The possibility of premature failure of an expensive battery forces the motorist to carefully monitor the operation of the voltage regulator relay and the state of the car's on-board electrical network. The voltage in it should not differ by more than ± 3% from the optimal value, which is determined for the given operating conditions of the battery and depends on the climatic zone, the location of the battery and its technical condition, and the mode of operation of the vehicle. The more precisely the optimal voltage is maintained when recharging the battery, the longer it will last.
The correct operation of the car generator is of great importance. With an increase in the generator voltage above the optimum by 10-12% (about 0.15 V), the service life of the battery and electric lamps is reduced by 2-2.5 times.
In order to accurately perform all the necessary adjustments, you need a special voltmeter that measures voltage in the range of 13-15 V with an accuracy of 0.1 V. It is difficult to buy such a device, but many will be able to make a similar one with a scale stretched in the range of 10-15 V. Increased measurement accuracy, a linear scale over the entire measurement range, the absence of its own power source, increased reliability (due to the protection elements provided in the device that do not affect measurement accuracy), the ability to adjust the "stretch" zone of the scale are the distinguishing features of this device. It is made on the basis of an operational amplifier and is a voltage difference meter.
Specifications of the voltmeter
- Range of measured voltages, V. . . 10 to 15
Achievable measurement error at a temperature of 20±5°C, not worse, % ...0.5
Discreteness, V. . . 0.05
Input resistance, not less than, kOhm. . . 0.75
Operating temperature range, °С. . . -10 to +35
Dimensions (with M906 microammeter), mm. . . 65x105x120
The voltmeter is powered directly from the measurement object. The initial offset relative to which the measurement is made is set by the resistance of the chain of resistors R3, R4 (see the circuit diagram in Fig. 1), and the feedback value (which determines the gain of the OUDA1 and, accordingly, the degree of "stretching" of the range) is set by the resistance of the chain of resistors R5, R6.
The reference voltage source on the zener diode VD3 also provides a potential shift at the non-inverting input DA by an amount equal to approximately half of the measured voltage drop, which is necessary for the operation of the op-amp with a unipolar supply.
The resistance of the resistor R7 depends on the sensitivity of the microammeter PA and the value of the maximum output voltage of the operational amplifier relative to the cathode of the zener diode VD3.
Diodes VD1, VD2 protect the op-amp, and VD4, VD5 protect the microammeter from overcurrent. VD1 prohibits the passage of negative current through the resistor R1 and the operational amplifier. It is possible to pass current through a zener diode VD3, biased in the forward direction, a diode VD2 and resistors R2-R4. Thus, a potential difference of not more than 0.7 V will be established between the DA inputs (pins 3.2). A similar voltage drop will be at pin 3 relative to pin 4 of the op-amp.
This ensures reliable protection of the op amp from errors when connecting the polarity.
The voltmeter uses fixed resistors of the MLT type, it is desirable to use multi-turn types SP5-2, SP5-3, SP5-14 as tuned resistors. It is permissible to use other types of op amps, for example, K140UD7 or K140UD1A, K553UD1 with the appropriate correction circuits. Diodes - any low-power silicon. The KS147A zener diode can be replaced with a KS156A, but, probably, then the temperature stability of the voltmeter will deteriorate and it will be necessary to clarify the values of the resistors R1-R3. Microammeter - M906 or M24 with a total deflection current of 50 μA and a scale corresponding to the selected measurement zone. It is also possible to use other pointer devices with a total deviation current of up to 1 mA, but in this case it is necessary to select the value of the resistor R5, based on the selected value of the voltage drop across it (about 1.5 V). You can also use the avometer in microammeter mode. Then this device will be made in the form of a prefix to the tester.
In the absence of defective elements and installation errors, the adjustment of the voltmeter is reduced to its calibration. This operation is performed using an adjustable power supply with an output voltage of 9-16 V and an exemplary voltmeter, preferably digital, for example B7-16, FZO, VR-11.
Trimmer resistors are set to the middle position and a voltage of 12-13 V is applied to the input of the voltmeter, controlling it using a standard instrument. The pointer of the adjusted voltmeter should deviate from zero. Then, a voltage of 10 V (± 0.05 V) is set at the output of the power source, and the voltmeter needle is set to zero division of the scale with resistor R4. Then, by increasing the measured voltage to 15 ± 0.05 V, the arrow is set to the final division of the scale with resistor R6. By repeating these operations for 10 V and 15 V, they achieve the most accurate setting of the voltmeter in the operating range of 13-14.5 V.
During the establishment of the relay-regulator, the voltage is measured directly at the battery terminals.
Figure 2 shows a printed circuit board with a layout of elements. The board is installed on the contact bolts of the M906 microammeter and placed with it in the box.
Rice. 2
V. Bakanov, E. Kachanov, Chernivtsi, Model Designer No. 12, 1990, p.27
List of radio elements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| DA | OU | K140UD6 | 1 | K140UD7, K140UD1A, K553UD1 | To notepad | |
| VD1, VD2, VD4, VD5 | Diode | KD521V | 4 | To notepad | ||
| VD3 | zener diode | KS147A | 1 | To notepad | ||
| C1 | electrolytic capacitor | 4.7uF 20V | 1 | To notepad | ||
| C2 | Capacitor | 0.1uF | 1 | To notepad | ||
| R1 | Resistor | 510 ohm | 1 | To notepad | ||
| R2 | Resistor | 15 kOhm | 1 | To notepad | ||
| R3 | Resistor | 8.2 kOhm | 1 | MLT, selection | To notepad | |
| R4 | Trimmer resistor | 4.7 kOhm | 1 |
Something often began to ask me questions on analog electronics. Did the session of the students take the balls? ;) Okay, it's time to move a little educational program. In particular, on the operation of operational amplifiers. What is it, what is it eaten with and how to calculate it.
What is this
An op-amp is a two-input amp, nevie... uhm... big signal gain and one output. Those. we have U out \u003d K * U in, and K is ideally equal to infinity. In practice, of course, there are more modest numbers. Let's say 1000000. But even such numbers explode the brain when trying to apply them directly. Therefore, as in kindergarten, one Christmas tree, two, three, many Christmas trees - we have a lot of reinforcement here;) And that's it.
And there are two entrances. And one of them is direct, and the other is inverse.
Moreover, the inputs are high-impedance. Those. their input impedance is infinity in the ideal case and VERY high in the real one. The account there goes to hundreds of Megaohms, and even to gigaohms. Those. it measures the voltage at the input, but it is minimally affected. And we can assume that the current in the op-amp does not flow.
The output voltage in this case is calculated as:
U out \u003d (U 2 -U 1) * K
Obviously, if the voltage at the direct input is greater than at the inverse, then the output is plus infinity. Otherwise, it will be minus infinity.
Of course, in a real circuit, there will be no plus and minus infinity, and they will be replaced by the highest and lowest power supply voltage of the amplifier. And we will get:
comparator
A device that allows you to compare two analog signals and make a verdict - which of the signals is greater. Already interesting. You can think of a lot of applications for it. By the way, the same comparator is built into most microcontrollers, and I showed how to use it using the AVR as an example in articles about creating . Also, the comparator is wonderfully used to create .
But the matter is not limited to one comparator, because if you introduce feedback, then a lot can be done from the op-amp.
Feedback
If we take the signal from the output and send it directly to the input, then feedback will occur.
positive feedback
Let's take and drive the signal directly from the output into the direct input.
- The voltage U1 is greater than zero - the output is -15 volts
- The voltage U1 is less than zero - at the output +15 volts
What happens if the voltage is zero? In theory, the output should be zero. But in reality, the voltage will NEVER be zero. After all, even if the charge of the right one outweighs the charge of the left by one electron, then this is already enough to roll the potential to the output at an infinite amplification. And at the output, a shaped hell will begin - signal jumps here and there at the speed of random disturbances induced at the inputs of the comparator.
Hysteresis is introduced to solve this problem. Those. a kind of gap between switching from one state to another. To do this, introduce positive feedback, like this:
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We consider that at the inverse input at this moment +10 volts. At the output from the op-amp, minus 15 volts. At the direct input, it is no longer zero, but a small part of the output voltage from the divider. Approximately -1.4 volts Now, until the voltage at the inverted input drops below -1.4 volts, the output of the op-amp will not change its voltage. And as soon as the voltage drops below -1.4, then the output of the op-amp will jump sharply to +15 and there will already be a +1.4 volt bias at the direct input.
And in order to change the voltage at the output of the comparator, the signal U1 will need to increase by as much as 2.8 volts to get to the upper bar of +1.4.
There is a kind of gap where there is no sensitivity, between 1.4 and -1.4 volts. The gap width is controlled by the ratios of the resistors in R1 and R2. The threshold voltage is calculated as Uout/(R1+R2) * R1 Let's say 1 to 100 will give +/-0.14 volts.
But still, the op-amp is more often used in the negative feedback mode.
negative feedback
Okay, let's put it another way:
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In the case of negative feedback, the op amp has an interesting property. It will always try to adjust its output voltage so that the voltages at the inputs are equal, resulting in a zero difference.
Until I read this in the great book from comrades Horowitz and Hill, I could not get into the work of the OU. But everything turned out to be simple.
Repeater
And we got a repeater. Those. at the input U 1 , at the inverse input U out = U 1 . Well, it turns out that U out \u003d U 1.
The question is what for we are such happiness? It was possible to throw the wire directly and no op-amp would be needed!
It is possible, but not always. Imagine such a situation, there is a sensor made in the form of a resistive divider:
 |
The lower resistance changes its value, the layout of the output voltage from the divider changes. And we need to take readings from it with a voltmeter. But the voltmeter has its own internal resistance, albeit large, but it will change the readings from the sensor. Moreover, if we do not want a voltmeter, but want a light bulb to change brightness? There is no way to connect a light bulb here! Therefore, the output is buffered by an operational amplifier. Its input resistance is huge and it will have minimal effect, and the output can provide a quite tangible current (tens of milliamps, or even hundreds), which is quite enough for the light bulb to work.
In general, applications for the repeater can be found. Especially in precision analog circuits. Or where the circuitry of one stage can affect the operation of another, to separate them.
Amplifier
And now let's do a feint with our ears - let's take our feedback and put it on the ground through a voltage divider:
 |
Now half the output voltage is applied to the inverted input. And the amplifier still needs to equalize the voltages at its inputs. What will he have to do? That's right - raise the voltage at your output twice as high as before to compensate for the divider that has arisen.
Now there will be U 1 on the straight line. On the inverse U out /2 \u003d U 1 or U out \u003d 2 * U 1.
Let's put a divisor with a different ratio - the situation will change in the same way. In order not to turn the voltage divider formula in your mind, I will immediately give it:
U out \u003d U 1 * (1 + R 1 / R 2)
It is mnemonically remembered what is divided into what is very simple:
It turns out that the input signal goes through the circuit of resistors R 2 , R 1 in U out . In this case, the direct input of the amplifier is set to zero. We recall the habits of the op-amp - it will try by hook or by crook to make sure that a voltage equal to the direct input is formed at its inverse input. Those. zero. The only way to do this is to lower the output voltage below zero so that zero occurs at point 1.
So. Imagine that U out =0. While equal to zero. And the input voltage, for example, is 10 volts relative to U out. The divisor of R 1 and R 2 will divide it in half. Thus, at point 1 there are five volts.
Five volts is not equal to zero and the op amp lowers its output until there is zero at point 1. To do this, the output should be (-10) volts. In this case, the difference will be 20 volts relative to the input, and the divider will provide us with exactly 0 at point 1. We got an inverter.
But you can also choose other resistors so that our divider gives out other coefficients!
In general, the formula for the gain for such an amplifier will be as follows:
U out \u003d - U in * R 1 / R 2
Well, a mnemonic picture for quickly memorizing xy from xy.
Let's say U 2 and U 1 will be 10 volts each. Then at the 2nd point there will be 5 volts. And the output will have to become such that at the 1st point it also becomes 5 volts. That is, zero. So it turns out that 10 volts minus 10 volts equals zero. Everything is right :)
If U 1 becomes 20 volts, then the output will have to drop to -10 volts.
Calculate for yourself - the difference between U 1 and U out will be 30 volts. The current through the resistor R4 will be (U 1 -U out) / (R 3 + R 4) = 30/20000 = 0.0015A, and the voltage drop across the resistor R 4 will be R 4 * I 4 = 10000 * 0.0015 = 15 volts . Subtract the 15 volt drop from the input 20 and get 5 volts.
Thus, our op-amp solved the arithmetic problem from 10 subtracted 20, getting -10 volts.
Moreover, in the problem there are coefficients determined by resistors. It's just that, for simplicity, the resistors are of the same value, and therefore all the coefficients are equal to one. But in fact, if we take arbitrary resistors, then the dependence of the output on the input will be as follows:
U out \u003d U 2 * K 2 - U 1 * K 1
K 2 \u003d ((R 3 + R 4) * R 6) / (R 6 + R 5) * R 4
K 1 \u003d R 3 / R 4
The mnemonics for memorizing the coefficient calculation formula is as follows:
Straight to the diagram. The numerator of the fraction is at the top, so we add the upper resistors in the current flow circuit and multiply by the lower one. The denominator is at the bottom, so add the lower resistors and multiply by the upper one.
Everything is simple here. Because point 1 is constantly reduced to 0, then we can assume that the currents flowing into it are always equal to U / R, and the currents entering node number 1 are summed up. The ratio of the input resistor to the feedback resistor determines the weight of the incoming current.
There can be as many branches as you like, but I drew only two.
U out \u003d -1 (R 3 * U 1 / R 1 + R 3 * U 2 / R 2)
The input resistors (R 1 , R 2 ) determine the amount of current, and hence the total weight of the incoming signal. If you make all the resistors equal, like mine, then the weight will be the same, and the multiplication factor of each term will be equal to 1. And U out \u003d -1 (U 1 + U 2)
Adder non-inverting
Everything is a bit more complicated, but it seems.
 |
Uout \u003d U 1 * K 1 + U 2 * K 2
K 1 \u003d R 5 / R 1
K 2 \u003d R 5 / R 2
Moreover, the feedback resistors must be such that the equation R 3 / R 4 \u003d K 1 + K 2 is observed
In general, on operational amplifiers, you can create any math, add, multiply, divide, count derivatives and integrals. And almost instantly. Analog computers are made at the OU. I even saw one of these on the fifth floor of SUSU — a fool the size of a room floor. Several metal cabinets. The program is typed by connecting different blocks with wires :)
This article focuses on two voltmeters implemented on the PIC16F676 microcontroller. One voltmeter has a voltage range of 0.001 to 1.023 volts, the other, with an appropriate 1:10 resistive divider, can measure voltages from 0.01 to 10.02 volts. The current consumption of the entire device with a stabilizer output voltage of +5 volts is approximately 13.7 mA. The voltmeter circuit is shown in Figure 1.
Two voltmeter circuit
Digital voltmeter, circuit operation
To implement two voltmeters, two outputs of the microcontroller are used, configured as input for the digital conversion module. The RA2 input is used to measure low voltages, in the region of a volt, and a 1:10 voltage divider is connected to the RA0 input, consisting of resistors R1 and R2, which allows you to measure voltages up to 10 volts. This microcontroller uses ten-bit ADC module and in order to implement a voltage measurement with an accuracy of 0.001 volts for a range of 1 V, it was necessary to apply an external reference voltage from the ION of the DA1 K157XP2 microcircuit. Since the power AND HE the microcircuit is very small, and in order to exclude the influence of external circuits on this ION, a buffer op-amp on the DA2.1 microcircuit was introduced into the circuit LM358N. This is a non-inverting voltage follower with 100% negative feedback - OOS. The output of this op-amp is loaded with a load consisting of resistors R4 and R5. From the trimmer resistor R4, a reference voltage of 1.024 V is applied to pin 12 of the microcontroller DD1, configured as a reference voltage input for operation ADC module. At this voltage, each bit of the digitized signal will be equal to 0.001 V. To reduce the effect of noise, another voltage follower, implemented on the second op amp of the DA2 chip, was used when measuring small voltage values. The OOS of this amplifier sharply reduces the noise component of the measured voltage value. The voltage of impulse noise of the measured voltage also decreases.
A two-line LCD was used to display information about the measured values, although one line would be enough for this design. But having the ability to display some more information in reserve is also not bad. The brightness of the indicator backlight is regulated by resistor R6, the contrast of the displayed characters depends on the value of the resistors of the voltage divider R7 and R8. The device is powered by a voltage regulator assembled on the DA1 chip. The +5 V output voltage is set by resistor R3. To reduce the total current consumption, the supply voltage of the controller itself can be reduced to a value at which the indicator controller would remain operational. When checking this circuit, the indicator worked steadily at a microcontroller supply voltage of 3.3 volts.
Voltmeter setting
Setting up this voltmeter requires at least a digital multimeter capable of measuring 1.023 volts to set the reference voltage of the reference. And so, using a control voltmeter, we set a voltage of 1.024 volts at pin 12 of the DD1 microcircuit. Then, at the input of the op-amp DA2.2, pin 5, we apply a voltage of a known value, for example, 1,000 volts. If the readings of the control and adjustable voltmeters do not match, then the trimming resistor R4, by changing the value of the reference voltage, achieves equivalent readings. Then, a control voltage of a known value is applied to the input U2, for example, 10.00 volts, and by selecting the resistance value of the resistor R1, it is possible and R2, or both can achieve equivalent readings of both voltmeters. This completes the adjustment.
Comparators
If you use an operational amplifier without negative feedback (NFB), then you can definitely say what happens. In order to understand how it works, you can do a few simple but visual experiments. To do this, you need a little: the actual operational amplifier, a power supply with a voltage of 9 ... 25V, several resistors, a pair of LEDs and a voltmeter ().
The simplest logic probe is assembled from LEDs and resistors, as shown in Figure 1.
When a positive voltage is applied to the input of the probe (you can even apply + U), the red LED lights up, and if the input is connected to a common wire, the green one lights up. With the help of such a probe, the output state of the op-amp under test becomes clear and understandable.
Any not very high-quality and expensive one is suitable as an experimental "rabbit", for example, KR140UD608 (708) in plastic cases or K140UD6 (7) in round metal cases.
Figure 1. Diagram of a simple logic probe
It should be noted that despite the different cases, the pinout of these microcircuits is the same and corresponds to that shown in the diagrams below. It often happens that the pinout of plastic and metal cases does not match, although in fact these are the same microcircuits. Now most of the operational amplifiers, especially imported ones, are produced in plastic cases, and everything works well and perfectly, and there is no confusion with pinouts. And earlier, such "plastic" microcircuits were contemptuously called "Shirpotrebovsky" by specialists.
Figure 2. Operational amplifier circuit
For the first experiments, we will assemble the circuit shown in Figure 2. Not much has been done here: the operational amplifier itself and the logic probe shown in Figure 1 are connected to a unipolar power supply. Supply voltage + U unipolar value 9 ... 30V. The magnitude of the voltage in our experiments is of no particular importance.
Here a completely legitimate question may arise: “Why is the probe logical, because the operational amplifier is an analog element?”. Yes, but in this case, the operational amplifier does not operate in gain mode, but in comparator mode, and has only two levels at the output. A voltage close to 0V is called a logical zero, and a voltage close to +U is called a logical one. In the case of a bipolar power supply, a logical zero corresponds to a voltage close to -U.
When the supply voltage is applied, one of the LEDs must necessarily light up. It is impossible to answer the question of which one, red or green, since everything depends on the parameters of a particular operational amplifier and on external conditions, for example, from network interference. If you take several of the same type of op amps, then the results will be very different.
The voltage at the output of the operational amplifier is controlled by a voltmeter: if the red LED is on, the voltmeter will show a voltage close to + U, and if the green LED is on, the voltage will be almost zero.
Now you can try to apply some voltage to the inputs and look at the indicators and the voltmeter how the operational amplifier will behave. The easiest way to apply voltage is to touch each input of the operational amplifier with one finger in turn, and with the other one of the power leads. In this case, the glow of the probe and the readings of the voltmeter should change. But these changes may not happen.
The thing is, some op amps are designed to have input voltages within certain limits: slightly higher than the voltage at pin 4 and slightly lower than the supply voltage at pin 7. This "somewhat lower, higher" is 1 …2V. To continue the experiments, having fulfilled the specified condition, you will have to assemble a slightly more complex circuit, shown in Figure 3.
Figure 3
Now the voltage is applied to the inputs using variable resistors R1, R2, the sliders of which should be set near the middle position before starting measurements. The voltmeter has now moved to a different location: it will show the voltage difference between the direct and inverse inputs.
It is better if this voltmeter is digital: the polarity of the voltage can change, a minus sign will appear on the indicator of the digital device, and the pointer device will simply go off scale in the opposite direction. (You can use a pointer voltmeter with a mid-scale point.) In addition, the input impedance of a digital voltmeter is much higher than that of a pointer meter, therefore, the measurement results will be more accurate. The output status will be determined by the LED indicator.
Here it is appropriate to give such advice: it is better to do these simple experiments with your own hands, and not just read and decide that everything is simple and clear. It's like reading a guitar tutorial without picking up the guitar once. So, let's begin.
The first thing to do is set the variable resistor sliders to about the middle position, while the voltage at the inputs of the operational amplifier is close to half the supply voltage. The sensitivity of the voltmeter should be made maximum, but perhaps not immediately, but gradually, so as not to burn the device.
Assume that the output of the op-amp is low and the green LED is lit. If this is not the case, then this state can be achieved by rotating the variable resistor R1 in such a way that the engine moves down the circuit - almost to 0V.
Now, using the variable resistor R1, we will begin to add voltage to the direct input of the operational amplifier (pin 3), observing the readings of the voltmeter. As soon as the voltmeter shows a positive voltage (the voltage at the direct input (pin 3) is greater than at the inverse input (pin 2)) the red LED will light up. Therefore, the voltage at the output of the operational amplifier is high or, as previously agreed, a logical unit.
A little help
More precisely, not even a logical unit, but a high level: a logical unit indicates the truth of the signal, they say, the event has occurred. But this truth, this logical unit, can also be expressed at a low level. As an example, we can recall the RS-232 interface, in which a logical one corresponds to a negative voltage, while a logical zero has a positive voltage. Although in other schemes, a logical unit is most often expressed by a high level.
Let's continue the scientific experiment. Let's start carefully and slowly rotate the resistor R1 in the opposite direction, following the readings of the voltmeter. At a certain point, it will show zero, but the red LED will still glow. It is unlikely that it will be possible to catch the position in which both LEDs are extinguished.
With further rotation of the resistor, the polarity of the voltmeter readings will also change to negative. This indicates that the voltage at the inverse input (2) is higher in absolute value than at the direct input (3). The green LED will turn on, which indicates a low level at the output of the operational amplifier. After that, you can continue to rotate the resistor R1 in the same direction, but no changes will occur: the green LED will not go out and will not even change the brightness at all.
This phenomenon occurs when the operational amplifier operates in comparator mode, i.e. without negative feedback (sometimes even with PIC). If the op-amp operates in a linear mode, covered by negative feedback (NFB), then when the engine of the resistor R1 rotates, the output voltage changes in proportion to the angle of rotation, read the voltage difference at the inputs, and not at all a step. In this case, the brightness of the LED can be changed smoothly.
From all of the above, we can conclude that the voltage at the output of the operational amplifier depends on the difference in voltages at the inputs. In the case when the voltage at the direct input is higher than at the inverse one, the output voltage is high. Otherwise (the voltage on the inverse is higher than on the direct), the output is a logic zero level.
At the very beginning of this experiment, it was recommended to set the sliders of resistors R1, R2 approximately to the middle position. And what will happen if you initially set them to a third of the turnover or two-thirds? Yes, actually nothing will change, everything will work the same way as described above. From this we can conclude that the signal at the output of the operational amplifier does not depend on the absolute value of the voltages at the direct and inverse inputs. It just depends on the voltage difference.
From all of the above, one more important conclusion can be drawn: an operational amplifier without feedback is a comparator - a comparing device. In this case, a reference or exemplary voltage is applied to one input, and a voltage, the value of which must be controlled, is supplied to the other. Which input to apply the reference voltage to is decided during the design of the circuit.
As an example, Figure 4 shows a circuit, at the input of which there are 2 internal comparators DA1 and DA2 at once.
Figure 4 NE555 integrated timer circuit
Their purpose is to manage the internal. The control logic is quite simple: a logical unit from the output of the DA2 comparator sets the trigger to one, and a logical unit from the output of the DA1 comparator resets the trigger.
A divider is assembled on resistors R1 ... R3, which supplies reference voltages to the inputs of the comparators. All three resistors have the same resistance (5Kom), forming voltages of 2/3 and 1/3 of the supply voltage, which are applied, respectively, to the inverting input DA1 and to the non-inverting input DA2.
In terms of what was written above, it turns out that the logical unit at the output of the DA1 comparator will turn out if the input voltage at the direct input exceeds the reference at the inverse (2 / 3Upit.), The trigger will reset to zero.
In order to set the trigger to 1, you need to get a high level at the output of the internal comparator DA2. This state will be reached when the voltage level at the inverse input DA2 is less than 1/3Upit. It is this reference voltage that is applied to the direct input of the comparator DA2.
The purpose of describing the NE555 integrated timer is not set here, just as an example of using the op-amp, input comparators are shown hidden inside the microcircuit. For those who are interested in using the 555 timer, you can recommend reading the article.
RF voltmeter with linear scale
Robert AKOPOV (UN7RX), Zhezkazgan, Karaganda region, Kazakhstan
One of the necessary devices in the arsenal of a shortwave radio amateur, of course, is a high-frequency voltmeter. Unlike a low-frequency multimeter or, for example, a compact LCD oscilloscope, such a device is rarely found on sale, and the cost of a new branded one is quite high. Therefore, when there was a need for such a device, it was built, moreover, with a dial milliammeter as an indicator, which, unlike a digital one, allows you to easily and visually evaluate changes in readings quantitatively, and not by comparing the results. This is especially important when setting up devices where the amplitude of the measured signal is constantly changing. At the same time, the measurement accuracy of the device when using a certain circuitry is quite acceptable.
There is a typo in the diagram in the magazine: R9 should be a resistance of 4.7 MΩ
RF voltmeters can be divided into three groups. The first ones are built on the basis of a broadband amplifier with the inclusion of a diode rectifier in the negative feedback circuit. The amplifier ensures the operation of the rectifier element in the linear section of the current-voltage characteristic. In devices of the second group, a simple detector with a high-resistance DC amplifier (HPA) is used. The scale of such an RF voltmeter at the lower measurement limits is non-linear, which requires the use of special calibration tables or individual calibration of the device. An attempt to somewhat linearize the scale and shift the sensitivity threshold down by passing a small current through the diode does not solve the problem. Before the beginning of the linear section of the I–V characteristic, these voltmeters are, in fact, indicators. Nevertheless, such devices, both in the form of finished designs and attachments to digital multimeters, are very popular, as evidenced by numerous publications in magazines and on the Internet.
The third group of instruments uses scale linearization, when the linearizing element is included in the DCF circuit to provide the necessary gain change depending on the input signal amplitude. Such solutions are often used in professional equipment units, for example, in broadband high-linear instrumentation amplifiers with AGC, or AGC units of broadband RF generators. It is on this principle that the described device is built, the circuit of which, with minor changes, is borrowed from.
With all the obvious simplicity, the RF voltmeter has very good parameters and, of course, a linear scale that eliminates calibration problems.
The measured voltage range is from 10 mV to 20 V. The operating frequency band is 100 Hz…75 MHz. The input resistance is at least 1 MΩ with an input capacitance of no more than a few picofarads, which is determined by the design of the detector head. The measurement error is no worse than 5%.
The linearizing unit is made on the DA1 chip. Diode VD2 in the negative feedback circuit helps to increase the gain of this stage of the UPT at low input voltages. The decrease in the output voltage of the detector is compensated, as a result, the readings of the device acquire a linear dependence. Capacitors C4, C5 prevent self-excitation of the UPT and reduce possible pickups. The variable resistor R10 serves to set the pointer of the measuring device PA1 to the zero mark of the scale before taking measurements. In this case, the input of the detector head must be closed. The power supply of the device has no special features. It is made on two stabilizers and provides a bipolar voltage of 2 × 12 V for powering operational amplifiers (the network transformer is not conventionally shown in the diagram, but is included in the assembly kit).
All parts of the device, with the exception of the parts of the measuring probe, are mounted on two printed circuit boards made of one-sided foil fiberglass. Below is a photograph of the UPT board, the power board and the measuring probe.
Milliammeter RA1 - M42100, with a current of full deflection of the needle 1 mA. Switch SA1 - PGZ-8PZN. Variable resistor R10 - SP2-2, all tuning resistors - imported multi-turn, for example 3296W. Resistors of non-standard ratings R2, R5 and R11 can be made up of two connected in series. Operational amplifiers can be replaced by others with high input impedance and preferably with internal correction (so as not to complicate the circuit). All fixed capacitors are ceramic. Capacitor C3 is mounted directly on the input connector XW1.
The D311A diode in the RF rectifier was chosen from the point of view of the optimal maximum allowable RF voltage and rectification efficiency at the upper measured frequency boundary.
A few words about the design of the instrument's measuring probe. The body of the probe is made of fiberglass in the form of a tube, on top of which a copper foil screen is put on.
Inside the case there is a board made of foil fiberglass, on which the probe parts are mounted. A ring of tinned foil strip approximately in the middle of the body is provided to make contact with the common wire of a detachable divider, which can be screwed on in place of the probe tip.
The adjustment of the device begins with the balancing of the op-amp DA2. To do this, switch SA1 is set to the "5 V" position, the input of the measuring probe is closed, and the pointer of the device PA1 is set to the zero mark of the scale with a trimming resistor R13. Then the device is switched to the “10 mV” position, the same voltage is applied to its input, and the arrow of the RA1 device is set to the last division of the scale with the resistor R16. Next, a voltage of 5 mV is applied to the input of the voltmeter, the arrow of the device should be approximately in the middle of the scale. The linearity of the readings is achieved by selecting the resistor R3. Even better linearity can be achieved by selecting the resistor R12, however, it should be borne in mind that this will affect the gain of the UPT. Next, the device is calibrated on all subranges with the corresponding tuning resistors. As a reference voltage when calibrating the voltmeter, the author used an Agilent 8648A generator (with a 50 Ohm load equivalent connected to its output), which has a digital output signal level meter.
The entire article from the magazine Radio No. 2, 2011 can be downloaded from here
LITERATURE:
1. Prokofiev I., Millivoltmeter-Q-meter. - Radio, 1982, No. 7, p. 31.
2. Stepanov B., RF head for a digital multimeter. - Radio, 2006, No. 8, p. 58, 59.
3. Stepanov B., Schottky diode RF voltmeter. - Radio, 2008, No. 1, p. 61, 62.
4. Pugach A., High-frequency millivoltmeter with a linear scale. - Radio, 1992, No. 7, p. 39.
The cost of printed circuit boards (probe, main board and power supply board) with a mask and marking: 80 UAH
