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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Dummy load for testing power supplies. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies Many radio amateurs, when collecting power supplies for various devices, are faced with the need to check them before using them for their intended purpose. The proposed device allows you to automatically determine the maximum load current of the source by a 5% drop in its output voltage, or to remove the load characteristic manually. Once I had a need to check the output parameters of the power supply. Not finding suitable load resistors in my stocks, I decided to assemble a transistorized adjustable load dummy. Since I could not find a description of the finished design, I decided to develop and assemble such a device myself. Technical specifications
The load dummy circuit is shown in fig. 1. It is controlled by the microcontroller DD1, thanks to which it became possible to display on the LCD HG1 the voltage of the source under test and the current given off by it. After turning on the equivalent, the microcontroller program displays its version number on the LCD for 3 s, after which it turns on the green LED HL2, signaling readiness for operation. Now you can connect the input of the equivalent to the output of the source under test. After a short press on the SB1 "+" button, the device will switch to manual mode, if you hold it down for at least 0,5 s, automatic mode will be turned on. In automatic mode, first of all, the voltage of the tested source is measured at idle, then the load current is gradually increased until the voltage drops by 5% or the current reaches the limit of 9 A. The voltage coming from the source under test is reduced by a resistive divider R1R2 to measure the value allowed for the built-in ADC in the DD1 microcontroller. The voltage follower on the DA2.1 op amp has a low output impedance, which is necessary for the correct operation of the ADC. The controlled load of the tested source is the transistor VT3. The constant component of the pulses generated by the microcontroller at the output of RC1.1, selected by the integrating circuit R5C3, is fed to its base through a follower on the op-amp DA1, a voltage divider R6R1 and an emitter follower on the transistor VT2. The greater the duty cycle of the pulses (the ratio of their duration to the repetition period), the greater the constant component, the more open the transistor VT3 and the greater the load current of the source under test. Proportional to this current, the voltage taken from the resistor R7, the amplifier on the op-amp DA2.2 brings to an acceptable value for the ADC of the microcontroller. In automatic mode, the program gradually increases the duration of the pulses, and the current increases until the voltage of the source under test decreases by 5% relative to the original. Further, the current growth stops, and the steady-state values of voltage and current can be read on the LCD. In manual mode, the load current is adjusted by pressing the buttons SB1 "+" and SB2 "-", reading the voltage and current values \u1b\uXNUMXbfrom the HGXNUMX indicator. In the absence of overcurrent, the output of RC7 is set to a high voltage level. Therefore, the field effect transistor VT2 is open and does not affect the operation of the device. But as soon as the current exceeds the limit value of 9 A, the microcontroller will set the output RC7 to a low voltage level and the transistor VT2 will close, breaking the load circuit of the source under test. An overload message will appear on the LCD. In order to return the equivalent to the operating mode after eliminating the cause of the overload, press the SB1 button. The microcontroller will again set the RC7 output to a high level, opening the transistor VT2. After measuring and displaying the voltage and current values on the LCD in the program, the BK1 sensor measures the temperature of the heat sink, on which the transistors VT2 and VT3 are installed. This turned out to be very important, since at a constant base current, the collector current of transistor VT3 increases strongly with increasing temperature. Depending on the measured value of the heat sink temperature, the program does the following: 1. If the temperature does not exceed 35 ° C, sets the outputs RC5 and RC6 of the microcontroller to low logic levels. Transistors VT4 and VT5 are closed, fan M1 is off. 2. If the temperature is in the range of 35 ... 56 ° C, sets the output RC5 high, and the output RC6 low level by opening transistor VT4 and turning on the first fan speed M1. 3. If the temperature is above 56 ° C, sets the output RC5 low, and the output RC6 high level, closing the transistor VT4, opening VT5 and thus including the second (increased) fan speed. 4. If the temperature has exceeded 70 ° C, it sets a low level at the output of RC7, thus closing the transistor VT2 and interrupting the load current of the source under test. In addition, it turns off the green LED HL2 and turns on the red HL1. The fan continues to operate, cooling the transistors, and the message "Overheating is being purged" appears on the LCD and the time is counted until the completion of this operation. After the message "Purge completed" the equivalent switches to normal mode by closing the load circuit of the tested source, turning off the red LED HL1 and turning on the green HL2. In addition to the measured values of current and voltage, the HG1 LCD displays the value of the CCPR1L register of the microcontroller, on which the duration of the generated pulses depends. It indirectly characterizes the degree of opening of the current-regulating transistor VT3. Every 250 µs, it is checked whether the current has exceeded 9 A. If this happens, the load circuit of the tested source is interrupted.
The device is assembled on a single-sided printed circuit board made of foil fiberglass, shown in Fig. 2. It can use any fixed resistors with a power of 0,125 W, such as MLT. Resistor R7 - SQP-10 or other 10 W wire. If you plan to use a device to test currents above 5 A, it is advisable to provide this resistor with a heat sink. Trimmer resistors R10 and R16 are imported PV37W. Capacitors C1 - C3, C5 - oxide company Jamicon, the rest - ceramic. Transistors VT2 and VT3 are installed separately from the board on the heat sink from the Pentium 4 processor. A two-speed M1 fan is also used from it. The wires connecting transistors VT2 and VT3 with the board and between themselves must have a cross section of at least 1 mm2. Next to the transistors on the heat sink is a temperature sensor BK1. Instead of the DS18S20 sensor indicated on the diagram, you can use the DS1820. No heat sink is required for DA3 and DA4 integrated regulators. The current consumed by the load dummy from its power source does not exceed 250 mA and is spent mainly on the backlight of the LCD display. When replacing the indicator of the type indicated in the diagram with WH1602D, it is possible to reduce the current consumption to 17 mA by selecting resistor R90. If you completely turn off the backlight, it will decrease even more. Establishing an equivalent is performed in the following order. First of all, a 10.12 V DC voltage source is connected to its input, the value of which is measured as accurately as possible with a digital voltmeter. By transferring the equivalent to manual mode, we make sure that the voltage value on its LCD coincides with the digital voltmeter readings. We eliminate the difference by selecting the resistor R1. To calibrate the current meter, we connect an ammeter in series between the voltage source and the load dummy. Having set the current in this circuit to about 2 A, we compare its readings with the value displayed on the equivalent LCD. With the help of a trimmer resistor R10, we achieve a match. Further, increasing and decreasing the current by pressing the buttons SB1 and SB2, we make sure that the readings coincide throughout the entire range of its change. After that, we fix the engine of the tuning resistor R10 with a quick-drying varnish. Finally, one piece of advice. After all the parts are soldered to the printed circuit board, it is necessary to carefully remove the flux (rosin) residues from it. As it turned out, the leaks they create between the printed conductors can disrupt the correct operation of the device. Having found such violations, I checked all the printed conductors of the board for mutual shorts and breaks, but did not find them. And after washing all the problems disappeared. I used the "Titan" thinner, which is available in aerosol form and perfectly removes flux residues. The voltage reduction thresholds of the device under test under load and the current protection operation set in the program can be changed, but this requires intervention in the source code of the program (the file rez.asm available in the application). Threshold information is recorded in its first lines, as shown in the table.
The values available there must necessarily be expressed as integers: current - in milliamps, voltage reduction - in percent. After making changes, the program should be retranslated and the resulting HEX file loaded into the microcontroller's memory. The PCB file in Sprint Layout format and the microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2013/06/rez.zip. Author: Kuldoshin
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