GWInstek PEL3031-E CP mode issues

[pwnell]

Hi,

Please take note that this is the first time I have ever used a DC electronic load - I am learning so that is my background.  I might be making a stupid mistake - if so, please enlighten me.

My issue is that when using CP mode, if I set it to something under about 1.2W, I never get the wattage I am asking for.  Above about 1.5W it is roughly drawing the power I am asking of it.  I initially tested Ni-MH batteries, but this behaviour confused me.  So I hooked up my bench power supply to eliminate any DUT issues.  Lab power supply was set to OCP of 2A and 1.4V which is plenty to do 2W with less than 2A current flowing.  See the video I took of testing from 0.1W up to 2W - why does the load not sink the wattage I ask of it below 1.5W?  Von = 0.0V.

https://www.waldonell.com/static/images/IMG_5328-small.MOV

I can add that CA seems to work fine.

[golden_labels]

Looking at PEL-3031E specs from RS Online, the range for CP mode is 3 W – 30 W (low mode) and 30 W – 300 W (high mode). If I am understanding that right, below 3 W is outside of specification.

Not displaying any warning is a really poor UI decision from the manufacturer, though.

[pwnell]

That would just be terrible because it allows me to enter 1mW…

[electr_peter]

Range: 3W - 30W///
Setting Range: 0 - 30.6W

It seems that this DC load cannot do <3W properly due to limited mechanism of CP control. In video it can be seen that from 1.5-2W DC load almost manages to work, but with all CP settings current is very unstable. IMO CP mode is implemented indirectly via digital control of CC mode with disadvantage of slow performance and erratic behaviour (i.e., check current*voltage value; change CC value; check current*voltage value; change CC value; ...). The point is that power is not directly controlled.

This is similar to how Rigol DL3000 works in CP mode, current is not exactly stable with 1.5W at 1.4V input (but it can work to ~100mW or less).

Very high end DC loads have better performance in CP mode (some even have hardware or analog CP mode, which works much better).

That would just be terrible because it allows me to enter 1mW…

1mW setting resolution is likely there just to match readback resolution, not that it is usable.

[pwnell]

Thank you , this was most useful. 

[pwnell]

I guess sometimes the answer is to read the datasheet first then get the right instrument.  No issues here so far:

https://www.waldonell.com/static/images/IMG_5333-small.mov

[electr_peter]

That's more like it. Sadly it is difficult to know from specs alone how DC load will behave, demo unit may be necessary for proper evaluation.