An electronic load meter using LM324 operational amplifier as the main control device

The entire instrument consists of electronic switch, ramp wave generation, current detection amplification, comparison adjustment, and PWM drive unit.

This instrument can test the discharge performance of 12~48V power supplies and batteries, with a maximum current of 20A. It is very convenient to operate; Kl controls the discharge input or cutout; W1 adjusts the current amplitude; W2 adjusts the undervoltage value.

The circuit is shown in the figure below. In the figure, ICIA, R1~R4, Ql, C3, C4, and D1 form a ramp wave generating circuit. Among them, the voltage divided by R2 and R3 is the reference voltage of the inverting input terminal of ICIA; while the non-inverting input terminal is connected to C3 for charging through R1.

Initially, ICIA outputs low level; when the voltage of C3 rises greater than the inverting reference, ICIA outputs high level; Q6 is turned on through D1 and R4, causing C3 to instantly discharge to 0V. At this time, the ICIA output flips to low level. Repeat the above process again.

Repeating this cycle, C3 continuously generates sawtooth wave pulses. The maximum amplitude of the sawtooth is slightly lower than the reference voltage of the inverting terminal. This pulse is sent to the inverting terminal of ICIB as a PWM period comparison pulse; and the non-inverting terminal of ICIB inputs a control signal: this signal is passed through The ICID unit compares and processes the current feedback signal and outputs it. It can be seen that if the control signal amplitude is smaller than the pulse signal amplitude, the ICIB outputs a low level; and the control signal is greater than the pulse signal amplitude, the ICIB outputs a high level. Therefore, changes in the control signal amplitude are converted into changes in output pulse width; and the greater the signal amplitude in each sawtooth wave cycle, the wider the ICIB output pulse. The smaller the signal amplitude, the narrower the ICIB output pulse width.

Obviously, the change in the pulse width of the ICID output changes the conduction time of the field effect transistor per cycle, thereby achieving the purpose of controlling the load current. This is the basic principle of various types of PWM control. The application of PWM mode not only enables the power device to operate in a high-frequency switching area with extremely low energy consumption, but also enables the power device to produce a control effect equivalent to linear operation.

R12, R13, R14, and ICIC form the current sampling amplification part, which is used to convert current changes into a feedback voltage of a certain amplitude. Its gain is 10 times, determined by adjusting the ratio of R13 and R12. The ICIC output signal is transported to the ICID unit as a current feedback signal for comparison processing.

The ICIC output is sent to the inverting terminal of ICID and subtracted from the current given signal at the non-inverting terminal. The resistance values ​​of R15, R16, R17 and R18 in the circuit are equal, forming a standard subtractor. The figure shows the given voltage minus the feedback voltage. This can be understood as once the given voltage is determined, if the current rises for some reason, the ICID output voltage drops; when the current drops due to a drop in battery voltage, the ICID output voltage rises. Due to the function of ICID, the amplitude change when the discharge current fluctuates is compensated to a large extent, making it close to a constant current discharge situation.

R7～R10, Ql, Q2, Q3, Q4 form a PWM drive circuit. When 1CIB outputs a high level, Q1, Q2, and Q3 are turned on, and Q3 is turned on so that Q5 (field effect power transistor) gets a gate voltage close to 15V and turns on. At this time, Q4 is reverse-biased and turned off. When the ICIB output flips to low power. Q1, Q2, Q3 are cut off, and the gate voltage of Q5 disappears. At this time, Q4 is forward-biased and turned on, causing the gate of Q5 to discharge quickly and turn off reliably!

The function of Q3 and Q4 on the tube is to use sufficient driving power to overcome the capacitance effect of the gate and source of the field tube to enable Q5 to switch reliably.

This is a classic circuit for field effect transistors. It is very reliable and has strong driving capability. The author has used it to drive high-power devices with hundreds of amps, and the effect is very good.

The circuit for battery under-voltage protection is composed of R20 and R19 and the voltage is sampled to the inverting input terminal of IC2. 6.3V consists of R21, W2, and R22 to form an undervoltage setting circuit, which sets the level signal to the non-inverting input terminal of IC2. When the power supply voltage sampling signal is greater than the setting signal; the output of IC2 is low level. At this time, T1 does not act. Once the power supply voltage sampling signal is less than the set signal, the output terminal of IC2 flips to high level, T1 is turned on and self-locked. T1 turns on, causing Q1 to lose bias and cut off, interrupting the transmission of the PWM drive signal, causing the electronic switch to turn off randomly, achieving the purpose of battery under-voltage protection. The self-locking of T1 is a necessary measure to prevent the battery terminal voltage from rising and causing the IC2 output to flip after the discharge is turned off, causing the system to re-enter the working area and frequent switching to cause machine vibration.

Q5 uses I×FK120N20 with parameters of 120A/200V/10mΩ and a 150X100 radiator; it is also equipped with a 12V/0.35A instrument fan for forced air cooling. It is also very necessary to protect the safety of field effect tubes.

R×1 in the figure is a high-power resistor with a resistance value of 0.75Ω and a power of 300W. Although the addition of this resistor greatly reduces the power consumption of the power VMOS tube, it also brings a problem: without adding R×1, very low voltage batteries can be tested, such as the 10A discharge of a 3.6V power lithium battery. Detection; after adding R×1, it is equivalent to an increase of 0.75Ω on-resistance of Q5, which can actually only reach the maximum discharge value of 4.8A. Therefore, when detecting large current of a single battery, R×1 should be short-circuited to achieve the desired effect.

For power supplies exceeding 20V, the effect of this resistance is significant. For example, if a 24V battery is discharged at 20A, the electronic switch must withstand a total power of 480W. It can be calculated that RX1 shares 300W of it. In fact, it is not easy to purchase power resistors that meet the power and resistance requirements, and they are very expensive. The following introduces a method suitable for making this resistor under amateur conditions: the material is made of 2 to 5kW electric heating wire, and multiple sections are intercepted and connected in parallel to achieve the required resistance value and power. Pay attention to making the resistance value of each section consistent as much as possible to ensure the same current flowing through it. As for the connection method, in order to reduce the system resistance as much as possible, the plugging method is not suitable and the welding method should be used as much as possible. But how to solve the problem that the heating wire is not easy to tin?

First, wrap the heating wire head tightly with a certain width of copper soft wire; then tin it in the conventional way; before tinting, bend the heating wire head to prevent the copper wire from slipping out.

In this way, the heating wire that is difficult to tin can be soldered and connected.

The homemade R×1 heats up very much when working at high current values.

The heat dissipation method can be forced air cooling, but it is not ideal. The best way is water cooling, that is, placing the heating wire in a water tank of a certain volume. However, the welding points must be protected with resin coating or heat shrink tube, and due to the electrochemical effect, the life of the heating wire cannot be very long. It would be ideal if you could find some transformer oil (or car tank antifreeze) to use as the cooling medium instead of water.

The display instrument uses a digital meter, preferably the one powered by 220V. Note: The ammeter and voltmeter cannot share the 5V power supply, otherwise it will cause inaccurate readings or even damage the meter head.

Use of the instrument: Connect the 220V power supply, set the discharge switch to the off position, counter-rotate W1 to the end, and connect the discharge power supply (battery and various power supplies).

Then set the discharge switch to the discharge position and adjust W1 to the appropriate discharge current value.

The undervoltage value is not determined: Connect an adjustable power supply, adjust the discharge current to 0.5~1A, and adjust the adjustable power supply to the undervoltage value, such as 21V; then carefully adjust the W2 knob until the ammeter reading disappears, proving that W2 has been adjusted to 21V undervoltage. voltage value; then turn off the power of the instrument (reset T1) and then restart the power, connect the battery under test, and adjust the appropriate current value. Of course, Tl must be reset every time after undervoltage protection.