I have an Aglient power supply Model number N5747A, and I want to use its current sinking ability to become an active load.
This is to test a DC/DC converter (5VDC Output), Gaia MGDS-35-H-C, I want to load the output of the DC/DC to 75% load.
Requirement of Electronic Load
I am using an active load because I need to vary the load from 1% to 100% in 15 steps. An active load will be smoother and more accurate.
According to the datasheet 75% load is around 5.25A. My idea is to hook up the Agilent power supply and set the current limit to 5.25A on the output of the DC/DC converter.
My question is, what kind of "fixed" resistance would I need, would it be just to protect the Agilent supply and DC/DC converter? I assume I would need some kind of power resistor.
The power supply will create a current loop and force the DC/DC converter to draw 75% of its maximum current. Am I correct in my understanding of this?
Below is a drawing for 100% load (7A) on the output of the DC/DC converter
Then the fixed resistance I chose was to maintain a safe operating voltage for the current sink. Example: 60V current source, to maintain 7A's of current through the loop a 4 Ohm resistor will operate the current source in the middle of its operating voltage range of around (28V – 5V )= 23V. If you chose an 8 Ohm resistor for example, then you'll be operating really close to the supply's limits (7A * 8 Ohms = 56V-5=51V). Also the fixed resistor limit in cause the supply goes to 0V (a dead short), so at least 5V/7A = 1 Ohm minimum (rounded up for safety) to put a fixed cap on the current that will flow.
Best Answer
Most power supplies do not have "current sinking" capability.
When discussing this, we talk about the four quadrants of the I-V space:
Where the response curve of the supply is plotted with I on the x-axis and V on the y-axis. For example an ideal voltage supply would be characterised by a horizontal line at constant voltage, no matter what the current.
Most power supplies can operate only in the I quadrant. This means that current is going out of the more positive terminal of the supply. Some can also (or you can flip the leads to work in) the III quadrant. In these quadrants, the supply is delivering power to its load.
Only specially designed supplies can operate in the II and IV quadrants. These are called "4 quadrant supplies". An example of this is the BOP series supplies from Kepco.
There are also "electronic loads" designed to operate in the II and IV quadrants, such as the Agilent N6063B.
However, it's not obvious from what you said about the goals of your test that you couldn't simply connect a (or a combination of) power resistors to the supply being tested instead of spending a lot of money on an electronic load.