Wednesday, February 22, 2012

On DC Source Voltage and Current Levels and (Compliance) Limits Part 1: When levels and limits are one and the same

I was having a discussion with a colleague about constant current operation versus constant voltage operation and the distinction between level settings and limit settings the other day. “The level and limit settings are really the same thing!” he claimed. I disagreed. We each then made ensuing arguments in defense of our positions.

He based his argument on the case of a DC power supply that has both constant voltage and constant current operation. I’ll agree that is a reasonable starting point. As a side note there is a general consensus here that if it isn’t a true, well regulated constant voltage or constant current, whether settable or fixed, then it is simply a limit, not a level setting, end of story. He continued “if the load on the power supply is such that it is operating in constant voltage, then the voltage setting is the level setting and the current setting is the limit setting. If the load increases such that the power supply changes over from constant voltage operation into constant current operation then the voltage setting is becomes the limit setting and the current setting becomes the level setting!” (See figure 1.) He certainly has a good point! For your more basic DC power supply that only operates in quadrant 1 capable of sourcing power only, the current and voltage settings usually interchangeably serve as both the level and compliance limit setting, depending on whether the DC power supply is operating in constant voltage or constant current. The level and compliance limit regulating circuits are one and the same. Likewise with the programming, there are only commands to set the voltage and current levels. There are not separate commands for the limits. I might be starting to lose grounds on this discussion!
Figure 1: Unipolar single quadrant DC source operation

However, all is not lost yet. The DC power supply world is often more complicated than just this unipolar single quadrant operation just presented. Watch for my second part on when the levels and limits are not necessarily one and the same.

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