The purpose of the dummy load is to increase the speed of the voltage fall time for high speed test applications. However, this function is not guaranteed to work for all applications.
Here we present a couple of examples of when NOT to use the dummy load:
1. Fast transients with frequencies bigger than 500 Hz.
The issue that can be observed at 500 Hz is that the dummy load overheats. At this speed, the dummy load does not have enough time to cool down, especially when we are dealing with big voltages. The dummy load function was originally designed for the purpose of providing the functional tests to comply with some automotive test standards, as described in the manual for the 9115-AT. But the dummy load became an extension also for the non-AT models in the series. That being said, those automotive test required the fastest time using the dummy load to be 5ms, only momentarily and not continuous. The load has not been specified yet, partly because initially was not intended to be used for other purposes other than the automotive, but it was later made available for the general user. The dummy load’s performance will greatly vary depending on environmental conditions because the limitation (cooling down the components) is temperature related. The conclusion is that the dummy load can be used for transients, but not for faster transients of 500 Hz.
2. Charging batteries with the dummy load on.
For example, it is not recommended for applications that included battery charging or power electric motors. Any other loads that may behave similarly to these load should not use this function. The issue observed with a battery, for example, will depend on the voltage level of the battery. If the voltage level of the battery is low, then what the dummy load will do is to drain the battery to very low levels or completely and that is typically not recommended to be done by the battery manufacturers. If the battery capacity is big and this function is enabled, a large current will flow through the dummy load and damage the output and dummy load from the power supply.
Note: The output impedance of the 9115 series is 0.0002Ω and when used with the dummy load that value decreases significantly, however, it is not specified at the moment.