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The production of electronic dummy loads

Published on Mar 16 2011 // Instrumentation and Tester Circuits

This dummy load to create an electronic alternative to traditional load resistance boxes, slide wire rheostat, etc., in particular, can set a constant current or constant voltage is applied to the traditional slip line field can not be resolved rheostat. For generators, AC / DC, DC / DC converters, uninterruptible power supply (UPS), batteries, batteries, transformers, chargers and other output characteristics to be tested. Leave up to the maximum load power 600W, leave the load resistance can be adjusted in the 30mΩ ~ 14.352kΩ.
First, the basic idea
False load power electronic devices, generally used in small power required to control the FET and IGBT control, and selection must be more than full load power margin, to avoid the use of burning; e-load a lot of heat generated when the work, need to install the radiator, and power devices and heat sink thermal resistance should be as small as possible between the necessary cooling fan can be installed; e-load power devices prone to parasitic self-oscillation, once the oscillation, not only fully working condition changed, and will burn out power devices. Therefore, anti-parasitic self-oscillation is very important and successful production of electronic dummy load determining factor. This production generates a reference voltage to the three operational amplifiers, respectively, through the constant voltage, constant current to achieve the basic functions of electronic dummy load. The total block diagram shown in Figure 1.

The production of electronic dummy loads circuit.

Schematic shown in Figure 2, the basic circuit for the addition and the two meter 5 dashed outside part of the circuit by the constant pressure, constant current circuit, over-current protection circuit, driver circuit. V = 12V input voltage, after the current limiting resistor R1 to the three-terminal adjustable shunt reference U1 (TL431) of the cathode K, R obtained by the reference-side output voltage VR is 2.5V, adjusted by sliding rheostat resistor R1 to R6, all the way through resistor R2 to provide voltage for the U3A, resistor R7 to U3C another path to provide voltage.

(Click to view original picture)
1. Constant voltage circuit
1 dashed box shown in Figure 2. When the load input voltage increases, U3A-phase input voltage increases. When the input phase voltage is greater than the inverting input voltage (reference voltage), U3A output high, the FET Q1, Q2, Q3, Q4 gate voltage VG G to generate voltage drop, making the drain D and source voltage VDS between S decreases, so as to achieve the purpose of constant pressure.

2. Constant current circuit
2 dashed box shown in Figure 2. When the load current increases, R19, R22, R25, R28 the voltage increases. The R18, R21, R24, R27 on the sampling voltage increases, that is U3C inverting input voltage increases, when U3C inverting input voltage is greater than the same-phase input voltage, U3C output low, field-effect tube Q1, Q2, Q3, Q4 gate G voltage VG decreases, Q1, Q2, Q3, Q4 RDS internal resistance increases, the load current decreases, so as to achieve the purpose of constant current.

3. Overcurrent protection circuit
3 dashed box shown in Figure 2. When the load current increases, R19, R22, R25, R28 the voltage increases, the R18, R21, R24, R27 on the sample voltage increases, U3B inverting input voltage is increased, but the current continues to increase. When the inverting terminal voltage is greater than the current set overcurrent protection reference voltage (phase-ended input voltage), U3B output low, FET Q1, Q2, Q3, Q4 G gate voltage VG decreases Q1, Q2, Q3, Q4 of the internal resistance RDS increases, the load current decreases, and thus play a role in over-current protection.

4. Driver
4 dashed box shown in Figure 2. Q1, Q2, Q3, Q4 IRF540 power FET used as a power device, but the multi-tube parallel, due to the extremely distributed capacitance between the capacitance and a corresponding increase in deterioration of the amplifier’s high frequency characteristics, easily lead to the amplifier through the feedback of high frequency parasitic oscillations. To this end, parallel composite pipe is generally not more than 4, and in each tube on the base or gate resistor in series anti-parasitic oscillation. R17, R20, R23, R26 for the driver resistance, R18, R21, R24, R27 for the sample voltage resistor, R19, R22, R25, R28 is the current limit resistor. C9 IRF540 MOSFET drain of a termination, the other end for the anti-shock.
Third, the circuit testing
E-load to be carried out after production testing, test circuit wiring diagram shown in Figure 2. 5 dashed and two meter section. 1 measuring power output voltage meter, power meter 2 measurement output current, the two sliding variable resistance are playing at 50%. J1 SPDT switch in a terminal when playing for the constant voltage mode, the selected power supply output voltage measured at 12.501V, shows off the electronic load has constant function. J1 SPDT switch when playing in the 2-side, as the constant current mode, the selected output current to maintain the measured power at 19.993A, shows that the electronic dummy loads have constant current function. Constant current mode can be used to test the power supply voltage source and load regulation. Sliding variable resistor R6 to change the pumping line position, can change the preset constant voltage and current values. Overcurrent protection can be preset current slide to change the variable resistor R14.
5 part in the dashed box diagram measurements listed power supply (other power can also be used as the measuring power), V ∞ is 15V AC voltage, the bridge rectifier D1, D2, D3, D4 rectified, obtained by the DC filter capacitor voltage, and then by three-terminal regulator U2 (ML7812) regulator, to ensure a stable +12 V DC voltage.