Simplification of the voltage indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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The author of the article brings to the attention of readers a modernized version of the once popular on-board voltage indicator.

The vehicle on-board voltage indicator [1], published almost ten years ago by E. Klimchuk, in my opinion, still remains one of the most successful designs for this purpose. This indicator does not require modifications to the car's dashboard; the readings are easy to read. The device allows you to reliably judge the main parameters of the system: battery-voltage stabilizer. The indicator has worked on my car for more than five years, confirming its usefulness, high stability and reliability.

However, if we apply a slightly different principle for implementing additional modes of operation of the control lamp and take advantage of the appearance on the market of elements that were previously inaccessible to radio amateurs, then it will be possible to significantly simplify the indicator, increasing its performance, especially when working together with a temperature-compensated voltage stabilizer [2] Number At the same time, the number of microcircuits is reduced from three to one, the number of passive elements is reduced, and the permissible supply voltage range has expanded to 3...30 V.

The schematic diagram of the voltage indicator is shown in Fig. 1. As in the prototype, to organize four modes of operation of the control lamp, two voltage comparators are used on the op-amp DA1.1 and DA1.2. The difference is that to obtain an additional switching threshold for the upper comparator in the circuit, not the high, but the low output voltage of the lower comparator in the circuit is used. Amplifier DA1.3 inverts the output signal of comparator DA1.2.

Thus, as the voltage at the terminals of the battery increases, logic combinations 1.1, 1.3, 01 and 11 are sequentially formed at the outputs of op amps DA00 and DA10.

A rectangular pulse generator is assembled on the DA1.4 op-amp, the repetition frequency of which depends on the ratings of the C2R15 circuit. The voltage “hysteresis” is provided by positive feedback through resistor R14. Typically, for such generators, the “hysteresis” is symmetrical with respect to the switching voltage of the op-amp, which is ensured by using the same resistance resistors in the voltage divider R11R12. In this case, the duty cycle of the pulses at the generator output is two.

When the ratio of the values ​​of the divider resistors changes, the “hysteresis”, without changing the loop width, ceases to be symmetrical, and therefore, the charging and discharging time of capacitor C2 turns out to be unequal, i.e., the duty cycle of the pulses changes. Moreover, if the comparator switching voltage exceeds half the supply voltage, the duty cycle increases. This principle is used to quickly recognize two generator operating modes of the test lamp.

A visual check of the indicator’s operation showed that at a certain optimal frequency of the generator, two modes can be obtained: in one, the lamp periodically goes out, and in the other, it periodically turns on. It was noticed that with a significant decrease in the frequency of lamp extinguishing (let’s call this mode that way), the duration of the lamp’s off state became such that the “integrity of the picture” in visual memory was disrupted, in other words, the process of the lamp’s transition from the on to the off state and back seemed to be divided into separate elements. This subjectively made both modes somewhat similar, and to determine the true one it was necessary to stop looking at the indicator for a second or two, concentrate and determine what is greater in the operation of the lamp - the sum of pauses or the sum of switches.

At the same time, by selecting the frequency, it was possible to ensure that both modes became an organic continuation of the neighboring main states of the control lamp - continuous glow and its complete absence.

So, if with the ignition on, but with the starter turned off and the engine not running (position I of the ignition switch), the lamp is constantly on, this indicates that the battery, if discharged, is moderately discharged.

If dips in brightness appear in the constant glow of the lamp, the battery needs to be recharged.

A similar picture is observed when the engine is running. If the voltage generated by the generator is within acceptable limits, the lamp is turned off and does not distract the driver. As soon as the voltage exceeds a level dangerous for electrical equipment, uniform short flashes of the control lamp will begin.

Naturally, all of the above is true with the appropriate choice of comparison thresholds, i.e., voltage values ​​at which the display modes change. With the values ​​of resistors R2, R4 and R9 indicated in the diagram, these thresholds are approximately equal to 12,2, 13,6 and 14,4 V.

It should be noted, however, that the generator frequency values ​​are still unequal, although they correspond to a combination favorable for psychological perception. Thus, the lamp extinguishing frequency is slightly lower than the switching frequency (with the ratings of passive elements indicated in the diagram - about 1,2 and 1,5 Hz, respectively).

Switching of the generator operating modes occurs as a result of changing the voltage polarity on the divider R11R12 - levels 01 and 10 at the outputs of the op amp DA1.1 and DA1.3. If the output levels coincide (11 and 00), the generator is inhibited and op-amp DA1.4 operates as a voltage follower, i.e., its output is either high or low voltage. When operating without load, the generator may be excited at a parasitic frequency.

A current amplifier loaded with an incandescent indicator lamp is assembled on transistor VT1. If an LED is used instead of a lamp, it is connected directly between resistor R16 and the common wire, the anode to the resistor.

A few words need to be said about the “hysteresis” of comparison thresholds. As in the original design, it can be adjusted by changing the ratio of the resistance values ​​of the resistor dividers R6R8 and R7R10. However, the indicator in question has a feature associated with a change in the generator load on op-amp DA1.4. Depending on the mode, the output current of the generator can vary from several microamps to several milliamps. This leads to a change in the voltage drop across resistor R13 of the smoothing filter C1R13 and, consequently, in the voltage thresholds. A similar effect, although weakly expressed, was observed in the prototype [1].

With the component ratings indicated in the diagram, the “hysteresis” of the first and third comparison thresholds does not exceed 20 mV, and of the second - about 250 mV! This is explained by the fact that the average current consumption in the generator and adjacent main modes is approximately the same, and voltage ripples are well suppressed by the C1R13 filter.

Significantly reducing the “hysteresis” of the second comparison threshold (to a value of less than 40 mV) is quite simple - just connect the positive output of the operational amplifier supply voltage (pin 4) to the right (according to the diagram) output of resistor R13. Nevertheless, I did not do this, since such dissimilarity seemed to me even preferable.

The fact is that the second comparison threshold separates two, in general, normal states of electrical equipment. On the other hand, slight fluctuations in the voltage in the on-board network are possible near this threshold (at idle speed of the engine or when the tension of the generator drive belt is weak), which, taking into account the thermal inertia of the lamp, makes it difficult to “read” the information. At the same time, the small “hysteresis” of the extreme values ​​of the controlled voltage ensures high accuracy of control, which is especially important when determining the degree of discharge of the battery.

Instead of the LM324DP microcircuit in the indicator, you can use its domestic analogue K1401UD2, you just need to keep in mind that it has the opposite arrangement of power pins: pin 4 must be supplied with -Up, and pin 11 - +Up [3]. The composite transistor VT1 can be replaced with a conventional one from the KT815 or KT817 series. Zener diode VD1 - any for stabilization voltage 4,7...7,5 V (for example, KS147G, KS156G, KC168A). It is advisable to use tantalum capacitor C1 (K53-1A, K53-18, etc.). Capacitor C2 (K73-17 for a rated voltage of 63 V) should be selected with the lowest possible temperature coefficient of capacitance.

All parts of the indicator are mounted on a printed circuit board made of foil fiberglass laminate 1,5 mm thick. The board drawing is shown in Fig. 2. The board is placed in a plastic box, which is mounted behind the instrument panel.

Setting up the indicator consists of setting the comparison thresholds using a selection of resistors R2, R4 and R9. How to do this is described in detail in [1]. I will only note that I consider it advisable to abandon the use of trimming resistors. As the practice of using the indicator has shown, there is no need to adjust the voltage thresholds.

In conclusion, it remains to add that it would be useful to try to slightly change the frequency of the generator to bring the display algorithm into more complete compliance with the individual characteristics of perception. It is advisable to do this with a lamp of the same type with which the indicator will work.

Literature

1. Klimchuk E. Voltage indicator. - Radio, 1993, No. 6, p. 35, 36.
2. Biryukov S. Simple temperature-compensated voltage regulator. - Radio, 1994, No. 1, p. 34, 35.
3. Petropavlovsky Yu. Components in household video equipment. - Radio, 2001, No. 7, p. 9-11.

Author: A.Martemyanov, Seversk, Tomsk region

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