For testing power supplies and transformers, an appropriate, preferably adjustable, load is indispensable. Often, a number of interlinked high-wattage resistors are used for this purpose, but that is not alway satisfactory, feasible or safe. The electronic load described here is a much more flexible and suitable solution.
The load is based on a number of power transistors. The output current, which is the sum of the collector currents of these transistors, is converted into heat that is lost by convection and radiation through a suitable heat sink. The base current of the transistors is arranged at a value that results in the required level of emitter-collector current. Since the base-emitter voltage that deter- mines the base current of a transistor varies with temperature (¤ 2-8 mV / °C), an opamp is used to iron out any consequent variations of the base current. The end result is an adjustable load with a ’resistance’ value ranging from almost zero to infinity and a thermal rating that depends solely on the power transistors and the manner in which these are cooled. The load may operate in either the constant-current mode or the constant resistance mode. In the first, the current remains constant irrespective of the applied voltage, while in the second it is directly proportional to the applied voltage. A waveform generator (triangular, sinusoidal and rectangular) enables the ’resistance’ to be modulated.
Circuit description
Each of the power transistors, T3-T7, in Fig. 1 can draw a collector current of up to 2 A with an appropriate heat sink. Resis- tors R24—R33 provide a measure of current feedback, which ensures equalization of the currents drawn by the individual power transistors.
Resistance simulation
Each group of power transistors, T3—T7 and T8—T12 respectively, is driven by one half of dual opamp IC2 via a driver transis- tor, T1 and T2 respectively. The inverting input of the opamps is connected to the emitter resistor of the first power transistor in each group. The non- inverting inputs are connected in parallel and linked to the pole of switch Sib. This pole receives one of four different control signals via the switch contacts.
When S1 is in position l (7), terminal "P” carries part of the voltage, as set by P2- R9, that exists between terminal "U" and earth. The opamp tries to reduce the ‘potential difference between its inputs to vir- tually zero. It will therefore increase the base currents of the power transistors, and thus the load current, until the voltage drop across R24 (R29) and the input voltage set by P2 are equal. When the input voltage rises, the potential at the non-inverting in- puts, and thus the load current, increases. This means that the circuit behaves as a resistance, the value of which may be set with the aid of P2.
Modulation and constant current
Opamps IC1a and IC1b form a simple function generator that produces rectangular waveforms when S1 is in position 2 (8) and triangular waveforms when the switch is in position 3 (9). The frequency is adjustable over the range 5-50 Hz by P3. The signal from the generator is amplified in ICM and fed, via SIB and "P", to the non·inverting inputs of control amplifier IC2 where it serves as reference voltage. Since this potential is no longer dependent on the input voltage to the load, an in- crease in the input level no longer leads to a higher load current. In fact, if the signal, whether triangular or rectangular, is used as the control signal, the circuit functions as a modulated constant-current source.
The load current is modulated in the same way as the control signal. The gain of IC1d, which determines the depth of modulation, is set by P1. Potentiometer P4 enables an offset to be added to the control sign al. This offset makes it possible to shift the modulation level with respect to zero. ln other words, P1 sets the level by which the current varies, while P4 determines between which values modulation is effected, for resistance, between 2.5 A and 3.0 A. This assumes, of course, that the unit or device under test can provide currents at those levels .
External modulation
Opamp IC1e serves as an inverting. unity-gain amplifier. Its output signal B available at contact 4 of Si. It may be fed with an external modulating signal via Ki. The input must be between 0 V and +10V. The con- trol characteristic may be set between 3 A / V and 1.5 A / V for each power transistor. If, for example, the voltage at K1 changes by 100 mV, the output current varies by 3 A with P1 set to maximum and by 1.5 A with Pi set to minimum.
Construction and alignment
The load is best built on the PCB shown in Fig. 2. This figure does not show the part of the board for power transistors T8—T12 and associated components since this is identical to that for T3—T7. Before any start can be made with populating, the board must be cut into four with a fine hacksaw. Screw each of the two long parts to a 5 mm thick aluminium bracket. Since the collectors of the power transistors will be at the same potential, there is no need for insulating washers, provided. that the heat sinks and aluminium brackets are isolated from the enclosure.
Do not fit the emitter resistors too close A to the board, because even in normal operation these get fairly hot. The same applies to R22 and Rza. The choice of enclosure depends in the first instance on the heat sinks used. The requirements of these are fairly stringent because they have to dissipate some 300 W. The dissipation may be increased to 1 kW if forced cooling is used. The only calibration is in the provision of a scale for P2. For that purpose, a laboratory ype power supply with variable output and capable of providing an output current of at least a few amperes is required. Measure the voltage and current for a number of positions of P2 and calculate the corresponding resistance. The resulting scale is linear for input voltages greater than about 4 V
Fit the potiometers and switch On the front panel of the enclosure together with two heavy-duty, spring- loaded, insulated terminals. Connect points "U" and "T" to their positive terminals, and the earth points to the negative terminal. To ensure that sufficient current flows through the power transistors, the load needs its own 12 V power supply, for which a simple unit with a 500 mA transformer and a Type 7812 voltage regulator will do fine.
The maximum values the power - transistors can tolerate are not those found in their data sheet, that is, 60 V 15 A, 115 W. Instead, the maxi- mum dissipation is determined from the safe operating area (SOA) shown in Fig. 3. This shows that the maximum collector current de- creases with a rising collector-emitter volt- age. Conversely, when a current of 15 A flows through the transistor, its collector- emitter voltage should not exceed 8 V It is imperative that at no time the C—E voltage exceed the limit for a given current and vice versa. But that is not all. The SOA characteristic in Fig. 3 refers to a maxi- mum dissipation of 115 W at a case tem- perature of 25°C. With rising temperature, the dissipation is degraded at a rate of 0.65 W/°C. This means that at a case tempera- ture of 80 OC the maximum dissipation is only 80 W and at 140 °C it is only 40 W.
The design of the load allows for each power transistor to tolerate a current of up to 2 A, so that the input voltage can go up to close to 60 V. lf that is not sufficient, the ZN3055 transistors should be re- placed by types with a higher rating.
