meter for M1.Using a lower FSD meter will improve sensitivity.
* The coil L1 can be made by making 6 turns of 20 SWG enameled copper wire on a inch plastic former.
* The antenna can be a telescopic whip antenna.

comprehensive note on DC voltmeters

DC Voltmeter Block Diagram

DC voltmeter mainly consists of a dc ampli­fier apart from the attenuator, as shown in figure.

DC voltmeters can further be divided into two categories.

1. Direct Coupled Amplifier DC Voltmeter.

This type of voltmeter is very common because of its low cost. This instrument can be used only to measure voltages of the order of milli-volts owing to limited amplifier gain. The circuit diagram for a direct coupled am­plifier dc voltmeter using cascaded transistors is shown in figure. An attenuator is used in input stage to select voltage range. A transistor is a current controlled device so resistance is inserted in series with the transistor Q₁ to select the voltage range. It can be seen from figure that sensitivity of voltmeter is 200 kilo ohms/volt neglecting small resistance offered by transistor Q₁. Other values of range selecting resistors are also so chosen that sensitivity remains the same for all ranges. So current drawn from the circuit is only 5micro Ampere.

Two transistors in cascaded connections are used instead of using a single transistor for amplification in order to keep the sensitivity of the circuit high. Transistors Q₁ and Q₂ are taken complement to each other and are directly coupled to minimize the number of components in the circuit. They form a direct coupled amplifier. A variable resistance R is put in the circuit for zero adjustment of the PMMC. It controls the bucking current from the supply E to buck out the quiescent current. The draw-back of such a voltmeter is that it has to work under specified ambient temperature to get the required accuracy otherwise excessive drift problem occurs during operation.

Another circuit diagram of a direct coupled amplifier dc voltmeter using FET in input stage is shown in figure. In this voltmeter, voltage to be measured is firstly attenuated with range selector switch to keep the input voltage of amplifier within its level. FET is used in the input stage of amplifier because of its high input impedance so that is does not load the circuit of which voltage is to be measured and it also keeps the sensitivity of voltmeter very high. As FET is a voltage controlled device so resistance network of attenuator is put in shunt in the circuit. Transistors Q₂ and Q₃ form the direct coupled dc amplifier whose output is finally supplied to PMMC meter. When transistors work within their dynamic region, the deflec­tion of meter remains proportional to the applied input voltage. This voltmeter can be used for measurement of voltages of the order of milli-volts because of sufficient gain of ampli­fier.

Apart from the high input imped­ance, this circuit has another advan­tage that when in­put voltage exceeds its limit, amplifier gets saturated which limits the current passing through the PMMC meter. So meter does not burn out.

2. Chopper Type DC Voltme­ter.

Chopper type dc amplifier is used in highly sensitive dc electronic volt­meters. Its block diagram is shown in figure. Firstly dc input voltage is converted into ac voltage by chopper modulator and then it is supplied to an ac amplifier, i Output of am­plifier is then demodulated to a dc voltage proportional to the original input voltage. Modulator chopper and demodulator chopper act in anti-synchronism.Chopper system may be either mechanical or electronic.

Circuit diagram of an electronic chopper employing photo diodes is shown in figure. Photo diodes change its resistance under different illumination conditions, this property f photo diode is used in chopper amplifier. Its resistance changes from the order of few mega-ohms to few hundred ohms when it is illuminated by a light source in the dark place.

Two neon lamps are used in this circuit, these are supplied by an oscillator for alter­nate half cycles. Two photo diodes are used in input stage which acts as half-wave modu­lators because of its alternate switching action by the neon lamps at the frequency of oscillator.

Output of chopper modulator is a square wave voltage (proportional to the input signal) which is supplied to the ac amplifier through a capacitor. Amplified output is again passed through a capacitor and then fed to chopper demodulator. Capacitor is used to remove dc drift from the signal. Chopper demodulator gives a dc output voltage (proportional to the input voltage) which is passed through the low pass filter to remove any residual ac component. Now this dc output voltage is supplied to the PMMC meter for measurement of input voltage.

In chopper amplifier dc voltmeter, input impedance is of the order of hundred mega-ohms and it has sensitivity of one micro-volt per scale division.

S:circuitstoday.com

comprehensive note on AC voltmeter

In electronic ac voltme­ters input signal is firstly rectified and then sup­plied to the dc amplifier, as shown in figure. Sometimes signal is firstly amplified by ac amplifier and then recti­fied before supplying it to dc meter, as shown in figure. In the former case the advantage is of economical amplifiers and the arrangement is usually used in low priced voltmeters.

Broadly ac voltmeters can be divided into three categories.

1. Average Reading AC Voltmeters.

Normally ac voltmeters are average responding type and the meter is calibrated in terms of the rms values for a sine wave. Since most of the voltage measurements involve sinusoidal waveform so this method of measuring rms value of ac voltages works satisfactorily and is less expensive than true rms respond­ing voltmeters. However, in case of measurement of non-sinusoidal waveform voltage, this meter will give high or low reading depending on the form factor of the waveform of the voltage to be measured.

The circuit diagram for an average reading voltme­ter using a vacuum tube diode is shown in figure. The arrangement requires a vacuum tube diode, an high resistance (of the order of 10⁵ Q.) R and PMMC instru­ment, all connected in series, as shown in fig. Resist­ance R is used to limit the current and make the plate

voltage-current characteristics linear. The linear plate characteristics are essential in order to make the current directly proportional to voltage. Also because of high series resistance R, plate resist­ance of vacuum tube diode becomes negligible and therefore variations in plate resistance do not cause non-linearity in voltage-current characteristics. In this way we get the scale of PMMC instrument uniform and independent of variations of tube plate resistance. Voltage across the high resistance is fed to dc amplifier and output of the amplifier is fed to PMMC instrument. Circuit diagram of an average reading ac voltmeter using semi-conductor diode is shown in figure. The diode conducts during the positive half cycle and does not conduct during the -ve half cycle, as shown in figure.

The average current through the meter will be given by the expression

I_{av ­}= V_av / 2R = 0.45 * [V_rms/R]

V_rms is the effective or rms value of applied voltage and 1.11 is the form factor of sinusoidal wave. R is multiplied by 2 because the voltmeter operates on half-wave recti­fication. It is to be worthnoting that this instrument can be used to indicate dc voltages but in such a case the instrument readings will have to be multiplied by 2 x 1.11, that is, as the diode conducts all the time. Circuit diagram of an average reading ac voltmeter using semi-conductor diodes as a full-wave rectifier is shown in figure. In this case average current through the meter will be V_rms/ 1.11R. Main advantages associated with these voltmeters are that they are simple in construc­tion, have high input impedance, low power consumption and uniform scale. Main disadvantage of these voltmeters is that these operate in audio-frequency range. In radio-frequency range, distributed capacitance of the high resistance R introduces error in the readings.

Another disadvantage of such a voltmeter is that due to non-linear volt-ampere char­acteristic for lower voltage the readings of the voltmeter at lower voltage are not correct.

2. Peak Reading AC Voltmeters.

The circuit diagrams for peak reading voltmeter using vacuum tube and semi-conductor diode are shown in figures respec­tively.

In this type of voltmeters capacitor C is charged to the peak value of .the applied voltage and capaci­tor -is discharged through the high resistance R between two peaks of the wave which results in a small fall in capacitor voltage. But this voltage is again built up during next peak of the wave, as shown in figure. So voltage across capacitor C and resistance R remains almost constant and equal to the peak value of the applied voltage.

Either the average voltage across R or the average current through R, can be used to indicate the peak value of applied voltage. In case the vacuum tube diode (or semi-conductor diode), series resistance R shunted by capacitance C and PMMC are connected in series across the source of unknown voltage, the current through the PMMC will indicate the peak value of applied voltage.

In case, the circuit shown in figure making use of rectifying diode, series resistance R, dc amplifier and PMMC is employed, the average voltage across R will indicate the peak value of applied voltage. This alternative is preferred, as explained earlier, the power consumption can be reduced by making series resistance R high. By making series resistance R high a less sensitive type of PMMC instrument can also be used. The high value input resistance also gives more linear relationship between peak applied voltage and the instrument indication. Also the performance of the diode with inputs consisting of pulses and modulated waves is improved.

The dc amplifier associated with the diode rectifier should be provided with stabilizing means in order to prevent drift in the indication of the output meter. Usually a voltage regulated supply combined with a compensating circuit is used.

The use of high series resistance R, associated with dc amplifier, no doubt results in a high input resistance but at the same time it implies that an applied voltage of sufficient amplitude is required so that the system acts as a peak voltage device. The main disadvantage of this system is with regards to measurement of low voltage. If the applied voltage is too small, then there is a flow of some current throughout the cycle of the voltage because of high velocity of emission of electrons, and the input resistance may be a few hundred ohms and it defeats the very purpose with which elec­tronic instruments are used.

S:circuitstoday.com

true RMS reading voltmeter

RMS value of the sinusoidal waveform is meas­ured by the average reading voltmeter of which scale is calibrated in terms of rms value. This method is quite simple and less expensive. But sometimes rms value of the non-sinusoidal waveform is required to be measured. For such a measurement a true rms reading voltmeter is required. True rms reading voltmeter gives a me­ter indication by sens­ing heating power of waveform which is proportional to the square of the rms value of the voltage.

Thermo-couple is used to measure the heating power of the input waveform of which heater is sup­plied by the amplified version of the input waveform. Output voltage of the thermo­couple is proportional to the square of the rms value of the input waveform. One more thermo-couple, called the balancing thermo-couple, is used in the same thermal environment in order to overcome the difficulty arising out of non-linear behaviour of the thermo-couple. Non-linearity of the input circuit thermo-couple is cancelled by the similar non-linear effects of the balancing thermo-couple. These thermo-couples form part of a bridge in the input circuit of a dc amplifier, as shown in block diagram.

AC waveform to be measured is applied to the heating element of the main thermo­couple through an ac amplifier. Under absence of any input waveform, output of both thermo-couples are equal so error signal, which is input to dc amplifier, is zero and therefore indicating meter connected to the output of dc amplifier reads zero. But on the application of input waveform, output of main thermo-couple upsets the balance and an error signal is produced, which gets amplified by the dc amplifier and fedback to the heating element of the balancing thermo-couple. This feedback current reduces the value of error signal and ultimately makes it zero to obtain the balanced bridge condition. In this balanced condition, feedback current supplied by the dc amplifier to the heating element of the balance thermo-couple is equal to the ac current flowing in the heating element of main thermo-couple. Hence this direct current is directly proportional to the rms value of the input ac voltage and is indicated by the meter connected in the output of the dc amplifier. The PMMC meter may be calibrated to read the rms voltage directly.

By this method, rms value of any voltage waveform can be measured provided that the peak excursions of the waveform do not exceed the dynamic range of the ac amplifier.

S:circuitstoday.com

(VTVM and VTM) Balanced bridge voltmeters

Balanced Bridge Voltmeters

Such voltmeters can be used for measurement of both dc and ac voltages and are very popular. These voltmeters are available in both versions, vacuum tube type and transistorized type and are described below:

(a) Balanced Bridge Triode VTVM.

The circuit diagram of a balanced bridge triode VTVM is shown in figure.

In this instrument two identical triodes T₁ and T₂ are used. Plate voltage is applied to both of the triodes through two, identical resistors R_L and a variable resistance R_Q. Resistance R₀ is so adjusted that plate voltages of both of the triodes become equal when there is no voltage applied on the grid of triode T₁. So, in this condition, meter connec­ted between the plates of the two triodes reads zero. The bias battery E_CC biases the cathodes of both of the triodes positive and so the bias of the grid of both of the triodes becomes negative with respect to their respective cathodes.

When a positive dc voltage is applied to the grid of triode T₁, potential of grid with respect to cathode increases and so the plate current increases. Due to increase in plate current of triode T₁ the voltage drop across the resistor R_L connected in series with triode T₁ increases and, therefore, plate voltage of triode T₁ falls. The increased current also flows through the resistor R_k which is common to both of the tubes T₁ and T₂. An increase in current through resistor R_k causes increase in voltage drop across it and, therefore, cathode of triode T₂ becomes more positive. It results in increase of -negative grid bias of tube T₂ and therefore, plate current of triode T₂ is reduced. With this voltage drop across resistor R_L connected with plate of triode T₂ falls and, therefore, plate voltage of triode T₂ increases. Thus with the application of dc voltage on the grid of triode T₁ voltages of the two triode plates do not remain same and a current flows through the PMMC meter, which is proportional to the applied dc voltage. So PMMC meter can be directly calibrated to read the applied input dc voltage. Range of this instrument can be extended by using a range selector switch and a potential divider network. For measuring ac voltage with this voltmeter, it is first rectified and then applied to this voltmeter. A resistance is connected in series with the meter for the purpose of setting the full scale reading of the meter accurately.

The balanced bridge VTVM has the following advantages over an ordinary VTVM (i.e. a VTVM using a single tube):

1. Effect of variations in the tube characteristics is reduced.
2. Fluctuations in the power supply have a smaller effect on the measuring circuit.
3. There is a lesser tendency for the meter zero to shift during the meter operation.

(b) Balanced Bridge TVM.

The circuit diagram for a balanced bridge TVM is shown in figure.

In balanced bridge TVM two identical transistors Q₁ and Q₂ are used. When the positive input dc voltage is applied to the base of transistor Q₁, emitter current of this transistor increases which causes the increase in voltage drop across resistor R_L and therefore, reduces the potential of collector of transistor Q₁ With the increase in emitter current, voltage drop across resistance R_E increases and so the emitter current of transistor Q₂ is reduced. This increases the potential of the collector of transistor Q₂ and a current proportional to the applied input voltage starts flowing in the ammeter connected between collectors of transistors Q₁ and Q₂.

Resistor R₁ is used to achieve zero position in ammeter under the absence of dc input voltage. Resistor R₂ is used for calibration purpose.

Main advantage of VTM is that if both transistors Q₁ and Q₂ are similar then with the change in ambient temperature, p values of both the transistors will change equally so there will be no effect on collector current in changed ambient temperature condition. Also with the change in ambient temperature, saturation current of both the transistors will change equally and therefore the meter reading will remain unaffected.

Resistor R_E also provides negative feedback for transistors Q₁ and Q₂ as any increase in ambient temperature also increases the emitter current which in turn is limited by the increase in the voltage drop across resistor R_E which is normally of high magnitude. Thus accuracy of calibration of TVM is maintained over a wide range of temperature.

Another advantage of TVM is that any fluctuation in power supply V_CC and V_EE does not affect its performance as it changes the biasing of both the transistors equally. Main disadvantage of balanced bridge TVM is that it offers less input impedance comparatively. But by using FET in the circuit of TVM, very high input impedance can be achieved. In the circuit of this TVM, two resistors R_L and the internal resistances of transistors Q₁ and Q₂ form the arms of a balanced bridge and the current flows through the meter when external voltage is applied to the base of transistor Q₁, which disturbs the balance of the bridge. That is why, this is called the balanced bridge TVM.

S:circuitstoday.com

versatile multimeter (or VOM)

It is one of the most versatile general purpose instruments capable of measuring dc and ac voltages as well as current and resistances. The solid-state electronic multimeter (or VOM) generally consists of the following elements.

1. A balanced bridge dc am­plifier and a PMMC meter.
2. An attenuator in input stage to select the proper voltage range.

3. A rectifier for convert­ing of an ac input volt­age to proportionate dc value.

4. An internal battery and additional circuitry for providing the capability of resistance measure­ment.

5. A function switch for selecting various meas­urement functions of the meter such as voltage, current or resistance.

In addition, the instrument is usually provided with a built-in power supply for operation on ac mains and, in most cases, one or more batteries for operation as a portable test instrument.

The schematic diagram of a balanced-bridge dc amplifier using two field effect transistors (FETs) is given in figure. It is to be noted that two FETs used in circuit should be reasonably well matched for current gain to ensure circuit thermal stability. The two FETs and the source resistors R_x and R₂, together with zero adjust control resistor R₃, constitute a bridge circuit. The PMMC meter is connected between the source terminals of the FETs, representing two opposite corners of the bridge.

In the absence of input signal, the gate terminals of the FETs are at ground potential and the transistors operate under identical quiescent conditions. Ideally no current should flow through the PMMC movement but in practice, on account of some mismatch between the two FETs and slight tolerance differences in the values of various resistors a current does flow and causes the meter movement to deflect from zero position. This current is reduced zero by the adjust control resistor R₃. Now the bridge is balanced. With a positive input signal applied to the gate of input transistor T₁ its drain current creases causing the voltage at the source terminal to rise. The resulting unbalance between the two transistors T₁ and T₂ source voltages is shown by the meter movement, whose scale is calibrated in terms of the magnitude of the applied input voltage. The maximum voltage that can be applied to the gate of input transistor T₁ is deter­mined by its operating range, which is usually of the order of a few volts. The range of the bridge can easily be extended by employing an input attenuator or a range switch, as illustrated in figure.

The unknown dc input voltage is applied through a large resistor in the probe body to a resistive voltage divider. Thus, with the range switch in the 3-V position as illustrated, the voltage at the gate of the input transistor T₁ is developed across 8 MQ resistor of the total resistance of 11.3 MQ and the circuit is so arranged that the PMMC meter gives full scale deflection with 3 V applied to the tip of the probe. With the range switch in 1,200 V position, the gate voltage is developed across 20 kilo ohm of the total divider resistance of 11.3 Mega ohms and an input voltage of 1,200 V will he required to cause the full-scale meter deflection.

When the multimeter’s func­tion switch is placed in the OHM position, the unknown resistor is connected in series with an internal battery, and the PMMC meter simply measure the voltage drop across it (unknown resistor, R_x).

A typical circuit is given in figure. In the given circuit, a separate divider network, employed only for the meas­urement of resist­ance, provides a number of different resistance ranges. With the unknown resistor R_x con­nected to the OHM terminals of the multimeter, the battery supplies current through one of the range resistors and the unknown resistor to ground. Voltage drop across unknown resistor R_x, V_x is applied to the input of the bridge amplifier and the PMMC movement deflects. Since the voltage drop across R_x is directly proportional to its resistance, the meter scale can be calibrated in terms of Kilo ohm.

The worth noting point is that this instrument indicates increasing resistance from left to right whereas the conventional meter, indicates increasing resistance from right to left. This is because the electronic multimeter reads a large resistance as a higher voltage, whereas the conventional multimeter indicates a higher resistance as a smaller current.

S:circuitstoday.com