Keywords: electronic load circuit
Author: Zhang Hanping
Electronic load instruments are an indispensable instrument for power supply and battery performance testing. As the name of the Siyi Electronics load, the electronic device is an analog load for detecting various types of power belt load characteristics and chemical power output performance. The battery capacity value can be accurately measured when the constant current test is synchronized.
Due to the need for work, I have exposed to multiple manufacturers electronic load instrument products. Although the grade is very different, the volume difference is very large, but the important part of the electronic load, that is, most of the electronic switching components consist of the VMOS power tube having a smaller consumption, and its working modes are PWM mode. For the convenience of battery capacity, most loaders are mostly working in constant current discharge mode. The electronic switch driver circuit has a single chip microcomputer, an operational amplifier, and a discrete component constitutes a control circuit. Advanced also has various functions display and setting functions. Therefore, the high price is extremely high, about a few thousand yuan to tens of thousands of yuan. Its price is a lot of electronic enthusiasts and small manufacturers.
It can be seen that if you open a lot of worship and display functions, it is sure to be welcomed by a lot of users as a strong and reliable, easy to operate, simple and reliable as a large-capacity load instrument for design indicators!
Based on the above-mentioned leading principles, the author designed an electronic load instrument that uses the LM324 amplifier as a master device. The entire instrument consists of electronic switches, ramp, current detection, comparison, and PWM drive units.
The instrument can discharge performance test of 12 ~ 48V power and battery, maximum current is 20A; the operation is very convenient; discharging or cut by K1; W1 adjusts the current amplitude size; W2 adjusts the undervoltage value.
The following is a brief introduction, and the circuit is shown in Figure 1. In the figure, IC1A, R1 ~ R4, Q1, C3, C4, D1 constitute a slanted wave generating circuit. Wherein, R2, R3 is divided into IC1A inverted input terminal reference voltages; while the phase input end C3 is charged via R1. At the beginning, IC1A outputs a low level; when the C3 voltage rises, the IC1A is output to high; the Q6 is turned on by D1, R4, and the C3 instant discharge is 0V. At this time, the IC1A output is flipped to a low level. The above process is repeated.
Thus, re-starting, C3 generates a class such as a zigzag pulse, the zigzag maximum amplitude is slightly lower than the inverting end reference voltage, the pulse IC1B inverted end as a PWM cycle ratio pair pulse; and IC1B input control signal: This signal is The IC1D unit is output after comparing the current feedback signal. It can be seen that if the control signal amplitude is smaller than the pulse signal amplitude, the IC1b outputs a low level; the control signal is greater than the pulse signal amplitude, the IC1b outputs a high level. Therefore, the control signal amplitude change is converted to the output pulse width; and the larger the signal amplitude in each serrated period, the wider the IC1B output pulse is wider. The smaller the signal amplitude, the narrower the IC1B output pulse.
Obviously, the pulse width of the IC1D output changes, enables the field effect tube to turn on time on time, to achieve the purpose of controlling the load current. This is the basic principle of various PWM controls. The application of the PWM mode is caused to operate both the power device in a high frequency switch area, and the power device generates the control effect when the power device is equipped.
R12, R13, R14, IC1C constitute a large amount of current samples, and the action is to convert current changes into a certain amplitude feedback voltage. Its gain is 10 times, and the adjustment R13 is determined by the R12 ratio. The IC1C output signal is transported as a current feedback signal to the IC1D unit as a comparison process.
The IC1C output is sent to the IC1D inverting end, which is subtracted for subtraction of the same phase end current. In the circuit, R15, R16, R17, R18 resistance is equal to constitute a standard subtractor. The feedback voltage is subtracted for a given voltage. This will be appreciated that once the given voltage is determined, if the current increases due to a reason, the ICID output voltage is lowered; the IC1D output voltage rises due to the decrease in the battery voltage drop. Due to the role of IC1D, the amplitude change when the discharge current fluctuation is largely compensated, so that it is close to constant current discharge.
R7 ~ R10, Q1, Q2, Q3, Q4 constitute a PWM drive circuit. When the 1C1B outputs high level, Q1, Q2, Q3 are turned on, and Q3 conducts Q5 (field effect power tube) to obtain a gate pressure close to 15 V, and Q4 is turned off. When the IC1B output is flipped to low, Q1, Q2, Q3, the cutoff, Q5 gate pressure disappears, at this time, Q4 is biased, so that Q5 gate is quickly discharged and reliable! Q3, Q4 pairs The role of tube is to overcome the gates of the field tube at sufficient driving power, and the capacitance effect of the source makes the Q5 reliable switch.
This is a classic circuit for the field effect tube, very reliable, very driving ability, the author used to drive hundreds of amps's high power devices, the effect is very good.
The circuit for battery undervoltage protection is composed of R20, R19, constitutive voltage sampling to IC2 inverting input. 6.3V consists of R21, W2, R22, and set the level signal to the IC2 in phase input. When the power supply voltage sampling signal is greater than the set signal; the IC2 output is low, at this time, T1 does not operate. Once the power supply voltage sampling signal is less than the set signal, the IC2 output is flipped to high, T1 is turned on and self-locking. T1 is turned on, so that the Q1 offs the bias, the transmission of the PWM drive signal is interrupted, so that the electronic switch is off, and the purpose of the undervoltage protection is achieved. The self-locking of T1 is to prevent closing discharge, the battery-end voltage rise makes the IC2 output flipped, and the system re-enters the work area and the necessary measures for transit motor vibration.
Q5 is selected for IXFK120N20, parameter 120A / 200V / 10MΩ, is applied with a 150x100 radiator; additionally configuring 12V / 0.35A instrument fan forced air cooling. It is also necessary for the security of the protection field effect tube.
RX1 in the figure is a high power resistance, the resistance is 0.75Ω, and the power is 300W. Although this resistance is greatly reduced by power consumption of power VMOS tube, it also brings a problem: in the case where RX1 is not added, the very low voltage battery, such as the 10A discharge detection of the power lithium battery of the 3.6V; After adding RX1, the equivalent Q5 increases the on-resistance of 0.75Ω, and can actually only reach the maximum 4.8A discharge value. Therefore, on the large current detection of the monomer battery, RX1 should be shorted to achieve the expected effect.
For power supplies more than 20V, this resistance is significant, such as a 24V battery as an example, at this time, the electronic switch must withstand a total of 480W power, and RX1 can be calculated 300W. In fact, the power resistance required for the proclaim consolidation is not easy, and the value is very expensive. Next, a method suitable for making this resistance under amateur conditions: the material is used for electric heating wires of 2 to 5 kW, and intercepting multi-segment parallel to achieve the need resistance and power. Note that the value of each paragraph is made to ensure that the current is the same. As for its coupling method, to minimize system resistance, it should not use the way to use the welding method. But how to solve the electric wire is not easy? First, the electric wire head is tightly wound with a copper cord; then tin in a conventional manner; the electric wire head is bent before tin, preventing the copper wire. In this way, it can be welded to the welding of the electric wire.
When the soil RX1 is working in a large current value, he is very hot. The heat dissipation method can be used forced air cooling, but is not ideal, the best way or water is cold, that is, the electric heating wire is placed in a certain amount of water tank. However, the welding point must be protected by resin or heat shrinkable tube, and the electric heating life is not very long due to electrochemical effects. If you can find some transformer oil (or car tank antifreeze) instead of water as a cooling medium, then it is ideal.
The display instrument selection digital header, preferably 220V power supply. Note: The current table and the voltage meter cannot share the 5V power supply, otherwise the reading is not allowed, and even damages the head.
Instrument: Touch 220V power supply, discharge switch position, W1 inverted to the end, connect the discharge power supply (battery and various types of power). The discharge switch is then discharged, and the W1 can be adjusted to the appropriate discharge current value.
The underflatable value is defined: connecting an adjustable power supply, discharge current is adjusted to 0.5 ~ 1A, adjust the adjustable power supply to the undervoltage value, such as 21V; then adjust the W2 knob to the current table reading, prove W2 has been adjusted to 21V Pressure value; then turn off the instrument power (reset to T1) and restart the power, connect the battery, adjust the appropriate current value. Of course, each time the undervoltage is protected, T1 must be reset.
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