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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal load equivalent. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Miscellaneous electrical devices When adjusting their designs, radio amateurs often need various load equivalents. The industry produces standard devices, but their prices "bite". Therefore, radio amateurs have to "sculpt" something "in haste". Such surrogate equivalents in the form of open, chaotically soldered resistors lying on a table without any insulation, often, in case of an accidental short circuit, instantly "bury" all the work spent on the development of some serious device, so I strongly recommend that you still allocate time, which , of course, is always lacking, and to assemble a suitable design that ensures safety for both the devices under test and people. For everyday work, in most cases it is enough to have a load equivalent in the form of a set of powerful wirewound resistors. You can make it yourself. To simplify the design and reduce the number of resistors used, it is better to take wire-wound adjustable resistors (Fig. 1) with a movable clamp fixed by a screw (PEVR type). For example, PEVR-25, designed for a power dissipation of 25 watts.
You can increase the power dissipated by resistors if you equip them with radiators. Both industrial-made radiators (Fig. 2) and self-made ones (Fig. 3) are suitable. The industrial radiator shown in Fig. 2 consists of two identical halves fastened together with a long screw with an M3 nut.
In the lower parts, along the edges of the radiator, there are two M3 threaded holes. With their help, the resistor can be mounted on a metal plate, which will improve cooling and, accordingly, further increase the allowable power dissipation. Homemade radiator consists of three parts. This is a metal tube 1 (Fig. 3) with an outer diameter of approximately 11 mm and an internal through longitudinal hole 3 with a diameter of 3 ... 4 mm. Instead of a tube, a rod is suitable, in which holes are drilled at the ends and threads are cut into them, or an axis with a length of at least 70 mm with threads cut at the edges.
The second part is a U-shaped plate 2 made of aluminum or duralumin with a thickness of at least 3.4 mm. Instead, you can use two duralumin corners measuring 30x30 mm. In the lower part of the U-shaped bracket or corners, you need to drill two holes 4 for the mounting screws in the housing. Additional parts are screws with M3 or M4 thread (not shown). To make an equivalent, it remains to choose a finished case of suitable dimensions. It's not worth the hassle here. The "volume reserve", like the ventilation holes in the case and even the fan from the computer, improve the cooling of the resistors. To connect to the devices under test, you can use special terminals that are fixed on the case in a convenient place (Fig. 4).
Such a load equivalent can be used when testing stereo audio equipment by connecting separate loads R1 and R2 to both UMZCH channels simultaneously (Fig. 1). By switching the outputs of the resistors R1, R2 through the corresponding terminals XT, you can get different values of load resistances. If you set the sliders R1 and R2 to the middle positions, controlling the resistance with a digital multimeter, at a nominal value of 8 ohms, we get Rn = 2; 4 and 8 ohms for each channel. Using resistors with a nominal value of 10 ohms and setting the sliders so that the resistance of one part of the resistors is 6 and the other is 4 ohms, we get fixed values of Rn 2,4; 4; 6 and 10 ohms. At lower resistance values, the load power increases due to the parallel connection of parts of the resistor. In the single-channel version, by appropriate switching, you can get much more options for fixed Rn. You can use both serial connection R1, R2, and parallel. In all cases, the power that can be dissipated by such a combined load is increased by several times. In addition, it is always possible to change these values by changing the position of the sliders. Author: V. Efremov, Essetuki, Stavropol region.
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