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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Voltage stabilizer with thermal compensation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Surge Protectors The voltage stabilizer is one of the most important components of the electrical system of a modern car. For this reason, articles on the design and operation of the node appeared on the pages of the Radio magazine more than once. And yet, apparently, it’s too early to put an end to this topic ... The most successful designs of the stabilizer published in "Radio", for example, [1; 2], allow you to maintain the optimal charge of the battery at different temperatures. The article [3] describes a voltage stabilizer with pulse-width control, which differs from similar ones by the constancy of the operating frequency. Along with the obvious advantages of these devices, they also have a significant drawback - a significant power of their own losses. In my version of the stabilizer, the power loss is reduced by a factor of three, which made it possible to eliminate the problem of heat removal from the output elements of the device. To ensure maximum thermal compensation, the temperature sensor is immersed directly in the battery electrolyte solution. The stabilizer is simpler in design, but has better voltage stabilization. It is known that in the "classic" models of VAZ cars, due to the relative remoteness of the stabilizer 121.3702 from the generator and battery, it is not possible to accurately monitor the voltage at the battery terminals due to the voltage drop on the connecting wires, connector contacts. Because of this, stabilization is very conditional. Measurements have shown that instability even in a new car can reach several hundred millivolts. The stabilizer brought to the attention of readers is intended for installation instead of node 121.3702 and has the following main technical characteristics:
When developing the stabilizer, the ideas proposed in [1-3], as well as the experience of operating a car in various weather conditions, were taken into account. The schematic diagram of the device is shown in fig. 1. Functionally, it consists of two parts - measuring A1 and regulating A2. The board with the measuring part is mounted near the battery, and with the regulating part - in place of the former stabilizer.
When the SA1 contacts are closed, the electronic switch opens, the role of which is played by the field-effect transistor VT1, and connects voltage and temperature sensors to the battery GB1, forming a bridge measuring element. The voltage sensor is a resistive divider R5R6, and the temperature sensor is a series circuit of VD1-VD4 diodes. The signal taken from the diagonal of the bridge is fed to the input of a differential amplifier. The amplified signal is converted into a pulse sequence with a variable duty cycle proportional to the signal level. The pulse frequency is determined by the auxiliary sawtooth voltage generator. Further, the signal after current amplification is fed to the output switch. The main link of the stabilizer is the pulse-width controller DD1, which includes the mentioned differential amplifier, generator, converter and current amplifier. The use of a push-pull synchronous switch, made on field-effect transistors VT3-VT5, can significantly reduce power losses. In a conventional electrical system, when the ignition is turned on, current begins to flow through the excitation winding of the generator, and if the engine start is delayed for one reason or another, energy is wasted on heating it. To eliminate this drawback, a blocking device is introduced into the described stabilizer, electrically connected to the oil pressure sensor. In other words, until the engine has entered the operating mode (and the indicator lamp "No oil pressure" is on on the instrument panel), no current is supplied to the excitation winding. In the initial state, the contacts of the SA1 ignition switch are open, and the contacts of the oil pressure sensor SF1 are closed. Switch VT1 is closed. When the ignition is turned on, transistors VT2 and VT1 open, the voltage from the battery GB1 is supplied to the voltage and temperature sensors. The use of a field-effect transistor with an induced channel for a switch is due, firstly, to the ease of opening and closing control, secondly, to the absence of residual voltage characteristic of bipolar transistors, and, thirdly, to the low resistance of the open channel. At the same time, the control lamp HL1 turns on on the dashboard of the car, indicating the absence of oil pressure. The current determined by the resistor R7 does not yet flow through the diodes VD1-VD4, as it closes through the internal diode of the controller DD1, connected between terminals 1 and 2, and closed contacts SF1 to a common wire. The description of the operating principle of the K1156EU1 controller and its electrical parameters are omitted here, but they can be found in [4; 5], since it is an analogue of the well-known Motorola uA78S40 controller. Since the voltage at the non-inverting input (pin 6) of the internal op-amp of the DD1 microcircuit, switched on by a differential amplifier, is greater than at the inverting input (pin 7), a high level is present at its OAout output (pin 4). A bias voltage equal to half the supply voltage is applied to the non-inverting input of the CMP (pin 9) of the comparator from the R12R13 divider, and since there is a high level at the inverting input (pin 10), the voltage at the output of the comparator is close to zero. The logic of the controller is such that if the output of the comparator is low, it is forbidden to turn on the internal output transistor of the current amplifier. This amplifier has a single-ended output, and the synchronous commutator requires bi-phase control for proper operation. For this purpose, a phase inverter on a VT3 field-effect transistor was introduced into the stabilizer. The voltage divider R15-R17 provides opening of transistors VT3, VT5, and VT4 is closed, since the voltage drop across the resistor R19 does not exceed the cutoff voltage. The voltage boost capacitor C3 is charged with current through the VD5 diode and the VT5 transistor to the supply voltage. After starting the engine, the contacts SF1 of the oil pressure sensor open and the lamp HL1 goes out. The current through the internal diode of the controller DD1 (pins 1 and 2) is interrupted and begins to flow through the temperature sensor VD1 - VD4, a voltage proportional to the temperature of the electrolyte is set on it. From this moment on, the voltage on the diagonal of the measuring bridge changes sign, and therefore the voltage at the output OAout of the controller becomes less than half of the supply voltage, the comparator switches to a high level state, and the current amplifier turns on. As a result, the transistors VT3 and VT5 are closed, and the closing of the transistor VT5 is accelerated due to the diode VD6. The voltage from the charged capacitor C3 through the resistor R18 is supplied to the gate of the transistor VT4 in the opening polarity, which leads to its opening. In fact, the voltage at the gate of the transistor VT4 in steady state is approximately equal to twice the supply voltage. In this state, the transistor remains for some time ton, determined by the capacitance of the capacitor C2 [4; 5]: ton = 25 103 C2, where ton is in microseconds, C2 is in microfarads. For reliable operation of the transistor VT4, it is necessary that the time constant of the discharge circuit traz3 of the capacitor C3 satisfies the condition: traz3 = (R18 + R19) -C3 >> ton It should be noted that this capacitor is recharged in operating mode through the load (field winding). The open-to-close time ratio at the controller output is internally limited to approximately 9:1. Therefore, after a certain time, the current amplifier closes, and the transistor VT3 opens. Transistor VT4 turns off and turns on VT5. This cycle (period) of switching ends. The duration of the open and closed states of transistors VT4 and VT5 is chosen such that the through current is minimal. Since the current in the excitation winding of the generator does not reach the required value in one switching period, the controller operates with the specified duty cycle for several cycles. The current in the winding and the voltage on the battery increase. As soon as the voltage in the measuring diagonal of the bridge approaches zero, the controller, changing the duty cycle, will maintain this state. In reality, taking into account the inertia of the system (inductance of the field winding, etc.) and phase shift, the shape of the charging voltage has a trapezoidal shape. On fig. 2 are presented for comparison of the inherent loss characteristic family of the automotive industrial stabilizer 121.3702 and that described above. The graphs show that for a stabilizer with SHI control, the loss power Ppot is smaller and constant throughout the entire range of changes in the load Рn and the crankshaft speed N of the engine. Accordingly, its efficiency is higher. The gain in the energy sector is also obvious in comparison with [1; 2]. All of the above confirms the expediency of using a synchronous switch on field-effect transistors.
The device uses precision resistors R5-R11 S2-29V, S2-14, etc. with TCR no worse than ±200-10-6 °C-1. It is permissible to use a tuning resistor SP5-6V or similar instead of R5 and R1; the remaining resistors are general purpose. Capacitors C1, C3 - K50-35, C2 - K73-17. Inductor L1 - DM0.1 inductance "! 60 μH. The field effect transistor BS250 can be replaced by any other p-channel transistor with an insulated gate and an open channel resistance of not more than 10 ohms. Instead of the BSS91, any n-channel medium-power insulated-gate field-effect transistor with a channel resistance of no more than 20 ohms will do. Powerful n-channel transistors VT4, VT5 must have a channel resistance of no more than 0,03 Ohm and a gate-source operating voltage of at least 20 V. It is most convenient to use transistors in small-sized DPAK (TO-252) packages, for example, Motorola MTD3302. Diodes KD102A can be replaced by KD103 with any letter index. Instead of K1156EU1, the KR1156EU1 controller is suitable if it is not intended to operate the car at temperatures below -15 ° C. Structurally, the measuring and control parts are assembled on two mounting plates, the connections are made with MGTF 0,07 wire. For circuits with high current, a mounting wire with a cross section of at least 0,75 mm2 is used. The boards are interconnected by a two-wire flexible cable РВШЭ1 in a shielding braid; wires are twisted into a cord. The same cord, but without a braid, was used to connect the measuring part to the battery. The measuring board must be placed in a suitable metal box. The design of the temperature sensor generally does not differ from that described in [2]. The flask with diodes is made of a polyethylene cable sheath. The diodes are immersed in the KPT-8 heat-conducting paste for better heat transfer from the walls to the inside of the diodes. A polyethylene tube of smaller diameter is put on the conductors (twisted pair) with an interference fit. With a soldering iron, heated to the melting point of polyethylene, the bottom of the flask is pre-brewed. Lastly, the junction of the flask and the cable tube is welded. The tightness of the seams must be high, since the flask will be immersed in the battery electrolyte during operation. To establish a voltage stabilizer, you will need a DC source with an output voltage regulated from 10 to 15 V at a load current of up to 3 A, a DC voltmeter with an accuracy class of at least 0,1, and a load resistor with a resistance of 5 ohms. In parallel with the source, it is necessary to connect an oxide capacitor with a capacity of at least 10000 microfarads. Temporarily, the resistor R6 is replaced with a variable having a resistance of 3 kOhm, and terminal 1 of the controller is connected to a common wire. First, a voltage of 15 V is supplied from the power source and the current consumed by the device is controlled - it should not exceed 50 mA. The temporary connection of terminal 1 with a common wire is opened and the supply voltage is reduced to 13,6 V. The variable resistor R6 is used to achieve the appearance of a pulse sequence at the DC and SC outputs of the controller, and an inverted pulse sequence with an amplitude equal to the supply voltage at the output of the stabilizer. Transistor VT4 should not heat up. The stabilizer is finally adjusted after it is installed on the car. The temperature sensor is immersed in the electrolyte solution through a hole in the cork of one of the middle cans of the battery. Connect all circuits according to the diagram, turn on the ignition and make sure that there is no voltage at the output of the stabilizer. The engine is started, and at idle with the consumers turned off, the charging voltage on the battery is set with a variable resistor R6 in accordance with the recommendations [1]. If the car has not been running for a long time, the ambient air and electrolyte temperatures can be considered equal. After setting the voltage, the variable resistor R6 is replaced by a constant one. By changing the speed of the engine crankshaft and the load of the generator, the instability of the charging voltage is controlled; it should be no worse than ± 0,02 V. When driving in winter conditions, it may sometimes be necessary to clarify the value of the resistor R7. It must be remembered that after adjusting the resistor R7, it is necessary to re-select R6. For the efficient operation of the stabilizer and prolongation of the battery life, it is desirable, firstly, to equalize the density of the electrolyte in all banks to ± 0,01 g/cm3, and the density must correspond to the climatic zone [6], and secondly, periodically wipe the battery cover with a weak an aqueous solution of ammonia (10%) to prevent current leakage through pollution, thirdly, paste over the battery case around the perimeter, if it is black, with aluminum foil (for example, Quintol or Moment glue) - this will lower the temperature electrolyte at 5 ... 10 ° C, which is especially important in summer. For a three-year period of operation of the stabilizer on a VAZ 2106 car, no remarks were noted in its operation, the electrolyte in the battery did not boil, there was no need to add water. At the annual technical inspection of the battery, I check the density of the electrolyte and the charging voltage. Literature
Author: V. Khromov, Krasnoyarsk
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