Electronic loads have many different applications, including testing power converters and modulating a current supply while performing other tests. They are easy-to-use and provide much higher throughput than resistors when varying loads are needed.
When choosing an electronic load, the most important specifications that you need to consider are the voltage rating, current rating, and power rating. This is different from selecting a power supply. When you select a power supply, you typically only need to know the maximum voltage and current at which you will be using the supply.
When you select an electronic load, however, you'll need to know how much current that the load will have to draw and at what voltage. Voltage times current equals power, so when selecting an electronic load, you must ensure that it can handle, not only the maximum voltage and current, but the power as well, so that your application does not overpower the load.
Each Sorensen and California Instruments electronic load has a unique power curve. Figure 1 shows a typical example. The voltage and current at which you operate the load cannot be outside the curve. Knowing the voltages and currents at which you must test a power source is very important to select the right load.
For example, when testing a 12V/30A power supply, you may think that you need an electronic load rated at 360 W. The supply may never actually operate at both 12 V and 30 A, though. The power supply may have a maximum current output of 7.5 A at 12 V, and only reach the maximum current output of 30 A at an output voltage of 3 V. In this case, an electronic load rated at 90W or more would be sufficient.
Using the electronic load as a current modulator
A special case for load sizing occurs when it is used as a fast current modulator to improve the performance of a power supply. In this case, it is possible to use an electronic load whose power rating is only a fraction of the power supply power rating.
The reason for this is that when current is set to its highest level, the voltage across the load will be minimal (~1-2V). On the other hand, when the voltage across the load is at the maximum value, the current will be very small.
In general, if the modulation is from zero current to some maximum, the load power can be sized at one-quarter of the operating voltage times the operating current, although the maximum voltage and current ratings must be respected . The maximum power occurs during the transition from low to high and at approximately half of the operating current and voltage.
For more information on selecting an electronic load, get our app note, “DC Electronic Load Selection.”