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AMATEUR RADIO By GARRY CRATT, VK2YBX Use this electronic load to check power supply performance Most amateurs use high-wattage resistors for checking out power supplies but that's often inconvenient. This electronic load can be used with power supplies of up to 30V output and is easily adjusted to give the required load current. When it comes to checking the performance of power supplies and batteries, one of the most useful pieces of test equipment is the resistive load. By using such a device, the performance of current limiting circuitry, battery capacity, power supply ripple and terminal voltage under load can all be checked. Unfortunately, due to the infinite number of voltage and current combinations that may require testing, the user is likely to end up with a mass of high wattage resistors wired in series or parallel. During testing, these may become quite hot and the whole arrangement can become unmanageable. This article describes the con- The electronic load is built into a standard metal box and this is fitted with large finned heatsinks for the power transistors. A multiturn pot on the front panel allows precise adjustment of the load current. 42 SILICON CHIP struction of an "electronic" load which can operate at any voltage up to about 30 volts, is capable of dissipating up to 10 amps, and is adjustable. In addition, the electronic load has provision to monitor terminal voltage under load using a standard multimeter. A digital multimeter is ideally suited for this application, although a plotter could also be used to monitor terminal voltage under load over a prescribed time interval. How it works Fig.1 shows the circuit details of the electronic load. Basically, it uses two transistors (Ql & QZ) which are wired as emitter followers, each driving a 0.470 resistive load. This resistive load consists of seven paralleled 3.30 5W resistors. In effect, the circuit is a large adjustable current "sink". We can use it to vary the amount of current pulled from a power supply. The bias to both transistors is controlled by multiturn potentiometer VR 1. This functions as the "load" control and allows the current dissipated in the emitter load resistors of each transistor to be varied as required. Thus, you can set the load to draw a specific current, which is monitored by the 10A ammeter on the front panel of the unit. This is a most useful feature, as the unit can be used to check a variety of power supplies, batteries and even solar cells under various load conditions. The 3.30 resistors have .been 10A d VIEWED FROM BELOW + 02 2N3055 VOLTAGE TEST Fig.t: the circuit uses two power transistors (Qt & Q2) which are wired as emitter follows, each driving a 0.470 resistive load. VRt varies the bias on both transistors so that the circuit behaves as an adjustable current sink. ELECTRONIC LOAD chosen to limit the maximum load current and to ensure that the two transistors are protected against thermal runaway. These resistors could be decreased in value to allow the load to handle more current but you would have to use larger heatsinks for the transistors than those shown in the photos. Construction Our unit was built into a standard metal project box with air vents to allow cooling of the 14 5W emitter resistors. Ventilation is of prime importance for this project and must be taken into account if you intend substituting for the case specified in the parts list. Fig.2 shows the wiring details. The 14 3.30 5W resistors are all mounted on a PC board coded SC 14106901 while the four smaller resistors are mounted on a second PC board coded SC 14106902. Note that 6 of the 5W resistors are "stacked" above other resistors on the board. Leave a few millimetres of space beneath the 5W resistors so that the air can circulate freely for cooling. The two transistors are mounted on heatsinks which in turn are .mounted on either side of the metal case. This external mounting allows adequate ventilation for the transistors. Both transistors must be electrically isolated from the heatsinks using mica washers and insulating bushes. Fig.3 shows the details. Smear heatsink compound on all mating surfaces before screwing the transistors down, then use your multimeter (switched to a high E------- 01 B C ~ V (O+ VOLTAGE TEST + METER Fig.2: use heavy duty cable to wire up the circuit and note that some of the 5W resistors are stacked one above the other. Take care with the connections to Qt & Q2. ohms range) to confirm that the transistors are correctly isolated from the heatsinks. Heavy-duty wiring leads can then be soldered to the emitter and base pins, while the collector connections can be made via solder lugs secured by the mounting screws. MAY1990 43 PARTS LIST 1 metal case, 150 x 76 x 134mm (Jaycar HB-5444 or DSE H-2743) 2 heatsinks (DSE H-37 40 or Jaycar HH-8560) 1 PC board , code SC14106901, 127 x 50mm 1 PC board, code SC14106902, 25 x 25mm 1 1OA panel meter 2 binding post terminals ( 1 red, 1 black) 2 banana sockets (1 red, 1 black) 1 5k0 multiturn pot (Geoff Wood) 1 vernier dial to suit pot (Geoff Wood Electronics) 2 2N3055 NPN transistors (Q1,Q2) 2 T03 mounting kits (mica washers plus insulating bushes) 2 T03 insulating caps 2 4700 ½W resistors 2 1 000 ½ W resistors 14 3.30 5W resistors 3 6mm-long threaded spacers 6 screws to suit spacers The prototype was built up on Verohoard hut the PCB version will he easier to build. The heatsinks are secured to the lid using self-tapping screws and the leads to the transistors run through the ventilation slots. The self-tapping screws used to secure the lid of the metal box were also used to secure the heatsinks. To do this, you will have to mark and drill the appropriate holes in the heatsink flanges. This done, mount the heatsinks in position, then drill additional holes through the lid at the top of each flange (two for each heatsink) to accept additional self-tapping screws. The case can now be drilled to accept the front panel components. These parts include the meter, the binding post terminals, banana sockets and the 10-turn pot. The meter cutout can be made by drilling a series of smaller holes around the circumference, and then filing the hole to a smooth shape. Both TO-3 transistors should be covered with plastic insulation caps, as the body (collector) of each transistor sits at the same potential TABLE 1 44 SILICON CHIP 0 HEATSINK 0 =--CASELID ~ - 4 - (§-INSULATING BUSH ~-- SOLDER LUG WASHER ,.-:1,,_ <at>--- ~ <at>....-SPRING WASHER <at> <at>--NUT Fig.3: the power transistors are isolated from the heatsinks using T0-3 mounting kits. Smear all mating surfaces with heatsink compound before mounting each transistor, then use your multimeter to confirm that its case is properly isolated from the heatsink. as the power supply or battery under test. A short circuit between either collector and a heatsink, which is at ground potential, would cause maximum current to be drawn and could damage the power supply being tested. We used a vernier coupled to the 10-turn potentiometer to give precise adjustment of the load current, as this control is quite sensitive. If you don't want to go to the trouble of obtaining etched PC boards, use Veroboard instead (as in the prototype). The two circuit boards are mounted inside the case on 10mm tapped spacers. Operation Finally, you must observe a few simple precautions when operating the unit. Because of the heatsinks specified, the maximum power that this unit can dissipate is about 60W Supply Voltage 3V 5V 9V 10V 12V 15V 18V 20V 22V Maximum Load 10A 10A 6.7A 6A 5A 4A 3.3A 3A 2.7A 2.4A 25V 30V 2A Mad It is a good idea to fit plastic insulating caps over the power transistors to prevent accidental shorts to the heatsinks. The heatsinks shown are capable of dissipating about 30W each (ie, 60W total). (or 30W for each transistor). This rating determines the maximum load current that should be drawn at a particular supply voltage. For example, if the supply voltage is 30V, then the maximum current that should be drawn is 2A (ie, P = IV = 30 x 2 = 60W). Similarly, if the supply voltage is 20V, then up to 3A may be drawn. Increasingly higher currents may be drawn at lower voltages, up to a maximum of lOA. Table 1 shows the maximum safe current that can be drawn at each voltage. Do not exceed these cur- Fig.4: you can use these artworks to etch your own PCBs or you can buy commercial boards from the usual retail outlets. rents - you could cause damage to the electronic load if you do. In practice, this all means that when testing a power supply, you should start off with VR1 set to maximum resistance. This equates to minimum load current. After that, VR1 is wound back (anticlockwise) until the load current is at the required value (while keeping in mind the maximum values listed in Table 1). ~ SC 41 in A t a ,a? p Polystryrene Capacitors - 1 0pF to 1mF Voltage Range 63VDCW to 10,000 VDCW Tolerance - 0.25% to 10% Allied Capacitors Australia manufactures capacitors to the specification of the customers using high quality, imported polystyrene and aluminium foil with a tolerance of 1 micron. Our capacitors are manufactured to the system of Total Quality Control. We can provide Just In Time delivery if required, together with a Certificate of Conformance if requested. Specific values between 1 0pF and 1uF are no more expensive than standard value components. You can now design circuits to use a single capacitor rather than a number in parallel or series to achieve a desired value. Personalised labelling is available at no extra cost. Your component code or name can be built into the capacitor. Minimum runs of only 25 allow you to specify a particular value for a prototype run. Delivery lead time for short runs is 3 to 4 working days, and for longer runs is less than 2 weeks . We are also able to produce capacitor styles for applications such as dual mount, end filled and mini style capacitors. Call us now on: (02) 938 4690 ALLIED CAPACITORS AUSTRALIA PO Box 740 Brookvale, NSW 2100